LIPIDS
Lipids are water-insoluble oily or fatty substances that can be extracted from cells with non-polar solvents. This is a heterogeneous group of compounds associated directly or indirectly with fatty acids.
Biological functions of lipids:
1) a source of energy that can be stored for a long time;
2) participation in the formation of cell membranes;
3) a source of fat-soluble vitamins, signaling molecules and essential fatty acids;
4) thermal insulation;
5) non-polar lipids serve as electrical insulators, ensuring the rapid propagation of depolarization waves along myelinated nerve fibers;
6) participation in the formation of lipoproteins.
Fatty acids are the structural components of most lipids. These are long-chain organic acids containing from 4 to 24 carbon atoms, they contain one carboxyl group and a long non-polar hydrocarbon tail. In cells, they are not found in a free state, but only in a covalently bound form. Natural fats usually contain fatty acids with an even number of carbon atoms, since they are synthesized from two-carbon units that form an unbranched chain of carbon atoms. Many fatty acids have one or more double bonds - unsaturated fatty acids.
The most important fatty acids (number of carbon atoms, name, melting point are given after the formula):
12, lauric, 44.2 o C
14, myristic, 53.9 o C
16, palmitic, 63.1 o C
18, stearic, 69.6 o C
18, oleic, 13.5 o C
18, linoleic, -5 o C
18, linolenic, -11 o C
20, arachidonic, -49.5 o C
General properties of fatty acids;
Almost all contain an even number of carbon atoms,
Saturated acids are twice as common in animals and plants as unsaturated acids.
Saturated fatty acids do not have a rigid linear structure, they are highly flexible and can adopt a variety of conformations,
In most fatty acids, the existing double bond is located between the 9th and 10th carbon atoms (Δ 9),
Additional double bonds are usually located between the Δ 9 -double bond and the methyl end of the chain,
Two double bonds in fatty acids are not conjugated, there is always a methylene group between them,
The double bonds of almost all natural fatty acids are found in cis-conformations, which leads to a strong bending of the aliphatic chain and a more rigid structure,
At body temperature, saturated fatty acids are in a solid, waxy state, while unsaturated fatty acids are liquids.
Sodium and potassium soaps of fatty acids are able to emulsify water-insoluble oils and fats, calcium and magnesium soaps of fatty acids dissolve very poorly and do not emulsify fats.
Unusual fatty acids and alcohols are found in bacterial membrane lipids. Many of the bacterial strains containing these lipids (thermophiles, acidophiles, and gallophiles) are adapted to extreme conditions.
isobranched
anteisobranched
cyclopropane-containing
ω-cyclohexyl-containing
isopranile
cyclopentanphytanyl
The composition of bacterial lipids is very diverse, and the spectrum of fatty acids of different species has become a taxonomic criterion for identifying organisms.
In animals, important derivatives of arachidonic acid are the histohormones prostaglandins, thromboxanes, and leukotrienes, united in the group of eicosanoids and having an extremely wide biological activity.
prostaglandin H 2
Lipid classification:
1. Triacylglycerides(fats) are esters of the alcohol glycerol and three molecules of fatty acids. They constitute the main component of the fat depots of plant and animal cells. The membranes do not contain Simple triacylglycerides contain residues of the same fatty acids in all three positions (tristearin, tripalmitin, triolein). Mixed contain different fatty acids. It is lighter than water in specific gravity, readily soluble in chloroform, benzene, and ether. Hydrolyzed by boiling with acids or bases, or by the action of lipase. In cells under normal conditions, the autoxidation of unsaturated fats is completely inhibited due to the presence of vitamin E, various enzymes and ascorbic acid. In specialized cells of the connective tissue of animal adipocytes, a huge amount of triacylglycerides can be stored in the form of fat droplets that fill almost the entire volume of the cell. In the form of glycogen, the body can store energy for no more than a day. Triacylglycerides can store energy for months, as they can be stored in very large quantities in almost pure, unhydrated form and, per unit weight, they store twice as much energy as carbohydrates. In addition, triacylglycerides under the skin form a heat-insulating layer that protects the body from the effects of very low temperatures.
neutral fat
The following constants are used to characterize the properties of fat:
Acid number - the number of mg KOH required to neutralize
free fatty acids contained in 1 g of fat;
Saponification number - the number of mg KOH required for hydrolysis
neutral lipids and neutralization of all fatty acids,
Iodine number - the number of grams of iodine associated with 100 g of fat,
characterizes the degree of unsaturation of a given fat.
2. Wax are esters formed by long-chain fatty acids and long-chain alcohols. In vertebrates, waxes secreted by the skin glands function as a protective coating that lubricates and softens the skin, and also protects it from water. Hair, wool, fur, animal feathers, as well as the leaves of many plants are covered with a wax layer. Waxes are produced and used in very large quantities by marine organisms, especially plankton, in which they serve as the main form of accumulation of high-calorie cellular fuel.
spermaceti, obtained from the brain of sperm whales
beeswax
3. Phosphoglycerolipids- serve as the main structural components of membranes and are never stored in large quantities. Be sure to contain polyhydric alcohol glycerin, phosphoric acid and fatty acid residues.
Phosphoglycerolipids can be divided into several types according to their chemical structure:
1) phospholipids - consist of glycerol, two fatty acid residues at the 1st and 2nd positions of glycerol and a phosphoric acid residue, to which the residue of another alcohol (ethanolamine, choline, serine, inositol) is associated. As a rule, the fatty acid in the 1st position is saturated, and in the 2nd - unsaturated.
phosphatidic acid - the starting material for the synthesis of other phospholipids, is found in tissues in small quantities
Phosphatidylethanolamine (Kefalin)
phosphatidylcholine (lecithin), it is practically absent in bacteria
phosphatidylserine
phosphatidylinositol is a precursor of two important second messengers (intermediaries) diacylglycerol and inositol-1,4,5-triphosphate
2) plasmalogens - phosphoglycerolipids, in which one of the hydrocarbon chains is a simple vinyl ether. Plasmalogens are not found in plants. Ethanolamine plasmalogens are widely distributed in myelin and in the sarcoplasmic reticulum of the heart.
ethanolamineplasmalogen
3) lysophospholipids - are formed from phospholipids during the enzymatic cleavage of one of the acyl residues. Snake venom contains phospholipase A 2, which forms lysophosphatides, which have a hemolytic effect;
4) cardiolipins - phospholipids of the inner membranes of bacteria and mitochondria, are formed by the interaction of two phosphatidic acid residues with glycerol:
cardiolipin
4. Phosphosphingolipids- the functions of glycerol in them are performed by sphingosine - an amino alcohol with a long aliphatic chain. Does not contain glycerin. They are present in large quantities in the membranes of cells of the nervous tissue and the brain. Phosphingolipids are rare in plant and bacterial cell membranes. Derivatives of sphingosine acylated at the amino group with fatty acid residues are called ceramides. The most important representative of this group is sphingomyelin (ceramide-1-phosphocholine). It is present in most membranes of animal cells, especially in the myelin sheaths of certain types of nerve cells.
sphingomyelin
sphingosine
5. Glycoglycerolipids - lipids that have a carbohydrate attached via a glycosidic bond in position 3 of glycerol do not contain a phosphate group. Glycoglycerolipids are widely present in chloroplast membranes, as well as in blue-green algae and bacteria. Monogalactosyldiacylglycerol is the most abundant polar lipid in nature, since it accounts for half of all lipids in the thylakoid membrane of chloroplasts:
monogalactosyldiacylglycerol
6. Glycosphingolipids- built from sphingosine, a fatty acid residue and an oligosaccharide. Contained in all tissues, mainly in the outer lipid layer of plasma membranes. They lack a phosphate group and carry no electrical charge. Glycosphingolipids can be further divided into two types:
1) cerebrosides are simpler representatives of this group. Galactocerebrosides are found mainly in the membranes of brain cells, while glucocerebrosides are present in the membranes of other cells. Cerebrosides containing two, three or four sugar residues are localized mainly in the outer layer of cell membranes.
galactocerebroside
2) gangliosides are the most complex glycosphingolipids. Their very large polar heads are formed by several sugar residues. They are characterized by the presence in the extreme position of one or more residues of N-acetylneuraminic (sialic) acid, which carries a negative charge at pH 7. In the gray matter of the brain, gangliosides make up about 6% of membrane lipids. Gangliosides are important components of specific receptor sites located on the surface of cell membranes. So they are located in those specific areas of nerve endings where the binding of neurotransmitter molecules occurs in the process of chemical transmission of an impulse from one nerve cell to another.
7. Isoprenoids- derivatives of isoprene (active form - 5-isopentenyl diphosphate), performing a wide variety of functions.
isoprene 5-isopentenyl diphosphate
The ability to synthesize specific isoprenoids is characteristic only of some species of animals and plants.
1) rubber - synthesized by several types of plants, primarily Hevea brazilian:
rubber fragment
2) fat-soluble vitamins A, D, E, K (due to the structural and functional affinity for steroid hormones, vitamin D is now classified as a hormone):
vitamin A
vitamin E
vitamin K
3) animal growth hormones - retinoic acid in vertebrates and neotenins in insects:
retinoic acid
neotenin
Retinoic acid is a hormonal derivative of vitamin A, stimulates cell growth and differentiation, neotenins are insect hormones, stimulate the growth of larvae and inhibit molting, are ecdysone antagonists;
4) plant hormones - abscisic acid, is a stress phytohormone that triggers a systemic immune response of plants, manifested in resistance to a variety of pathogens:
abscisic acid
5) terpenes - numerous aromatic substances and essential oils of plants with bactericidal and fungicidal action; compounds of two isoprene units are called monoterpenes, of three - sesquiterpenes, of six - triterpenes:
camphor thymol
6) steroids are complex fat-soluble substances, the molecules of which contain cyclopentanperhydrophenanthrene (in essence, triterpene). The main sterol in animal tissues is the alcohol cholesterol (cholesterol). Cholesterol and its esters with long-chain fatty acids are important components of plasma lipoproteins, as well as the outer cell membrane. Because the four fused rings create a rigid structure, the presence of cholesterol in the membranes regulates membrane fluidity at extreme temperatures. Plants and microorganisms contain related compounds - ergosterol, stigmasterol and β-sitosterol.
cholesterol
ergosterol
stigmasterin
β-sitosterol
Bile acids are formed from cholesterol in the body. They ensure the solubility of cholesterol in bile and promote the digestion of lipids in the intestine.
cholic acid
Steroid hormones are also formed from cholesterol - lipophilic signaling molecules that regulate metabolism, growth and reproduction. There are six main steroid hormones in the human body:
cortisol aldosterone
testosterone estradiol
progesterone calcitriol
Calcitriol is a hormonally active vitamin D that differs from vertebrate hormones, but is also built on the basis of cholesterol. Ring B opens by a light-dependent reaction.
A derivative of cholesterol is the molting hormone of insects, spiders and crustaceans - ecdysone. Steroid hormones that perform a signaling function are also found in plants.
7) lipid anchors that hold molecules of proteins or other compounds on the membrane:
ubiquinone
As we can see, lipids are not polymers in the literal sense of the word, however, both metabolically and structurally, they are close to the polyhydroxybutyric acid present in bacteria, an important reserve substance. This highly reduced polymer consists solely of D-β-hydroxybutyric acid units connected by an ester bond. Each chain contains about 1500 residues. The structure is a compact right-handed helix, with about 90 such chains stacked to form a thin layer in bacterial cells.
poly-D-β-hydroxybutyric acid
Amino acids are called carboxylic acids containing an amino group and a carboxyl group. Natural amino acids are 2-aminocarboxylic acids, or α-amino acids, although there are such amino acids as β-alanine, taurine, γ-aminobutyric acid. The generalized formula for an α-amino acid looks like this:
α-amino acids at the 2 carbon atom have four different substituents, that is, all α-amino acids, except for glycine, have an asymmetric (chiral) carbon atom and exist in the form of two enantiomers - L- and D-amino acids. Natural amino acids belong to the L-series. D-amino acids are found in bacteria and peptide antibiotics.
All amino acids in aqueous solutions can exist as bipolar ions, and their total charge depends on the pH of the medium. The pH value at which the total charge is zero is called the isoelectric point. At the isoelectric point, the amino acid is a zwitterion, that is, its amine group is protonated, and the carboxyl group is dissociated. In the neutral pH region, most amino acids are zwitterions:
Amino acids do not absorb light in the visible region of the spectrum, aromatic amino acids absorb light in the UV region of the spectrum: tryptophan and tyrosine at 280 nm, phenylalanine at 260 nm.
Amino acids are characterized by some chemical reactions that are of great importance for laboratory practice: a colored ninhydrin test for the α-amino group, reactions characteristic of sulfhydryl, phenolic and other groups of amino acid radicals, acetylation and the formation of Schiff bases by amino groups, esterification by carboxyl groups.
The biological role of amino acids:
1) are structural elements of peptides and proteins, the so-called proteinogenic amino acids. The composition of proteins includes 20 amino acids that are encoded by the genetic code and are included in proteins during translation, some of them can be phosphorylated, acylated or hydroxylated;
2) can be structural elements of other natural compounds - coenzymes, bile acids, antibiotics;
3) are signaling molecules. Some of the amino acids are neurotransmitters or precursors of neurotransmitters, hormones and histohormones;
4) are essential metabolites, for example, some amino acids are precursors of plant alkaloids, or serve as nitrogen donors, or are vital components of nutrition.
The classification of proteinogenic amino acids is based on the structure and polarity of the side chains:
1. Aliphatic amino acids:
glycine, gly, G, Gly
alanine, ala, A, Ala
valine, shaft, V, Val*
Leucine lei, L, Leu*
isoleucine, ile, I, Ile*
These amino acids do not contain heteroatoms or cyclic groups in the side chain and are characterized by a pronounced low polarity.
cysteine, cis, C, Cys
methionine, meth, M, Met*
3. Aromatic amino acids:
phenylalanine, hair dryer, F, Phe*
tyrosine, shooting gallery, Y, Tyr
tryptophan, three, W, Trp*
histidine, gis, H, His
Aromatic amino acids contain mesomeric resonantly stabilized cycles. In this group, only the amino acid phenylalanine exhibits low polarity, tyrosine and tryptophan are characterized by noticeable polarity, and histidine even by high polarity. Histidine can also be classified as a basic amino acid.
4. Neutral amino acids:
serine, ser, S, Ser
threonine, tre, T, Thr*
asparagine, asn, N, Asn
glutamine, gln, Q,Gln
Neutral amino acids contain hydroxyl or carboxamide groups. Although the amide groups are non-ionic, the molecules of asparagine and glutamine are highly polar.
5. Acidic amino acids:
aspartic acid (aspartate), asp, D, Asp
glutamic acid (glutamate), deep, E Glu
The carboxyl groups of the side chains of acidic amino acids are completely ionized over the entire range of physiological pH values.
6. Basic amino acids:
lysine, l from, K, Lys*
arginine, arg, R, Arg
The side chains of basic amino acids are completely protonated in the neutral pH region. A highly basic and very polar amino acid is arginine containing a guanidine moiety.
7. Imino acid:
proline, about, P, Pro
The side chain of proline consists of a five-membered ring, including an α-carbon atom and an α-amino group. Therefore, proline, strictly speaking, is not an amino acid, but an imino acid. The nitrogen atom in the ring is a weak base and does not protonate at physiological pH values. Due to the cyclic structure, proline causes bends in the polypeptide chain, which is very important for the structure of collagen.
Some of the listed amino acids cannot be synthesized in the human body and must be supplied with food. These essential amino acids are marked with asterisks.
As mentioned above, proteinogenic amino acids are the precursors of some valuable biologically active molecules.
Two biogenic amines β-alanine and cysteamine are part of coenzyme A (coenzymes are derivatives of water-soluble vitamins that form the active center of complex enzymes). β-Alanine is formed by the decarboxylation of aspartic acid, and cysteamine by the decarboxylation of cysteine:
β-alanine cysteamine
The glutamic acid residue is part of another coenzyme - tetrahydrofolic acid, a derivative of vitamin B c.
Other biologically valuable molecules are conjugates of bile acids with the amino acid glycine. These conjugates are stronger acids than basic acids, are formed in the liver and are present in bile as salts.
glycocholic acid
Proteinogenic amino acids are the precursors of some antibiotics - biologically active substances synthesized by microorganisms and inhibiting the reproduction of bacteria, viruses and cells. The most famous of them are penicillins and cephalosporins, which make up the group of β-lactam antibiotics and are produced by molds of the genus Penicillium. They are characterized by the presence in the structure of a reactive β-lactam ring, with the help of which they inhibit the synthesis of cell walls of gram-negative microorganisms.
general formula of penicillins
From amino acids by decarboxylation, biogenic amines are obtained - neurotransmitters, hormones and histohormones.
The amino acids glycine and glutamate are themselves neurotransmitters in the central nervous system.
Derivatives of amino acids are also alkaloids - natural nitrogen-containing compounds of the main nature, formed in plants. These compounds are extremely active physiological compounds widely used in medicine. Examples of alkaloids are the phenylalanine derivative papaverine, the isoquinoline alkaloid of the hypnotic poppy (an antispasmodic), and the tryptophan derivative physostigmine, an indole alkaloid from Calabar beans (an anticholinesterase drug):
papaverine physostigmine
Amino acids are extremely popular objects of biotechnology. There are many options for the chemical synthesis of amino acids, but the result is racemates of amino acids. Since only L-isomers of amino acids are suitable for food industry and medicine, racemic mixtures must be separated into enantiomers, which presents a serious problem. Therefore, the biotechnological approach is more popular: enzymatic synthesis using immobilized enzymes and microbiological synthesis using whole microbial cells. In both latter cases, pure L-isomers are obtained.
Amino acids are used as food additives and feed components. Glutamic acid enhances the taste of meat, valine and leucine improve the taste of baked goods, glycine and cysteine are used as antioxidants in canning. D-tryptophan can be used as a sugar substitute as it is many times sweeter. Lysine is added to feed for farm animals, since most vegetable proteins contain a small amount of the essential amino acid lysine.
Amino acids are widely used in medical practice. These are amino acids such as methionine, histidine, glutamic and aspartic acids, glycine, cysteine, valine.
In the last decade, amino acids have been added to skin and hair care products.
Chemically modified amino acids are also widely used in industry as surfactants in the synthesis of polymers, in the production of detergents, emulsifiers, and fuel additives.
Modern protein nutrition is impossible to imagine without considering the role of individual amino acids. Even with an overall positive protein balance, the animal's body may experience a lack of protein. This is due to the fact that the absorption of individual amino acids is interconnected in each other, a lack or excess of one amino acid can lead to a lack of another.
Some amino acids are not synthesized in the human body and animals. They are called indispensable. There are only ten such amino acids. Four of them are critical (limiting) - they most often limit the growth and development of animals.
Methionine and cystine are the main limiting amino acids in poultry diets, and lysine in pig diets. The organism must receive a sufficient amount of the main limiting acid in the diet so that other amino acids can be effectively used for protein synthesis.
This principle is illustrated by the Liebig barrel, where the fill level of the barrel represents the level of protein synthesis in the animal's body. The shortest board in the barrel "limits" the ability to hold liquid in it. If this board is extended, then the volume of liquid held in the barrel will increase to the level of the second limiting board.
The most important factor determining the productivity of animals is the balance of amino acids contained in it in accordance with physiological needs. Numerous studies have shown that in pigs, depending on the breed and sex, the need for amino acids differs quantitatively. But the ratio of essential amino acids for the synthesis of 1 g of protein is the same. This ratio of essential amino acids to lysine, as the main limiting amino acid, is called the "ideal protein" or "ideal amino acid profile". (
Lysine
is part of almost all proteins of animal, plant and microbial origin, however, proteins of cereal crops are poor in lysine.
- Lysine regulates the reproductive function, with a lack of it, the formation of sperm and eggs is disrupted.
- Necessary for the growth of young animals, the formation of tissue proteins. Lysine takes part in the synthesis of nucleoproteins, chromoproteins (hemoglobin), thereby regulating the pigmentation of animal hair. Regulates the amount of protein breakdown products in tissues and organs.
- Promotes calcium absorption
- Participates in the functional activity of the nervous and endocrine systems, regulates the metabolism of proteins and carbohydrates, however, reacting with carbohydrates, lysine becomes inaccessible to absorption.
- Lysine is the initial substance in the formation of carnitine, which plays an important role in fat metabolism.
Methionine and cystine sulfur-containing amino acids. At the same time, methionine can be transformed into cystine, so these amino acids are normalized together, and in case of a deficiency, methionine supplements are introduced into the diet. Both of these amino acids are involved in the formation of derivatives of the skin - hair, feather; Together with vitamin E, they regulate the removal of excess fat from the liver, and are necessary for the growth and reproduction of cells, red blood cells. With a lack of methionine, cystine is inactive. However, a significant excess of methionine in the diet should not be allowed.
Methionine
promotes the deposition of fat in the muscles, is necessary for the formation of new organic compounds of choline (vitamin B4), creatine, adrenaline, niacin (vitamin B5), etc.
Methionine deficiency in diets leads to a decrease in the level of plasma proteins (albumins), causes anemia (a decrease in the level of hemoglobin in the blood), while a lack of vitamin E and selenium contributes to the development of muscular dystrophy. An insufficient amount of methionine in the diet causes stunting of young animals, loss of appetite, decreased productivity, increased feed costs, fatty liver, impaired kidney function, anemia and malnutrition.
An excess of methionine impairs the use of nitrogen, causes degenerative changes in the liver, kidneys, pancreas, increases the need for arginine, glycine. With a large excess of methionine, an imbalance is observed (the balance of amino acids is disturbed, which is based on sharp deviations from the optimal ratio of essential amino acids in the diet), which is accompanied by metabolic disorders and inhibition of the growth rate in young animals.
Cystine is a sulfur-containing amino acid, interchangeable with methionine, participates in redox processes, metabolism of proteins, carbohydrates and bile acids, promotes the formation of substances that neutralize intestinal poisons, activates insulin, together with tryptophan, cystine participates in the synthesis in the liver of bile acids necessary for absorption products of digestion of fats from the intestines, is used for the synthesis of glutathione. Cystine has the ability to absorb ultraviolet rays. With a lack of cystine, cirrhosis of the liver, a delay in feathering and feather growth in young animals, fragility and loss (plucking) of feathers in an adult bird, and a decrease in resistance to infectious diseases are noted.
tryptophan
determines the physiological activity of digestive tract enzymes, oxidative enzymes in cells and a number of hormones, participates in the renewal of blood plasma proteins, determines the normal functioning of the endocrine and hematopoietic apparatuses, the reproductive system, the synthesis of gamma globulins, hemoglobin, nicotinic acid, ocular purpura, etc. in the diet of tryptophan, the growth of young animals slows down, egg production of laying hens decreases, feed costs for products increase, endocrine and sex glands atrophy, blindness occurs, anemia develops (the number of red blood cells and hemoglobin levels in the blood decrease), resistance and immune properties of the body decrease, fertilization and hatchability of eggs . In pigs fed a diet poor in tryptophan, feed intake is reduced, perverted appetite, roughening of the bristles and emaciation appear, fatty liver is noted. Deficiency of this amino acid also leads to sterility, irritability, convulsions, cataract formation, negative nitrogen balance and weight loss. Tryptophan, being a precursor (provitamin) of nicotinic acid, prevents the development of pellagra.
Amino acids are called carboxylic acids containing an amino group and a carboxyl group. Natural amino acids are 2-aminocarboxylic acids, or α-amino acids, although there are such amino acids as β-alanine, taurine, γ-aminobutyric acid. The generalized formula for an α-amino acid looks like this:
α-amino acids at the 2 carbon atom have four different substituents, that is, all α-amino acids, except for glycine, have an asymmetric (chiral) carbon atom and exist in the form of two enantiomers - L- and D-amino acids. Natural amino acids belong to the L-series. D-amino acids are found in bacteria and peptide antibiotics.
All amino acids in aqueous solutions can exist as bipolar ions, and their total charge depends on the pH of the medium. The pH value at which the total charge is zero is called the isoelectric point. At the isoelectric point, the amino acid is a zwitterion, that is, its amine group is protonated, and the carboxyl group is dissociated. In the neutral pH region, most amino acids are zwitterions:
Amino acids do not absorb light in the visible region of the spectrum, aromatic amino acids absorb light in the UV region of the spectrum: tryptophan and tyrosine at 280 nm, phenylalanine at 260 nm.
Amino acids are characterized by some chemical reactions that are of great importance for laboratory practice: a colored ninhydrin test for the α-amino group, reactions characteristic of sulfhydryl, phenolic and other groups of amino acid radicals, acetylation and the formation of Schiff bases by amino groups, esterification by carboxyl groups.
The biological role of amino acids:
are structural elements of peptides and proteins, the so-called proteinogenic amino acids. The composition of proteins includes 20 amino acids that are encoded by the genetic code and are included in proteins during translation, some of them can be phosphorylated, acylated or hydroxylated;
can be structural elements of other natural compounds - coenzymes, bile acids, antibiotics;
are signaling molecules. Some of the amino acids are neurotransmitters or precursors of neurotransmitters, hormones and histohormones;
are essential metabolites, for example, some amino acids are precursors of plant alkaloids, or serve as nitrogen donors, or are vital components of nutrition.
The classification of proteinogenic amino acids is based on the structure and polarity of the side chains:
1. Aliphatic amino acids:
glycine, gly,G,Gly
alanine, ala, A,Ala
valine, shaft,V,Val*
leucine, lei,L,Leu*
isoleucine, ile, I,Ile*
These amino acids do not contain heteroatoms or cyclic groups in the side chain and are characterized by a pronounced low polarity.
cysteine, cis,C,Cys
methionine, meth,M,Met*
3. Aromatic amino acids:
phenylalanine, hair dryer,F,Phe*
tyrosine, shooting gallery,Y,Tyr
tryptophan, three,W,Trp*
histidine, gis,H,His
Aromatic amino acids contain mesomeric resonantly stabilized cycles. In this group, only the amino acid phenylalanine exhibits low polarity, tyrosine and tryptophan are characterized by noticeable polarity, and histidine even by high polarity. Histidine can also be classified as a basic amino acid.
4. Neutral amino acids:
serine, ser,S, Ser
threonine, tre,T,Thr*
asparagine, asn, N, Asn
glutamine, gln, Q,Gln
Neutral amino acids contain hydroxyl or carboxamide groups. Although the amide groups are non-ionic, the molecules of asparagine and glutamine are highly polar.
5. Acidic amino acids:
aspartic acid (aspartate), asp,D,Asp
glutamic acid (glutamate), deep, E,Glu
The carboxyl groups of the side chains of acidic amino acids are completely ionized over the entire range of physiological pH values.
6. Basic amino acids:
lysine, l from, K, Lys*
arginine, arg,R,Arg
The side chains of basic amino acids are completely protonated in the neutral pH region. A highly basic and very polar amino acid is arginine containing a guanidine moiety.
7. Imino acid:
proline, about,P,Pro
The side chain of proline consists of a five-membered ring, including an α-carbon atom and an α-amino group. Therefore, proline, strictly speaking, is not an amino acid, but an imino acid. The nitrogen atom in the ring is a weak base and does not protonate at physiological pH values. Due to the cyclic structure, proline causes bends in the polypeptide chain, which is very important for the structure of collagen.
Some of the listed amino acids cannot be synthesized in the human body and must be supplied with food. These essential amino acids are marked with asterisks.
As mentioned above, proteinogenic amino acids are the precursors of some valuable biologically active molecules.
Two biogenic amines β-alanine and cysteamine are part of coenzyme A (coenzymes are derivatives of water-soluble vitamins that form the active center of complex enzymes). β-Alanine is formed by the decarboxylation of aspartic acid, and cysteamine by the decarboxylation of cysteine:
β-alanine 
cysteamine
The glutamic acid residue is part of another coenzyme - tetrahydrofolic acid, a derivative of vitamin B c.
Other biologically valuable molecules are conjugates of bile acids with the amino acid glycine. These conjugates are stronger acids than basic acids, are formed in the liver and are present in bile as salts.
glycocholic acid
Proteinogenic amino acids are the precursors of some antibiotics - biologically active substances synthesized by microorganisms and inhibiting the reproduction of bacteria, viruses and cells. The most famous of them are penicillins and cephalosporins, which make up the group of β-lactam antibiotics and are produced by molds of the genus Penicillium. They are characterized by the presence in the structure of a reactive β-lactam ring, with the help of which they inhibit the synthesis of cell walls of gram-negative microorganisms.
general formula of penicillins
From amino acids by decarboxylation, biogenic amines are obtained - neurotransmitters, hormones and histohormones.
The amino acids glycine and glutamate are themselves neurotransmitters in the central nervous system.
dopamine (neurotransmitter) norepinephrine (neurotransmitter)
adrenaline (hormone) histamine (mediator and histohormone)
serotonin (neurotransmitter and histohormone) GABA (neurotransmitter)
thyroxine (hormone)
A derivative of the amino acid tryptophan is the best-known naturally occurring auxin, indoleacetic acid. Auxins are plant growth regulators, they stimulate the differentiation of growing tissues, the growth of cambium, roots, accelerate the growth of fruits and the fall of old leaves, their antagonists are abscisic acid.
indoleacetic acid
Derivatives of amino acids are also alkaloids - natural nitrogen-containing compounds of the main nature, formed in plants. These compounds are extremely active physiological compounds widely used in medicine. Examples of alkaloids are the phenylalanine derivative papaverine, the isoquinoline alkaloid of the hypnotic poppy (an antispasmodic), and the tryptophan derivative physostigmine, an indole alkaloid from Calabar beans (an anticholinesterase drug):
papaverine physostigmine
Amino acids are extremely popular objects of biotechnology. There are many options for the chemical synthesis of amino acids, but the result is racemates of amino acids. Since only L-isomers of amino acids are suitable for food industry and medicine, racemic mixtures must be separated into enantiomers, which presents a serious problem. Therefore, the biotechnological approach is more popular: enzymatic synthesis using immobilized enzymes and microbiological synthesis using whole microbial cells. In both latter cases, pure L-isomers are obtained.
Amino acids are used as food additives and feed components. Glutamic acid enhances the taste of meat, valine and leucine improve the taste of baked goods, glycine and cysteine are used as antioxidants in canning. D-tryptophan can be used as a sugar substitute as it is many times sweeter. Lysine is added to feed for farm animals, since most vegetable proteins contain a small amount of the essential amino acid lysine.
Amino acids are widely used in medical practice. These are amino acids such as methionine, histidine, glutamic and aspartic acids, glycine, cysteine, valine.
In the last decade, amino acids have been added to skin and hair care products.
Chemically modified amino acids are also widely used in industry as surfactants in the synthesis of polymers, in the production of detergents, emulsifiers, and fuel additives.
PROTEINS
Proteins are macromolecular substances consisting of amino acids linked by peptide bonds.
It is proteins that are the product of genetic information transmitted from generation to generation and carry out all life processes in the cell.
Protein Functions:
catalytic function. The most numerous group of proteins are enzymes - proteins with catalytic activity that speed up chemical reactions. Examples of enzymes are pepsin, alcohol dehydrogenase, glutamine synthetase.
Structural function. Structural proteins are responsible for maintaining the shape and stability of cells and tissues, these include keratins, collagen, fibroin.
transport function. Transport proteins carry molecules or ions from one organ to another or across membranes within a cell, such as hemoglobin, serum albumin, ion channels.
protective function. Proteins of the homeostasis system protect the body from pathogens, foreign information, blood loss - immunoglobulins, fibrinogen, thrombin.
regulatory function. Proteins carry out the functions of signaling substances - some hormones, histohormones and neurotransmitters, are receptors for signaling substances of any structure, provide further signal transmission in the biochemical signaling chains of the cell. Examples are the growth hormone somatotropin, the hormone insulin, H- and M-cholinergic receptors.
motor function. With the help of proteins, the processes of contraction and other biological movement are carried out. Examples are tubulin, actin, myosin.
spare function. Plants contain storage proteins, which are valuable nutrients; in animals, muscle proteins serve as reserve nutrients that are mobilized in case of emergency.
Proteins are characterized by the presence of several levels of structural organization.
primary structure A protein is the sequence of amino acid residues in a polypeptide chain. A peptide bond is a carboxamide bond between the α-carboxyl group of one amino acid and the α-amino group of another amino acid.
alanylphenylalanylcysteylproline
The peptide bond has several features:
a) it is resonantly stabilized and therefore is located practically in the same plane - it is planar; rotation around the C-N bond requires a lot of energy and is difficult;
b) the -CO-NH- bond has a special character, it is less than usual, but more than double, that is, there is a keto-enol tautomerism:
c) substituents in relation to the peptide bond are in trance-position;
d) the peptide backbone is surrounded by side chains of various nature, interacting with the surrounding solvent molecules, free carboxyl and amino groups are ionized, forming cationic and anionic centers of the protein molecule. Depending on their ratio, the protein molecule receives a total positive or negative charge, and is also characterized by one or another pH value of the medium when the isoelectric point of the protein is reached. Radicals form salt, ether, disulfide bridges inside the protein molecule, and also determine the range of reactions inherent in proteins.
At present, it has been agreed to consider polymers consisting of 100 or more amino acid residues as proteins, polymers consisting of 50-100 amino acid residues as polypeptides, and polymers consisting of less than 50 amino acid residues as low molecular weight peptides.
Some low molecular weight peptides play an independent biological role. Examples of some of these peptides:
Glutathione, γ-glu-cis-gly, is one of the most widespread intracellular peptides involved in redox processes in cells and the transport of amino acids across biological membranes.
Carnosine - β-ala-gis - a peptide contained in the muscles of animals, eliminates the products of lipid peroxidation, accelerates the breakdown of carbohydrates in the muscles and is involved in energy metabolism in the muscles in the form of phosphate.
Vasopressin is a hormone of the posterior pituitary gland involved in the regulation of water metabolism in the body:
Phalloidin is a poisonous fly agaric polypeptide, in negligible concentrations causes the death of the body due to the release of enzymes and potassium ions from cells:
Gramicidin is an antibiotic that acts on many gram-positive bacteria, changes the permeability of biological membranes for low molecular weight compounds and causes cell death:
Met-enkephalin - thyr-gli-gli-fen-met - a peptide synthesized in neurons and relieves pain.
Secondary structure of a protein is a spatial structure formed as a result of interactions between the functional groups of the peptide backbone.
The peptide chain contains many CO and NH groups of peptide bonds, each of which is potentially capable of participating in the formation of hydrogen bonds. There are two main types of structures that allow this to happen: the α-helix, in which the chain coils like a telephone cord, and the β-pleated structure, in which elongated sections of one or more chains are stacked side by side. Both of these structures are very stable.
The α-helix is characterized by extremely dense packing of the twisted polypeptide chain, each turn of the right-handed helix has 3.6 amino acid residues, the radicals of which are always directed outward and slightly backward, that is, to the beginning of the polypeptide chain.
The main characteristics of the α-helix:
The α-helix is stabilized by hydrogen bonds between the hydrogen atom at the nitrogen of the peptide group and the carbonyl oxygen of the residue, four positions away from the given one along the chain;
all peptide groups participate in the formation of a hydrogen bond, this ensures maximum stability of the α-helix;
all nitrogen and oxygen atoms of the peptide groups are involved in the formation of hydrogen bonds, which significantly reduces the hydrophilicity of the α-helical regions and increases their hydrophobicity;
α-helix is formed spontaneously and is the most stable conformation of the polypeptide chain, corresponding to a minimum of free energy;
in a polypeptide chain of L-amino acids, the right-handed helix, commonly found in proteins, is much more stable than the left-handed one.
The possibility of forming an α-helix is due to the primary structure of the protein. Some amino acids prevent the peptide backbone from twisting. For example, adjacent carboxyl groups of glutamate and aspartate mutually repel each other, which prevents the formation of hydrogen bonds in the α-helix. For the same reason, the chain coiling is difficult in places of positively charged lysine and arginine residues located close to each other. However, proline plays the greatest role in breaking the α-helix. Firstly, in proline, the nitrogen atom is part of a rigid ring, which prevents rotation around the N-C bond, and secondly, proline does not form a hydrogen bond due to the absence of hydrogen at the nitrogen atom.
β-folding is a layered structure formed by hydrogen bonds between linearly arranged peptide fragments. Both chains may be independent or belong to the same polypeptide molecule. If the chains are oriented in the same direction, then such a β-structure is called parallel. In the case of the opposite direction of the chains, that is, when the N-terminus of one chain coincides with the C-terminus of the other chain, the β-structure is called antiparallel. Energetically, antiparallel β-folding with almost linear hydrogen bridges is more preferable.
parallel β-folding antiparallel β-folding
Unlike the α-helix, which is saturated with hydrogen bonds, each section of the β-folding chain is open to the formation of additional hydrogen bonds. The amino acid side radicals are oriented almost perpendicular to the leaf plane, alternately up and down.
In those areas where the peptide chain bends rather steeply, there is often a β-loop. This is a short fragment in which 4 amino acid residues are bent 180 o and stabilized by one hydrogen bridge between the first and fourth residues. Large amino acid radicals interfere with the formation of the β-loop, so it most often includes the smallest amino acid, glycine.
Suprasecondary protein structure is some specific order of alternation of secondary structures. A domain is understood as a separate part of a protein molecule, which has a certain degree of structural and functional autonomy. Now domains are considered to be fundamental elements of the structure of protein molecules, and the ratio and nature of the layout of α-helices and β-layers provides more for understanding the evolution of protein molecules and phylogenetic relationships than a comparison of primary structures. The main task of evolution is the construction of new proteins. There is an infinitesimal chance of synthesizing such an amino acid sequence by chance that would satisfy the packaging conditions and ensure the fulfillment of functional tasks. Therefore, there are often proteins with different functions, but similar in structure to such an extent that it seems that they had a common ancestor or evolved from each other. It seems that evolution, faced with the need to solve a certain problem, prefers not to design proteins for this first, but to adapt already well-established structures for this, adapting them for new purposes.
Some examples of frequently repeated supra-secondary structures:
αα' - proteins containing only α-helices (myoglobin, hemoglobin);
ββ' – proteins containing only β-structures (immunoglobulins, superoxide dismutase);
βαβ' is the structure of the β-barrel, each β-layer is located inside the barrel and is associated with an α-helix located on the surface of the molecule (triose phosphoisomerase, lactate dehydrogenase);
"zinc finger" - a protein fragment consisting of 20 amino acid residues, the zinc atom is associated with two cysteine and two histidine residues, resulting in a "finger" of about 12 amino acid residues, can bind to the regulatory regions of the DNA molecule;
"leucine zipper" - interacting proteins have an α-helical region containing at least 4 leucine residues, they are located 6 amino acids apart from each other, that is, they are located on the surface of every second turn and can form hydrophobic bonds with leucine residues of another protein . With the help of leucine zippers, for example, molecules of strongly basic histone proteins can be combined into complexes, overcoming a positive charge.
Tertiary structure of a protein- this is the spatial arrangement of the protein molecule, stabilized by bonds between the side radicals of amino acids.
Types of bonds that stabilize the tertiary structure of a protein:
electrostatic hydrogen hydrophobic disulfide
interaction communication interaction communication
Depending on the folding of the tertiary structure, proteins can be classified into two main types - fibrillar and globular.
Fibrillar proteins are water-insoluble long filamentous molecules, the polypeptide chains of which are extended along one axis. These are mainly structural and contractile proteins. A few examples of the most common fibrillar proteins are:
α-Keratins. Synthesized by epidermal cells. They account for almost all the dry weight of hair, wool, feathers, horns, nails, claws, needles, scales, hooves, and tortoise shell, as well as a significant part of the weight of the outer layer of the skin. This is a whole family of proteins, they are similar in amino acid composition, contain many cysteine residues and have the same spatial arrangement of polypeptide chains. In hair cells, keratin polypeptide chains are first organized into fibers, from which structures are then formed like a rope or a twisted cable, which eventually fills the entire space of the cell. At the same time, the hair cells become flattened and finally die, and the cell walls form a tubular sheath around each hair, called the cuticle. In α-keratin, the polypeptide chains are in the form of an α-helix, twisted one around the other into a three-core cable with the formation of cross disulfide bonds. N-terminal residues are located on the same side (parallel). Keratins are insoluble in water due to the predominance of amino acids with non-polar side radicals in their composition, which are turned towards the aqueous phase. During perm, the following processes occur: first, disulfide bridges are destroyed by reduction with thiols, and then, when the hair is given the necessary shape, it is dried by heating, while due to oxidation with air oxygen, new disulfide bridges are formed that retain the shape of the hairstyle.
β-Keratins. These include silk and cobweb fibroin. They are antiparallel β-folded layers with a predominance of glycine, alanine and serine in the composition.
Collagen. The most common protein in higher animals and the main fibrillar protein of connective tissues. Collagen is synthesized in fibroblasts and chondrocytes - specialized connective tissue cells, from which it is then pushed out. Collagen fibers are found in the skin, tendons, cartilage and bones. They do not stretch, surpass steel wire in strength, collagen fibrils are characterized by transverse striation. When boiled in water, fibrous, insoluble and indigestible collagen is converted to gelatin by hydrolysis of some of the covalent bonds. Collagen contains 35% glycine, 11% alanine, 21% proline and 4-hydroxyproline (an amino acid found only in collagen and elastin). This composition determines the relatively low nutritional value of gelatin as a food protein. Collagen fibrils are made up of repeating polypeptide subunits called tropocollagen. These subunits are arranged along the fibril in the form of parallel bundles in a head-to-tail fashion. The shift of the heads gives the characteristic transverse striation. Voids in this structure, if necessary, can serve as a site for the deposition of crystals of hydroxyapatite Ca 5 (OH)(PO 4) 3 , which plays an important role in bone mineralization.
Tropocollagen subunits are composed of three polypeptide chains tightly twisted into a three-stranded rope, different from α- and β-keratins. In some collagens, all three chains have the same amino acid sequence, while in others only two chains are identical, and the third one differs from them. The tropocollagen polypeptide chain forms a left-handed helix, with only three amino acid residues per turn due to chain bends caused by proline and hydroxyproline. Three chains are interconnected, in addition to hydrogen bonds, by a covalent-type bond formed between two lysine residues located in adjacent chains:
As we get older, more and more cross-links are formed in and between the tropocollagen subunits, making collagen fibrils stiffer and more brittle, and this changes the mechanical properties of cartilage and tendons, makes bones more brittle, and reduces the transparency of the cornea of the eye.
Elastin. Contained in the yellow elastic tissue of the ligaments and the elastic layer of connective tissue in the walls of large arteries. The main subunit of elastin fibrils is tropoelastin. Elastin is rich in glycine and alanine, contains a lot of lysine and little proline. The helical sections of elastin stretch when stretched, but return to their original length when the load is removed. The lysine residues of the four different chains form covalent bonds with each other and allow elastin to reversibly stretch in all directions.
Globular proteins are proteins whose polypeptide chain is folded into a compact globule, capable of performing a wide variety of functions.
The tertiary structure of globular proteins is most conveniently considered using the example of myoglobin. Myoglobin is a relatively small oxygen-binding protein found in muscle cells. It stores bound oxygen and promotes its transfer to the mitochondria. The myoglobin molecule contains one polypeptide chain and one hemogroup (heme) - a complex of protoporphyrin with iron. The main properties of myoglobin:
a) the myoglobin molecule is so compact that only 4 water molecules can fit inside it;
b) all polar amino acid residues, with the exception of two, are located on the outer surface of the molecule, and all of them are in a hydrated state;
c) most of the hydrophobic amino acid residues are located inside the myoglobin molecule and, thus, are protected from contact with water;
d) each of the four proline residues in the myoglobin molecule is located at the bend of the polypeptide chain, serine, threonine and asparagine residues are located at other places of the bend, since such amino acids prevent the formation of an α-helix if they are with each other;
e) a flat hemogroup lies in a cavity (pocket) near the surface of the molecule, the iron atom has two coordination bonds directed perpendicular to the heme plane, one of them is connected to the histidine residue 93, and the other serves to bind the oxygen molecule.
Starting from the tertiary structure, the protein becomes capable of performing its biological functions. The functioning of proteins is based on the fact that when the tertiary structure is laid on the surface of the protein, sites are formed that can attach other molecules, called ligands, to themselves. The high specificity of the interaction of the protein with the ligand is provided by the complementarity of the structure of the active center with the structure of the ligand. Complementarity is the spatial and chemical correspondence of interacting surfaces. For most proteins, tertiary structure is the maximum level of folding.
Quaternary protein structure- characteristic of proteins consisting of two or more polypeptide chains interconnected exclusively by non-covalent bonds, mainly electrostatic and hydrogen. Most often proteins contain two or four subunits, more than four subunits usually contain regulatory proteins.
Proteins having a quaternary structure are often called oligomeric. Distinguish between homomeric and heteromeric proteins. Homeric proteins are proteins in which all subunits have the same structure, for example, the catalase enzyme consists of four absolutely identical subunits. Heteromeric proteins have different subunits, for example, the RNA polymerase enzyme consists of five subunits of different structure that perform different functions.
The interaction of one subunit with a specific ligand causes conformational changes in the entire oligomeric protein and changes the affinity of other subunits for ligands; this property underlies the ability of oligomeric proteins to allosteric regulation.
The quaternary structure of a protein can be considered using the example of hemoglobin. It contains four polypeptide chains and four heme prosthetic groups, in which the iron atoms are in the ferrous form Fe 2+ . The protein part of the molecule - globin - consists of two α-chains and two β-chains, containing up to 70% α-helices. Each of the four chains has a characteristic tertiary structure, and one hemogroup is associated with each chain. The hemes of different chains are relatively far apart and have different angles of inclination. Few direct contacts are formed between two α-chains and two β-chains, while numerous contacts of the α 1 β 1 and α 2 β 2 type formed by hydrophobic radicals form between the α- and β-chains. A channel remains between α 1 β 1 and α 2 β 2.
Unlike myoglobin, hemoglobin is characterized by a significantly lower affinity for oxygen, which allows it, at low partial pressures of oxygen existing in tissues, to give them a significant part of the bound oxygen. Oxygen is more easily bound by hemoglobin iron at higher pH values and low CO 2 concentrations, characteristic of the lung alveoli; the release of oxygen from hemoglobin is favored by lower pH values and high concentrations of CO 2 inherent in tissues.
In addition to oxygen, hemoglobin carries hydrogen ions, which bind to histidine residues in the chains. Hemoglobin also carries carbon dioxide, which attaches to the terminal amino group of each of the four polypeptide chains, resulting in the formation of carbaminohemoglobin:
In erythrocytes, the substance 2,3-diphosphoglycerate (DFG) is present in sufficiently high concentrations, its content increases with ascent to high altitude and during hypoxia, facilitating the release of oxygen from hemoglobin in tissues. DFG is located in the channel between α 1 β 1 and α 2 β 2 interacting with positively infected groups of β-chains. When oxygen is bound by hemoglobin, DPG is displaced from the cavity. The erythrocytes of some birds do not contain DPG, but inositol hexaphosphate, which further reduces the affinity of hemoglobin for oxygen.
2,3-diphosphoglycerate (DPG)
HbA - normal adult hemoglobin, HbF - fetal hemoglobin, has a greater affinity for O 2, HbS - hemoglobin in sickle cell anemia. Sickle cell anemia is a serious hereditary disease associated with a genetic abnormality of hemoglobin. In the blood of sick people, there is an unusually large number of thin sickle-shaped red blood cells, which, firstly, are easily torn, and secondly, clog the blood capillaries. At the molecular level, hemoglobin S differs from hemoglobin A in one amino acid residue in position 6 of the β-chains, where valine is located instead of a glutamic acid residue. Thus, hemoglobin S contains two negative charges less, the appearance of valine leads to the appearance of a “sticky” hydrophobic contact on the surface of the molecule, as a result, during deoxygenation, deoxyhemoglobin S molecules stick together and form insoluble abnormally long filamentous aggregates, leading to deformation of erythrocytes.
There is no reason to think that there is an independent genetic control over the formation of levels of protein structural organization above the primary one, since the primary structure determines both secondary, tertiary, and quaternary (if any). The native conformation of a protein is the most thermodynamically stable structure under the given conditions.
Lecture #1
TOPIC: "Amino acids".
Lecture plan:
1. Characterization of amino acids
2. Peptides.
Characterization of amino acids.
Amino acids are organic compounds, derivatives of hydrocarbons, whose molecules include carboxyl and amino groups.
Proteins are made up of amino acid residues linked by peptide bonds. To analyze the amino acid composition, protein hydrolysis is carried out, followed by the isolation of amino acids. Let us consider the main patterns characteristic of amino acids in proteins.
It has now been established that the composition of proteins includes a constantly frequently occurring set of amino acids. There are 18 of them. In addition to those indicated, 2 more amino acid amides were found - asparagine and glutamine. All of them were named major(frequent) amino acids. They are often figuratively referred to "magic" amino acids. In addition to major amino acids, there are also rare ones, those that are not often found in the composition of natural proteins. They are called minor.
Almost all amino acids in proteins are α - amino acids(the amino group is located at the first carbon atom after the carboxyl group). Based on the foregoing, for most amino acids the general formula is valid:
NH 2 -CH-COOH
Where R are radicals having different structures.
Consider the formulas of protein amino acids, Table. 2.
Everybody α - amino acids, except for aminoacetic (glycine), have an asymmetric α is a carbon atom and exists as two enantiomers. With rare exceptions, natural amino acids belong to the L-series. Only in the composition of the cell walls of bacteria and in antibiotics were found amino acids of the D genetic series. The rotation angle value is 20-30 0 degrees. Rotation can be right (7 amino acids) and left (10 amino acids).
H— *—NH 2 H 2 N—*—H
D - configuration L-configuration
(natural amino acids)
Depending on the predominance of amino or carboxyl groups, amino acids are divided into 3 subclasses:
acidic amino acids. Carboxyl (acid) groups predominate over amino groups (basic), for example, aspartic, glutamic acids.
Neutral amino acids The number of groups are equal. Glycine, alanine, etc.
Basic amino acids. Basic (amino groups) predominate over carboxyl (acidic), for example, lysine.
In physical and a number of chemical properties, amino acids differ sharply from the corresponding acids and bases. They are more soluble in water than in organic solvents; well crystallized; have high density and exceptionally high melting points. These properties indicate the interaction of amine and acid groups, as a result of which amino acids in the solid state and in solution (in a wide pH range) are in the zwitterionic form (i.e. as internal salts). The mutual influence of groups is especially pronounced in α-amino acids, where both groups are in close proximity.
H 2 N - CH 2 COOH ↔ H 3 N + - CH 2 COO -
zwitterion
Zwitter - ionic structure of amino acids is confirmed by their large dipole moment (not less than 5010 -30 C m), as well as the absorption band in the IR spectrum of a solid amino acid or its solution.
Amino acids are able to enter into polycondensation reactions, leading to the formation of polypeptides of different lengths, which constitute the primary structure of the protein molecule.
H 2 N–CH(R 1)-COOH + H 2 N– CH(R 2) – COOH → H 2 N – CH(R 1) – CO-NH– CH(R 2) – COOH
dipeptide
The C-N bond is called peptide connection.
In addition to the 20 most common amino acids discussed above, some other amino acids have been isolated from the hydrolysates of some specialized proteins. All of them are, as a rule, derivatives of ordinary amino acids, i.e. modified amino acids.
4-hydroxyproline , found in the fibrillar protein collagen and some plant proteins; 5-oxylysin found in collagen hydrolysates, desmosy n and isodesmosine isolated from hydrolysates of fibrillar elastin protein. It appears that these amino acids are found only in this protein. Their structure is unusual: the 4th lysine molecules, connected by their R-groups, form a substituted pyridine ring. It is possible that due to this very structure, these amino acids can form 4 radially divergent peptide chains. The result is that elastin, unlike other fibrillar proteins, is able to deform (stretch) in two mutually perpendicular directions. Etc.
From the listed protein amino acids, living organisms synthesize a huge number of diverse protein compounds. Many plants and bacteria can synthesize all the amino acids they need from simple inorganic compounds. In the body of humans and animals, about half of the amino acids are also synthesized. The other part of the amino acids can enter the human body only with food proteins.
- essential amino acids - are not synthesized in the human body, but come only with food. Essential amino acids include 8 amino acids: valine, phenylalanine, isoleucine, leucine, lysine, methionine, threonine, tryptophan, phenylalanine.
- nonessential amino acids - can be synthesized in the human body from other components. Non-essential amino acids include 12 amino acids.
For a person, both types of amino acids are equally important: both replaceable and irreplaceable. Most of the amino acids are used to build the body's own proteins, but the body cannot exist without essential amino acids. Proteins, which contain essential amino acids, should be about 16-20% in the diet of adults (20-30g with a daily protein intake of 80-100g). In the nutrition of children, the proportion of protein increases to 30% for schoolchildren, and up to 40% for preschoolers. This is due to the fact that the child's body is constantly growing and, therefore, needs a large amount of amino acids as a plastic material for building proteins in muscles, blood vessels, the nervous system, skin and all other tissues and organs.
In these days of fast food and the general passion for fast food, the diet is very often dominated by foods high in easily digestible carbohydrates and fats, and the proportion of protein foods is noticeably reduced. With a lack of any amino acids in the diet or during starvation in the human body for a short time, proteins of connective tissue, blood, liver and muscles can be destroyed, and the “building material” obtained from them - amino acids go to maintain the normal functioning of the most important organs - the heart and brain. The human body can be deficient in both essential and non-essential amino acids. Deficiency of amino acids, especially essential ones, leads to poor appetite, stunted growth and development, fatty liver and other severe disorders. The first "heralds" of lack of amino acids may be a decrease in appetite, deterioration of the skin, hair loss, muscle weakness, fatigue, decreased immunity, anemia. Such manifestations can occur in individuals who, in order to reduce weight, follow a low-calorie unbalanced diet with a sharp restriction of protein products.
More often than others, manifestations of a lack of amino acids, especially essential ones, are encountered by vegetarians who deliberately avoid including complete animal protein in their diet.
An excess of amino acids is quite rare these days, but it can cause the development of serious diseases, especially in children and adolescents. The most toxic are methionine (provokes the risk of heart attack and stroke), tyrosine (may provoke the development of arterial hypertension, lead to disruption of the thyroid gland) and histidine (may contribute to copper deficiency in the body and lead to the development of aortic aneurysm, joint diseases, early gray hair). , severe anemia). Under normal conditions of the functioning of the body, when there is a sufficient amount of vitamins (B 6, B 12, folic acid) and antioxidants (vitamins A, E, C and selenium), an excess of amino acids quickly turns into useful components and does not have time to “damage” the body. With an unbalanced diet, there is a deficiency of vitamins and trace elements, and an excess of amino acids can disrupt the functioning of systems and organs. This option is possible with long-term observance of protein or low-carbohydrate diets, as well as with uncontrolled intake of protein-energy products by athletes (amino acid-vitamin cocktails) to increase weight and develop muscles.
Among the chemical methods, the most common method amino acid score (scor - score, count). It is based on the comparison of the amino acid composition of the protein of the evaluated product with the amino acid composition standard (ideal) protein. After a quantitative chemical determination of the content of each of the essential amino acids in the protein under study, the amino acid score (AC) is determined for each of them according to the formula
AC = (m ak . research / m ak . ideal ) 100
m acc. research - the content of an essential amino acid (in mg) in 1 g of the protein under study.
m acc. ideal - the content of an essential amino acid (in mg) in 1 g of a standard (ideal) protein.
FAO/WHO amino acid sample
Simultaneously with the determination of the amino acid score, limiting essential amino acid for a given protein , that is the one for which the speed is the least.
Peptides.
Two amino acids can be covalently linked via peptide connection with the formation of a dipeptide.
Three amino acids can be linked via two peptide bonds to form a tripeptide. Several amino acids form oligopeptides, a large number of amino acids form polypeptides. Peptides contain only one -amino group and one -carboxyl group. These groups can be ionized at certain pH values. Like amino acids, they have characteristic titration curves and isoelectric points at which they do not move in an electric field.
Like other organic compounds, peptides participate in chemical reactions that are determined by the presence of functional groups: a free amino group, a free carboxy group, and R-groups. Peptide bonds are susceptible to hydrolysis by a strong acid (eg, 6M HCl) or a strong base to form amino acids. Hydrolysis of peptide bonds is a necessary step in determining the amino acid composition of proteins. Peptide bonds can be broken by the action of enzymes proteases.
Many naturally occurring peptides are biologically active at very low concentrations.
Peptides are potentially active pharmaceutical preparations, there are three ways getting them:
1) excretion from organs and tissues;
2) genetic engineering;
3) direct chemical synthesis.
In the latter case, high requirements are placed on the yield of products at all intermediate stages.
Amino acids are called organic carboxylic acids in which at least one of the hydrogen atoms of the hydrocarbon chain is replaced by an amino group. Depending on the position of the -NH 2 group, α, β, γ, etc. are distinguished. L-amino acids. To date, up to 200 different amino acids have been found in various objects of the living world. The human body contains about 60 different amino acids and their derivatives, but not all of them are part of proteins.
Amino acids are divided into two groups:
- proteinogenic (part of proteins)
Among them are the main ones (there are only 20 of them) and rare ones. Rare protein amino acids (for example, hydroxyproline, hydroxylysine, aminocitric acid, etc.) are actually derivatives of the same 20 amino acids.
The remaining amino acids are not involved in the construction of proteins; they are in the cell either in a free form (as metabolic products), or are part of other non-protein compounds. For example, the amino acids ornithine and citrulline are intermediates in the formation of the proteinogenic amino acid arginine and are involved in the urea synthesis cycle; γ-amino-butyric acid is also in free form and plays the role of a mediator in the transmission of nerve impulses; β-alanine is part of the vitamin - pantothenic acid.
- non-proteinogenic (not involved in the formation of proteins)
Non-proteinogenic amino acids, unlike proteinogenic ones, are more diverse, especially those found in fungi and higher plants. Proteinogenic amino acids are involved in the construction of many different proteins, regardless of the type of organism, and non-proteinogenic amino acids can even be toxic to an organism of another species, that is, they behave like ordinary foreign substances. For example, canavanine, diencolic acid, and β-cyano-alanine isolated from plants are poisonous to humans.
Structure and classification of proteinogenic amino acids
The radical R in the simplest case is represented by a hydrogen atom (glycine), and may have a complex structure. Therefore, α-amino acids differ from each other primarily in the structure of the side radical, and, consequently, in the physicochemical properties inherent in these radicals. There are three classifications of amino acids:
The given physiological classification of amino acids is not universal, in contrast to the first two classifications, and to some extent conditional, since it is valid only for organisms of this species. However, the absolute indispensability of eight amino acids is universal for all types of organisms (Table 2 shows data for some representatives of vertebrates and insects [show] ).
| Table 2. Essential (+), non-essential (-), and semi-essential (±) amino acids for some vertebrates and insects (according to Lubka et al., 1975) | |||||||
| Amino acids | Man | Rat | Mouse | Hen | Salmon | Mosquito | Bee |
| Glycine | - | - | - | + | - | + | - |
| Alanya | - | - | - | - | - | - | - |
| Valine | + | + | + | + | + | + | + |
| Leucine | + | + | + | + | + | + | + |
| Isoleucine | + | + | + | + | + | + | + |
| Cysteine | - | - | - | - | - | - | - |
| Methionine | + | + | + | + | + | + | + |
| Serene | - | - | - | - | - | - | - |
| Threonine | + | + | + | + | + | + | + |
| Aspartic acid | - | - | - | - | - | - | - |
| Glutamic acid | - | - | - | - | - | - | - |
| Lysine | + | + | + | + | + | + | + |
| Arginine | ± | ± | + | + | + | + | + |
| Phenylalanine | + | + | + | + | + | + | + |
| Tyrosine | ± | ± | + | + | - | - | - |
| Histidine | ± | + | + | + | + | + | + |
| tryptophan | + | + | + | + | + | + | + |
| Proline | - | - | - | - | - | - | - |
For rats and mice, there are already nine essential amino acids (histidine is added to the eight known ones). Normal growth and development of a chicken is possible only in the presence of eleven essential amino acids (histidine, arginine, tyrosine are added), that is, amino acids semi-essential for humans are absolutely indispensable for chicken. For mosquitoes, glycine is absolutely essential, and tyrosine, on the contrary, is a non-essential amino acid.
This means that for different types of organisms, significant deviations in the need for individual amino acids are possible, which is determined by the characteristics of their metabolism.
The composition of essential amino acids that has developed for each type of organism, or the so-called auxotrophy of the organism in relation to amino acids, most likely reflects its desire for minimal energy costs for the synthesis of amino acids. Indeed, it is more profitable to receive a finished product than to produce it yourself. Therefore, organisms that consume essential amino acids spend about 20% less energy than those that synthesize all amino acids. On the other hand, in the course of evolution, no life forms have been preserved that would be completely dependent on the supply of all amino acids from the outside. It would be difficult for them to adapt to changes in the external environment, given that amino acids are the material for the synthesis of such a substance as protein, without which life is impossible.
Physico-chemical properties of amino acids
Acid-base properties of amino acids . According to their chemical properties, amino acids are amphoteric electrolytes, that is, they combine the properties of both acids and bases.
Acid groups of amino acids: carboxyl (-COOH -> -COO - + H +), protonated α-amino group (-NH + 3 -> -NH 2 + H +).
The main groups of amino acids: dissociated carboxyl (-COO - + H + -> -COOH) and α-amino group (-NH 2 + H + -> NH + 3).
For each amino acid, there are at least two acid dissociation constants pKa - one for the -COOH group, and the second for the α-amino group.
In an aqueous solution, the existence of three forms of amino acids is possible (Fig. 1.)
It has been proven that in aqueous solutions amino acids are in the form of a dipole; or zwitterion.
Effect of medium pH on the ionization of amino acids . Changing the pH of the medium from acidic to alkaline affects the charge of dissolved amino acids. In an acidic environment (pH<7) все аминокислоты несут положительный заряд (существуют в виде катиона), так как избыток протонов в среде подавляет диссоциацию карбоксильной группы:
In an acidic environment, amino acids in an electric field move towards the cathode.
In an alkaline environment (pH> 7), where there is an excess of OH - ions, amino acids are in the form of negatively charged ions (anions), since the NH + 3 group dissociates:
In this case, the amino acids move in the electric field towards the anode.
Therefore, depending on the pH of the medium, amino acids have a total zero, positive or negative charge.
The state in which the charge of an amino acid is zero is called isoelectric. The pH value at which such a state occurs and the amino acid does not move in the electric field either to the anode or to the cathode is called the isoelectric point and is denoted pH I. The isoelectric point very accurately reflects the acid-base properties of different groups in amino acids and is one of the important constants that characterize an amino acid.
The isoelectric point of non-polar (hydrophobic) amino acids approaches a neutral pH value (from 5.5 for phenylalanine to 6.3 for proline), for acidic it has low values (3.2 for glutamic acid, 2.8 for aspartic acid). The isoelectric point for cysteine and cystine is 5.0, which indicates the weak acidic properties of these amino acids. The main amino acids - histidine and especially lysine and arginine - have an isoelectric point significantly higher than 7.
In the cells and intercellular fluid of the human and animal body, the pH of the medium is close to neutral, so the basic amino acids (lysine, arginine) carry a total positive charge (cations), acidic amino acids (aspartic and glutamine) have a negative charge (anions), and the rest exist in the form dipole. Acidic and basic amino acids are more hydrated than all other amino acids.
Stereoisomerism of amino acids
All proteinogenic amino acids, with the exception of glycine, have at least one asymmetric carbon atom (C*) and are optically active, with most of them being left-handed. They exist as spatial isomers, or stereoisomers. According to the arrangement of substituents around the asymmetric carbon atom, stereo-isomers are classified into the L- or D-series.
L- and D-isomers relate to each other as an object and its mirror image, therefore they are also called mirror isomers or enantiomers. The amino acids threonine and isoleucine each have two asymmetric carbon atoms, so they each have four stereoisomers. For example, threonine, in addition to L- and D-threonine, has two more, which are called diastereomers or alloforms: L-allotreonine and D-allotreonine.
All amino acids that make up proteins belong to the L-series. It was believed that D-amino acids do not occur in nature. However, polypeptides have been found in the form of polymers of D-glutamic acid in capsules of spore-bearing bacteria (anthrax, potato and hay bacillus); D-glutamic acid and D-alanine are part of the mucopeptides of the cell wall of some bacteria. D-Amino acids are also found in antibiotics produced by microorganisms (see Table 3).
Perhaps D-amino acids were more suitable for the protective functions of organisms (this is the purpose of the capsule of bacteria and antibiotics), while L-amino acids are needed by the body to build proteins.
Distribution of individual amino acids in different proteins
To date, the amino acid composition of many proteins of microbial, plant and animal origin has been deciphered. Most often found in proteins are alanine, glycine, leucine, series. However, each protein has its own amino acid composition. For example, protamines (simple proteins found in fish milk) contain up to 85% arginine, but they lack cyclic, acidic and sulfur-containing amino acids, threonine and lysine. Fibroin - natural silk protein, contains up to 50% glycine; Collagen, a tendon protein, contains rare amino acids (hydroxylysine, hydroxyproline) that are absent in other proteins.
The amino acid composition of proteins is determined not by the availability or indispensability of a particular amino acid, but by the purpose of the protein, its function. The sequence of amino acids in a protein is determined by the genetic code.
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