Twenty amino acids are required for protein synthesis. Acid-base properties of amino acids Amino acids in acidic and alkaline media
Proteins form the material basis of the chemical activity of the cell. The functions of proteins in nature are universal. Name proteins,most accepted in Russian literature, corresponds to the term proteins(from the Greek. proteios- first). To date, great success has been achieved in establishing the ratio of the structure and functions of proteins, the mechanism of their participation in the most important processes of the body's vital activity, and in understanding the molecular basis of the pathogenesis of many diseases.
Depending on the molecular weight, peptides and proteins are distinguished. Peptides have a lower molecular weight than proteins. For peptides, a regulatory function is more characteristic (hormones, inhibitors and activators of enzymes, carriers of ions through membranes, antibiotics, toxins, etc.).
12.1. α -Amino acids
12.1.1. Classification
Peptides and proteins are built from α-amino acid residues. The total number of naturally occurring amino acids exceeds 100, but some of them are found only in a certain community of organisms, 20 of the most important α-amino acids are constantly found in all proteins (Scheme 12.1).
α-Amino acids are heterofunctional compounds, the molecules of which contain both an amino group and a carboxyl group at the same carbon atom.
Scheme 12.1.Essential α-amino acids *
* Abbreviations are used only for recording amino acid residues in peptide and protein molecules. ** Essential amino acids.
The names of α-amino acids can be constructed according to the substituent nomenclature, but their trivial names are more often used.
The trivial names for α-amino acids are usually associated with the source of the excretion. Serine is a part of silk fibroin (from lat. serieus- silky); tyrosine was first isolated from cheese (from the Greek. tyros- cheese); glutamine - from cereal gluten (from it. Gluten- glue); aspartic acid - from asparagus sprouts (from lat. asparagus- asparagus).
Many α-amino acids are synthesized in the body. Some amino acids necessary for protein synthesis are not formed in the body and must be supplied from the outside. These amino acids are called irreplaceable(see diagram 12.1).
Essential α-amino acids include:
valine isoleucine methionine tryptophan
leucine lysine threonine phenylalanine
α-Amino acids are classified in several ways depending on the characteristic underlying their division into groups.
One of the classification features is the chemical nature of the radical R. According to this feature, amino acids are divided into aliphatic, aromatic and heterocyclic (see Scheme 12.1).
Aliphaticα -amino acids.This is the largest group. Inside it, amino acids are subdivided using additional classification features.
Depending on the number of carboxyl groups and amino groups in the molecule, there are:
Neutral amino acids - one NH group each2 and COOH;
Essential amino acids - two NH groups2 and one group
UNOO;
Acidic amino acids - one NH 2 group and two COOH groups.
It can be noted that in the group of aliphatic neutral amino acids, the number of carbon atoms in the chain does not exceed six. At the same time, there is no amino acid with four carbon atoms in the chain, and amino acids with five and six carbon atoms have only a branched structure (valine, leucine, isoleucine).
The aliphatic radical may contain "additional" functional groups:
Hydroxyl - serine, threonine;
Carboxyl - aspartic and glutamic acids;
Thiol - cysteine;
Amide - asparagine, glutamine.
Aromaticα -amino acids.This group includes phenylalanine and tyrosine, constructed in such a way that the benzene rings in them are separated from the common α-amino acid fragment by the methylene group -CH2-.
Heterocyclic α -amino acids.Histidine and tryptophan belonging to this group contain heterocycles - imidazole and indole, respectively. The structure and properties of these heterocycles are discussed below (see 13.3.1; 13.3.2). The general principle of construction of heterocyclic amino acids is the same as for aromatic ones.
Heterocyclic and aromatic α-amino acids can be considered as β-substituted alanine derivatives.
Heroic also includes amino acid proline,in which the secondary amino group is included in the pyrrolidine
In the chemistry of α-amino acids, much attention is paid to the structure and properties of "side" radicals R, which play an important role in the formation of the structure of proteins and their performance of biological functions. Of great importance are characteristics such as the polarity of the "side" radicals, the presence of functional groups in the radicals and the ability of these functional groups to ionize.
Depending on the side radical, amino acids with non-polar(hydrophobic) radicals and amino acids c polar(hydrophilic) radicals.
The first group includes amino acids with aliphatic side radicals - alanine, valine, leucine, isoleucine, methionine - and aromatic side radicals - phenylalanine, tryptophan.
The second group includes amino acids that have polar functional groups in the radical that are capable of ionization (ionic) or incapable of passing into an ionic state (nonionic) under the conditions of the organism. For example, in tyrosine the hydroxyl group is ionogenic (has a phenolic character), in serine it is non-ionic (has an alcoholic nature).
Polar amino acids with ionogenic groups in radicals under certain conditions can be in the ionic (anionic or cationic) state.
12.1.2. Stereoisomerism
The basic type of construction of α-amino acids, that is, the bond of one and the same carbon atom with two different functional groups, a radical and a hydrogen atom, in itself predetermines the chirality of the α-carbon atom. The exception is the simplest amino acid glycine H2 NCH 2 COOH without a chiral center.
The configuration of α-amino acids is determined by the configuration standard, glycerolic aldehyde. The location in the standard projection Fischer formula of the amino group on the left (like the OH group in l-glycerolic aldehyde) corresponds to the l-configuration, on the right - the d-configuration of the chiral carbon atom. By R,In the S-system, the α-carbon atom of all α-amino acids of the l-series has the S-, and the d-series has the R-configuration (the exception is cysteine, see 7.1.2).
Most α-amino acids contain one asymmetric carbon atom in the molecule and exist as two optically active enantiomers and one optically inactive racemate. Almost all natural α-amino acids belong to the l-series.
The amino acids isoleucine, threonine and 4-hydroxyproline contain two chiral centers in the molecule.
Such amino acids can exist as four stereoisomers, which are two pairs of enantiomers, each of which forms a racemate. Only one of the enantiomers is used to build proteins in animal organisms.
The stereoisomerism of isoleucine is analogous to the previously discussed stereoisomerism of threonine (see 7.1.3). Of the four stereoisomers, proteins include l-isoleucine with the S-configuration of both asymmetric carbon atoms C-α and C-β. Another pair of enantiomers, which are diastereomeric with respect to leucine, use the prefix hello-.
Cleavage of racemates. The source of obtaining α-amino acids of the l-series are proteins, which are subjected to hydrolytic cleavage for this purpose. Due to the great need for individual enantiomers (for the synthesis of proteins, medicinal substances, etc.), chemicalmethods for the cleavage of synthetic racemic amino acids. Preferred enzymaticdigestion method using enzymes. At present, chromatography on chiral sorbents is used to separate racemic mixtures.
12.1.3. Acid-base properties
The amphotericity of amino acids is due to acidic (COOH) and basic (NH2) functional groups in their molecules. Amino acids form salts with both alkalis and acids.
In the crystalline state, α-amino acids exist as dipolar ions H3N + - CHR-COO- (the commonly used notation
the structure of the amino acid in non-ionized form is only for convenience).
In an aqueous solution, amino acids exist in the form of an equilibrium mixture of a dipolar ion, cationic and anionic forms.
The equilibrium position depends on the pH of the medium. All amino acids are dominated by cationic forms in strongly acidic (pH 1-2) and anionic - in strongly alkaline (pH\u003e 11) media.
The ionic structure determines a number of specific properties of amino acids: high melting point (above 200 ° C), solubility in water and insolubility in non-polar organic solvents. The ability of most amino acids to dissolve well in water is an important factor in ensuring their biological functioning; it is associated with the absorption of amino acids, their transport in the body, etc.
Fully protonated amino acid (cationic form) from the standpoint of Brønsted's theory is a diacid,
By donating one proton, such a dibasic acid turns into a weak monobasic acid - a dipolar ion with one acidic group NH3 + . Deprotonation of the dipolar ion leads to the formation of the anionic form of the amino acid - the carboxylate ion, which is the Brønsted base. The values \u200b\u200bcharacterize
the acidic properties of the carboxyl group of amino acids are usually in the range from 1 to 3; meaning pK a2 characterizing the acidity of the ammonium group - from 9 to 10 (table. 12.1).
Table 12.1.Acid-base properties of the most important α-amino acids
The position of equilibrium, i.e., the ratio of various forms of amino acids, in an aqueous solution at certain pH values \u200b\u200bdepends significantly on the structure of the radical, mainly on the presence of ionogenic groups in it, which play the role of additional acid and basic centers.
The pH value at which the concentration of dipolar ions is maximum and the minimum concentrations of the cationic and anionic forms of the amino acid are equal is calledisoelectric point (p /).
Neutralα -amino acids.These amino acids matterpIslightly lower than 7 (5.5-6.3) due to the greater ability to ionize the carboxyl group under the influence of the - / - effect of the NH 2 group. For example, alanine has an isoelectric point at pH 6.0.
Sourα -amino acids.These amino acids have an additional carboxyl group in the radical and are in a fully protonated form in a strongly acidic medium. Acidic amino acids are tribasic (Brøndsted) with three meaningspK a,as can be seen from the example of aspartic acid (p / 3.0).
For acidic amino acids (aspartic and glutamic), the isoelectric point is much below pH 7 (see Table 12.1). In the body at physiological pH values \u200b\u200b(for example, blood pH 7.3-7.5), these acids are in the anionic form, since both carboxyl groups are ionized in them.
The mainα -amino acids.In the case of basic amino acids, the isoelectric points are in the pH range above 7. In a strongly acidic medium, these compounds are also tribasic acids, the stages of ionization of which are shown by the example of lysine (p / 9.8).
In the body, the main amino acids are in the form of cations, that is, both amino groups are protonated in them.
In general, no α-amino acid in vivois not at its isoelectric point and does not enter the state corresponding to the lowest solubility in water. All amino acids in the body are in ionic form.
12.1.4. Analytically important reactions α -amino acids
α-Amino acids as heterofunctional compounds enter into reactions characteristic of both carboxyl and amino groups. Some of the chemical properties of amino acids are due to the functional groups in the radical. This section discusses reactions that are of practical importance for the identification and analysis of amino acids.
Esterification.When amino acids react with alcohols in the presence of an acid catalyst (for example, gaseous hydrogen chloride), esters in the form of hydrochlorides are obtained in good yield. To isolate free esters, the reaction mixture is treated with gaseous ammonia.
Esters of amino acids do not have a dipolar structure, therefore, unlike the original acids, they dissolve in organic solvents and are volatile. Thus, glycine is a crystalline substance with a high melting point (292 ° C), and its methyl ether is a liquid with a boiling point of 130 ° C. Amino acid esters can be analyzed by gas-liquid chromatography.
Reaction with formaldehyde. The reaction with formaldehyde, which underlies the quantitative determination of amino acids by the method formol titration(Sorensen method).
The amphotericity of amino acids does not allow direct titration with alkali for analytical purposes. The interaction of amino acids with formaldehyde gives relatively stable amino alcohols (see 5.3) - N-hydroxymethyl derivatives, the free carboxyl group of which is then titrated with alkali.
Qualitative reactions. The peculiarity of the chemistry of amino acids and proteins lies in the use of numerous qualitative (color) reactions, which previously formed the basis of chemical analysis. Currently, when research is carried out using physicochemical methods, many qualitative reactions continue to be used for the detection of α-amino acids, for example, in chromatographic analysis.
Chelation. With cations of heavy metals, α-amino acids as bifunctional compounds form intracomplex salts, for example, with freshly prepared copper hydroxide (11) under mild conditions, well crystallizing chelate
blue copper (11) salts (one of the non-specific methods for detecting α-amino acids).
Ninhydrine reaction. The general qualitative reaction of α-amino acids is the reaction with ninhydrin. The reaction product has a blue-violet color, which is used for visual detection of amino acids on chromatograms (on paper, in a thin layer), as well as for spectrophotometric determination on amino acid analyzers (the product absorbs light in the range of 550-570 nm).
Deamination. Under laboratory conditions, this reaction is carried out by the action of nitrous acid on α-amino acids (see 4.3). In this case, the corresponding α-hydroxy acid is formed and gaseous nitrogen is released, by the volume of which the amount of the amino acid reacted is judged (the Van Slike method).
Xanthoprotein reaction. This reaction is used to detect aromatic and heterocyclic amino acids - phenylalanine, tyrosine, histidine, tryptophan. For example, when concentrated nitric acid acts on tyrosine, a yellow nitro derivative is formed. In an alkaline medium, the color turns orange due to ionization of the phenolic hydroxyl group and an increase in the contribution of the anion to the conjugation.
There are also a number of private reactions that allow the detection of individual amino acids.
Tryptophandetected by reaction with p- (dimethylamino) benzaldehyde in sulfuric acid medium by the appearing red-violet color (Ehrlich reaction). This reaction is used for the quantitative analysis of tryptophan in protein breakdown products.
Cysteineis detected by several qualitative reactions based on the reactivity of the mercapto group contained therein. For example, when a protein solution with lead acetate (CH3COO) 2Pb is heated in an alkaline medium, a black precipitate of lead sulfide PbS is formed, which indicates the presence of cysteine \u200b\u200bin the proteins.
12.1.5. Biologically important chemical reactions
A number of important chemical transformations of amino acids are carried out in the body under the action of various enzymes. Such transformations include transamination, decarboxylation, elimination, aldol cleavage, oxidative deamination, and oxidation of thiol groups.
Transamination is the main pathway for the biosynthesis of α-amino acids from α-oxo acids. The donor of the amino group is the amino acid present in the cells in sufficient or excess amount, and its acceptor is the α-oxo acid. In this case, the amino acid is converted into an oxo acid, and the oxo acid into an amino acid with the corresponding structure of radicals. As a result, transamination is a reversible process of interchange of amino and oxo groups. An example of such a reaction is the production of l-glutamic acid from 2-oxoglutaric acid. The donor amino acid can be, for example, l-aspartic acid.
α-Amino acids contain an electron-withdrawing amino group in the α-position to the carboxyl group (more precisely, the protonated amino group NH3 +), in connection with which they are capable of decarboxylation.
Eliminationcharacteristic of amino acids in which the side radical in the β-position to the carboxyl group contains an electron-withdrawing functional group, for example, hydroxyl or thiol. Their cleavage leads to intermediate reactive α-enamino acids, which readily transform into tautomeric imino acids (analogy with keto-enol tautomerism). α-Imino acids as a result of hydration at the C \u003d N bond and subsequent elimination of the ammonia molecule are converted into α-oxo acids.
This type of transformation is called elimination-hydration.An example is the preparation of pyruvic acid from serine.
Aldol cleavage occurs in the case of α-amino acids in which the β-position contains a hydroxyl group. For example, serine is cleaved to form glycine and formaldehyde (the latter is not released in free form, but immediately binds to the coenzyme).
Oxidative deamination can be carried out with the participation of enzymes and the coenzyme NAD + or NADP + (see 14.3). α-Amino acids can be converted to α-oxo acids not only through transamination, but also by oxidative deamination. For example, α-oxoglutaric acid is formed from l-glutamic acid. The first stage of the reaction is the dehydrogenation (oxidation) of glutamic acid to α-iminoglutaric acid.
acid. In the second stage, hydrolysis occurs, as a result of which α-oxoglutaric acid and ammonia are obtained. The stage of hydrolysis proceeds without the participation of an enzyme.
The reaction of reductive amination of α-oxo acids proceeds in the opposite direction. The α-oxoglutaric acid always present in cells (as a product of carbohydrate metabolism) is converted in this way to L-glutamic acid.
Oxidation of thiol groups underlies the interconversion of cysteine \u200b\u200band cystine residues, which provide a number of redox processes in the cell. Cysteine, like all thiols (see 4.1.2), readily oxidizes to form a disulfide, cystine. The disulfide bond in cystine is easily reduced to form cysteine.
Due to the ability of the thiol group for easy oxidation, cysteine \u200b\u200bperforms a protective function when exposed to substances with a high oxidizing ability. In addition, it was the first drug to exhibit anti-radiation effects. Cysteine \u200b\u200bis used in pharmaceutical practice as a drug stabilizer.
The conversion of cysteine \u200b\u200bto cystine leads to the formation of disulfide bonds, for example, in reduced glutathione
(see 12.2.3).
12.2. Primary structure of peptides and proteins
It is conventionally believed that peptides contain up to 100 amino acids in a molecule (which corresponds to a molecular weight of up to 10 thousand), and proteins - more than 100 amino acid residues (a molecular weight from 10 thousand to several million).
In turn, in the group of peptides, it is customary to distinguish oligopeptides(low molecular weight peptides) containing no more than 10 amino acid residues in the chain, and polypeptides,the chain of which includes up to 100 amino acid residues. Macromolecules with the number of amino acid residues approaching or slightly exceeding 100 do not distinguish between polypeptides and proteins; these terms are often used interchangeably.
Formally, the peptide and protein molecule can be represented as the product of the polycondensation of α-amino acids, which proceeds with the formation of a peptide (amide) bond between monomer units (Scheme 12.2).
The construction of the polyamide chain is the same for the whole variety of peptides and proteins. This chain has an unbranched structure and consists of alternating peptide (amide) groups —CO — NH— and fragments —CH (R) -.
One end of the chain, which contains an amino acid with a free NH group2, called the N-end, the other - the C-end,
Scheme 12.2.The principle of building a peptide chain
which contains an amino acid with a free COOH group. Peptide and protein chains are recorded from the N-terminus.
12.2.1. The structure of the peptide group
In the peptide (amide) group —CO — NH—, the carbon atom is in the state of sp2 hybridization. The lone pair of electrons of the nitrogen atom enters into conjugation with the π-electrons of the double bond C \u003d O. From the standpoint of the electronic structure, the peptide group is a three-center p, π-conjugated system (see 2.3.1), the electron density in which is shifted towards the more electronegative oxygen atom. The atoms C, O, and N, forming a conjugated system, are in the same plane. The distribution of the electron density in the amide group can be represented using the boundary structures (I) and (II) or the shift of the electron density as a result of the + M- and - M-effects of the NH and C \u003d O groups, respectively (III).
As a result of conjugation, some alignment of the bond lengths occurs. The C \u003d O double bond is extended to 0.124 nm versus the usual length of 0.121 nm, and the C-N bond becomes shorter - 0.132 nm compared to 0.147 nm in the usual case (Fig. 12.1). The planar conjugated system in the peptide group is the reason for the difficulty of rotation around the C-N bond (the rotation barrier is 63-84 kJ / mol). Thus, the electronic structure predetermines a rather rigid flatthe structure of the peptide group.
As seen from Fig. 12.1, the α-carbon atoms of amino acid residues are located in the plane of the peptide group on opposite sides of the C-N bond, i.e., in a more favorable trans-position: the side radicals R of amino acid residues in this case will be the farthest from each other in space.
The polypeptide chain has a surprisingly uniform structure and can be represented as a series of angled each
Figure: 12.1.Planar arrangement of the peptide group -CO-NH- and α-carbon atoms of amino acid residues
to the other of the planes of peptide groups connected to each other through α-carbon atoms by bonds Сα-N and Сα-Сsp2 (fig.12.2). Rotation around these single bonds is very limited due to difficulties in the spatial arrangement of side radicals of amino acid residues. Thus, the electronic and spatial structure of the peptide group largely determines the structure of the polypeptide chain as a whole.
Figure: 12.2.Mutual position of planes of peptide groups in a polypeptide chain
12.2.2. Composition and amino acid sequence
With a uniformly constructed polyamide chain, the specificity of peptides and proteins is determined by two most important characteristics - amino acid composition and amino acid sequence.
The amino acid composition of peptides and proteins is the nature and quantitative ratio of the α-amino acids included in them.
The amino acid composition is established by analyzing peptide and protein hydrolysates, mainly by chromatographic methods. Currently, this analysis is carried out using amino acid analyzers.
Amide bonds are capable of hydrolysis in both acidic and alkaline media (see 8.3.3). Peptides and proteins are hydrolyzed with the formation of either shorter chains - this is the so-called partial hydrolysis,or a mixture of amino acids (in ionic form) - complete hydrolysis.Usually, the hydrolysis is carried out in an acidic medium, since many amino acids are unstable under the conditions of alkaline hydrolysis. It should be noted that the amide groups of asparagine and glutamine also undergo hydrolysis.
The primary structure of peptides and proteins is the amino acid sequence, i.e. the order of alternation of α-amino acid residues.
The primary structure is determined by sequential cleavage of amino acids from either end of the chain and their identification.
12.2.3. Structure and nomenclature of peptides
Peptide names are constructed by sequentially listing amino acid residues starting from the N-terminus, with the addition of a suffix-il, except for the last C-terminal amino acid, for which its full name is retained. In other words, the names
amino acids that have entered the formation of a peptide bond due to their "own" COOH group end in the name of the peptide in -il: alanyl, valyl, etc. (the names “aspartyl” and “glutamyl” are used for the residues of aspartic and glutamic acids, respectively). The names and symbols of amino acids indicate their belonging tol -series, unless otherwise indicated (d or dl).
Sometimes, in the abbreviated notation with the symbols H (as part of the amino group) and OH (as part of the carboxyl group), the unsubstituted functional groups of terminal amino acids are specified. It is convenient to depict functional peptide derivatives in this manner; for example, the C-terminal amino acid amide of the above peptide is written H-Asn-Gly-Phe-NH2.
Peptides are found in all organisms. Unlike proteins, they have a more heterogeneous amino acid composition, in particular, they quite often include amino acidsd -series. Structurally, they are also more diverse: they contain cyclic fragments, branched chains, etc.
One of the most common representatives of tripeptides - glutathione- is found in the body of all animals, plants and bacteria.
Cysteine \u200b\u200bin glutathione makes it possible for glutathione to exist in both reduced and oxidized forms.
Glutathione is involved in a number of redox processes. It acts as a protector of proteins, i.e., a substance that protects proteins with free SH thiol groups from oxidation with the formation of -S-S- disulfide bonds. This applies to those proteins for which such a process is undesirable. In these cases, glutathione takes over the action of an oxidizing agent and thus “protects” the protein. During the oxidation of glutathione, intermolecular crosslinking of two tripeptide fragments occurs due to a disulfide bond. The process is reversible.
12.3. Secondary structure of polypeptides and proteins
For high molecular weight polypeptides and proteins, along with the primary structure, higher levels of organization are also characteristic, which are called secondary, tertiaryand quaternarystructures.
The secondary structure is described by the spatial orientation of the main polypeptide chain, while the tertiary structure is described by the three-dimensional architecture of the entire protein molecule. Both secondary and tertiary structures are associated with the ordered arrangement of the macromolecular chain in space. The tertiary and quaternary structure of proteins is considered in the course of biochemistry.
It was shown by calculation that for the polypeptide chain one of the most favorable conformations is the arrangement in space in the form of a right-handed helix, called α-helix(Figure 12.3, a).
The spatial arrangement of the α-helical polypeptide chain can be imagined by imagining that it wraps around a certain
Figure: 12.3.α-Helical conformation of the polypeptide chain
cylinder (see Fig. 12.3, b). On average, there are 3.6 amino acid residues per turn of the helix, the helix pitch is 0.54 nm, and the diameter is 0.5 nm. In this case, the planes of two adjacent peptide groups are located at an angle of 108 °, and the side radicals of amino acids are located on the outer side of the helix, i.e., they are directed, as it were, from the surface of the cylinder.
The main role in fixing this chain conformation is played by hydrogen bonds, which in the α-helix are formed between the carbonyl oxygen atom of each first and the hydrogen atom of the NH-group of every fifth amino acid residue.
The hydrogen bonds are directed almost parallel to the axis of the α-helix. They keep the chain twisted.
Usually, protein chains are not completely spiralized, but only partially. Proteins such as myoglobin and hemoglobin contain rather long α-helical regions, such as a myoglobin chain
spiralized by 75%. In many other proteins, the proportion of helical regions in the chain may be small.
Another type of secondary structure of polypeptides and proteins is β-structure,also called folded sheet,or folded layer.Elongated polypeptide chains are laid in folded sheets, linked by a multitude of hydrogen bonds between the peptide groups of these chains (Fig. 12.4). Many proteins simultaneously contain α-helical and β-sheet structures.
Figure: 12.4.Secondary structure of the polypeptide chain in the form of a folded sheet (β-structure)
Lecture number 1
TOPIC: "Amino acids".
Lecture plan:
1. Characteristics of amino acids
2. Peptides.
Characteristics of amino acids.
Amino acids are organic compounds, derivatives of hydrocarbons, the molecules of which 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's consider the main patterns characteristic of amino acids of proteins.
It has now been established that proteins contain a frequently occurring set of amino acids. There are 18 of them. In addition to the above, 2 more amino acid amides were found - asparagine and glutamine. They all got the name major (common) amino acids. They are often figuratively called "Magic" amino acids. In addition to major amino acids, there are also rare ones, those that are not often found in natural proteins. They are called minor.
Almost all amino acids of proteins belong to α - amino acids (the amino group is located at the first carbon atom after the carboxyl group). Based on the foregoing, the general formula is valid for most amino acids:
NH 2 -CH-COOH
Where R - radicals with different structures.
Consider the formulas of protein amino acids, table. 2.
All α - amino acids, except for aminoacetic (glycine), have an asymmetric α is a carbon atom and exist 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 D amino acids of the genetic series found. The rotation angle value is 20-30 0 degrees. Rotation can be to the right (7 amino acids) and to the 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 prevail over amino (basic) groups, for example, aspartic, glutamic acids.
Neutral amino acids The number of groups is equal. Glycine, Alanine, etc.
Essential amino acids.Basic (amino) groups prevail over carboxyl (acidic), for example, lysine.
In terms of physical and a number of chemical properties, amino acids differ sharply from the corresponding acids and bases. They dissolve better in water than in organic solvents; crystallize well; have a high density and extremely 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 - the ionic structure of amino acids is confirmed by their large dipole moment (not less than 5010 -30 C m), as well as an 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 communication.
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 common amino acids, i.e. modified amino acids.
4-hydroxyproline , found in fibrillar protein collagen and some plant proteins; 5-hydroxylysine is found in collagen hydrolysates, desmosy n and isodesmosine isolated from hydrolysates of fibrillar protein elastin. It seems that these amino acids are only found in this protein. Their structure is unusual: 4 lysine molecules connected by their R-groups form a substituted pyridine ring. It is possible that due to just such a structure, these amino acids can form 4 radially diverging peptide chains. The result is that elastin, unlike other fibrillar proteins, is able to deform (stretch) in two mutually perpendicular directions. Etc.
Living organisms synthesize a huge number of various protein compounds from the listed protein amino acids. Many plants and bacteria can synthesize all the amino acids they need from simple inorganic compounds. In the human and animal body, about half of the amino acids is also synthesized. Another part of the amino acids can enter the human body only with dietary 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. The nonessential amino acids include 12 amino acids.
Both types of amino acids are equally important for humans: nonessential 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 (20-30 g with a daily protein intake of 80-100 g). In children's nutrition, the proportion of protein rises 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 of muscles, blood vessels, nervous system, skin and all other tissues and organs.
In our days of fast food and the general passion for fast food, the diet is very often dominated by foods with a high content of easily digestible carbohydrates and fats, and the proportion of protein products 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 are used to maintain the normal functioning of the most important organs - the heart and the brain. The human body may be deficient in both essential and non-essential amino acids. A deficiency of amino acids, especially essential ones, leads to a deterioration in appetite, delayed growth and development, fatty degeneration of the liver and other serious disorders. The first "messengers" of a lack of amino acids can be a decrease in appetite, deterioration of the skin, hair loss, muscle weakness, fatigue, decreased immunity, anemia. Such manifestations can occur in persons who, in order to reduce weight, follow a low-calorie unbalanced diet with a sharp restriction of protein products.
More often than others, vegetarians are faced with manifestations of a lack of amino acids, especially essential ones, who deliberately avoid the inclusion of 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 adolescence. The most toxic are methionine (provokes the risk of heart attack and stroke), tyrosine (can provoke the development of arterial hypertension, lead to disruption of the thyroid gland) and histidine (can contribute to copper deficiency in the body and lead to the development of aortic aneurysms, joint diseases, early graying , severe anemia). Under normal conditions of the body's functioning, when there is a sufficient amount of vitamins (B 6, B 12, folic acid) and antioxidants (vitamins A, E, C and selenium), the excess of amino acids quickly turns into useful components and does not have time to "damage" the body. With an unbalanced diet, a deficiency of vitamins and minerals occurs, and an excess of amino acids can disrupt the functioning of systems and organs. This option is possible with long-term adherence to protein or low-carbohydrate diets, as well as with uncontrolled intake of protein-energy products (amino acid-vitamin cocktails) by athletes to increase weight and develop muscles.
Among chemical methods, the most common method is amino acid scoring (scor - counting, counting). It is based on a comparison of the amino acid composition of the protein of the evaluated product with the amino acid composition standard (ideal) protein. After quantitative determination by chemical means of the content of each of the essential amino acids in the protein under study, the amino acid rate (AS) for each of them is determined by the formula
AC \u003d (m ak . research / m ak . perfect ) 100
m ac. research - the content of essential amino acids (in mg) in 1 g of the protein under study.
m ac. ideal - the content of essential amino acids (in mg) in 1 g of standard (ideal) protein.
FAO / WHO Amino Acid Sample
Simultaneously with the determination of the amino acid rate, limiting essential amino acid for a given protein , i.e the one for which the speed is the smallest.
Peptides.
Two amino acids can be covalently linked by peptide connection with the formation of a dipeptide.
Three amino acids can be linked through 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, which are determined by the presence of functional groups: free amino group, free carboxy group and R-groups. Peptide bonds are susceptible to hydrolysis with a strong acid (eg 6M HC1) 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 destroyed by enzymes proteases.
Many naturally occurring peptides have biological activity at very low concentrations.
Peptides are potentially active pharmaceuticals, 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 imposed on the yield of products at all intermediate stages.
General characteristics (structure, classification, nomenclature, isomerism).
The main structural unit of proteins is a-amino acids. There are approximately 300 amino acids in nature. The proteins contain 20 different a-amino acids (one of them, proline, is not amino-, and iminoacid). All other amino acids exist in a free state or as part of short peptides or complexes with other organic substances.
a-Amino acids are derivatives of carboxylic acids, in which one hydrogen atom, at the a-carbon atom is replaced by an amino group (–NH 2), for example:
Amino acids differ in the structure and properties of the radical R. The radical can represent residues of fatty acids, aromatic rings, heterocycles. Due to this, each amino acid is endowed with specific properties that determine the chemical, physical properties and physiological functions of proteins in the body.
It is thanks to amino acid radicals that proteins have a number of unique functions that are not characteristic of other biopolymers, and have a chemical identity.
Amino acids with the b- or g-position of the amino group are found much less frequently in living organisms, for example:
Classification and nomenclature of amino acids.
There are several types of classifications of amino acids that make up a protein.
A) One of the classifications is based on the chemical structure of amino acid radicals. There are amino acids:
1. Aliphatic - glycine, alanine, valine, leucine, isoleucine:
2. Hydroxyl-containing - serine, threonine:
4. Aromatic - phenylalanine, tyrosine, tryptophan:
5.With anion-forming groups in the side chains - aspartic and glutamic acids:
6. and amides - aspartic and glutamic acids - asparagine, glutamine.
7. The main ones are arginine, histidine, lysine.
8. Imino acid - proline
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B) The second type of classification is based on the polarity of the R-groups of amino acids.
Distinguish polar and non-polar amino acids. Nonpolar radicals have nonpolar C – C, C – H bonds, there are eight such amino acids: alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, proline.
All other amino acids are to polar (in the R-group there are polar bonds C – O, C – N, –OH, S – H). The more amino acids with polar groups in a protein, the higher its reactivity. Protein functions largely depend on the reactivity. Enzymes are characterized by a particularly large number of polar groups. Conversely, there are very few of them in such a protein as keratin (hair, nails).
C) Amino acids are classified based on the ionic properties of the R-groups(Table 1).
Sour (at pH \u003d 7, the R-group can carry a negative charge) these are aspartic, glutamic acids, cysteine \u200b\u200band tyrosine.
The main (at pH \u003d 7, the R-group can carry a positive charge) is arginine, lysine, histidine.
All other amino acids belong to neutral (the R group is uncharged).
Table 1 - Classification of amino acids based on polarity
R-groups.
R-groups
Aspartic acid
Glutamic acid
4. Positively charged
R-groups
Histidine
D) Amino acids are divided according to the number of amine and carboxyl groups:
– for monoamine monocarboxyliccontaining one carboxyl and one amine group;
- monoaminodicarboxylic (two carboxyl and one amine group);
- diaminomonocarboxylic (two amine and one carboxyl group).
E) According to the ability to be synthesized in the human and animal body, all amino acids are divided:
– replaceable,
- irreplaceable,
- partially irreplaceable.
Essential amino acids cannot be synthesized in humans and animals, they must be supplied with food. There are eight absolutely essential amino acids: valine, leucine, isoleucine, threonine, tryptophan, methionine, lysine, phenylalanine.
Partially irreplaceable - they are synthesized in the body, but in insufficient quantities, therefore, they must partially come from food. These amino acids are arganine, histidine, tyrosine.
Essential amino acids are synthesized in the human body in sufficient quantities from other compounds. Plants can synthesize all amino acids.
Isomerism
In the molecules of all natural amino acids (with the exception of glycine) at the a-carbon atom, all four valence bonds are occupied by various substituents, such a carbon atom is asymmetric, and is called chiral atom... As a result, amino acid solutions have optical activity - they rotate the plane of plane-polarized light. The number of possible stereoisomers is exactly 2 n, where n is the number of asymmetric carbon atoms. For glycine, n \u003d 0, for threonine, n \u003d 2. All the other 17 protein amino acids each contain one asymmetric carbon atom; they can exist in the form of two optical isomers.
As a standard in determining L and D-configurations of amino acids use the configuration of the stereoisomers of glyceraldehyde.
The location in the Fischer projection formula of the NH 2 -group on the left corresponds to L-configuration, and on the right - D-configurations.
It should be noted that the letters L and D denote the belonging of a particular substance by its stereochemical configuration to L or D row, regardless of the direction of rotation.
In addition to the 20 standard amino acids found in almost all proteins, there are also non-standard amino acids that are components of only some types of proteins - these amino acids are also called modified (hydroxyproline and hydroxylysine).
Receiving methods
- Amino acids are of extremely great physiological importance. Proteins and polypeptides are built from amino acid residues.
With the hydrolysis of protein substances animal and plant organisms are formed amino acids.
Synthetic methods for producing amino acids:
– Action of ammonia on halogenated acids
- α-Amino acids are obtained the action of ammonia on oxynitriles
Lecture number 3Topic: "Amino acids - structure, classification, properties, biological role"
Amino acids are nitrogen-containing organic compounds, the molecules of which contain an amino group -NH2 and a carboxyl group -COOH
The simplest representative is aminoethanic acid H2N - CH2 - COOH
Amino acid classification
There are 3 main classifications of amino acids:
Physicochemical - based on differences in physical and chemical properties of amino acids
Hydrophobic amino acids (non-polar). The components of the radicals usually contain hydrocarbon groups, where the electron density is evenly distributed and there are no charges and poles. They may also contain electronegative elements, but they are all in a hydrocarbon environment.
Hydrophilic uncharged (polar) amino acids ... The radicals of such amino acids contain polar groups: -OH, -SH, -CONH2
Negatively charged amino acids... These include aspartic and glutamic acids. They have an additional COOH group in the radical - they acquire a negative charge in a neutral medium.
Positively charged amino acids : arginine, lysine and histidine. They have an additional NH 2 -group (or imidazole ring, like histidine) in the radical - in a neutral medium they acquire a positive charge.
Irreplaceable amino acids, they are also called "essential". They cannot be synthesized in the human body and must be supplied with food. Their 8 and 2 more amino acids are partially irreplaceable.
Partially irreplaceable: arginine, histidine.
Replaceable (can be synthesized in the human body). There are 10 of them: glutamic acid, glutamine, proline, alanine, aspartic acid, asparagine, tyrosine, cysteine, serine and glycine.
Amino acids are classified according to structural characteristics.
1. Depending on the relative position of the amino and carboxyl groups, amino acids are subdivided into α-, β-, γ-, δ-, ε- etc.
The need for amino acids decreases:
With congenital disorders associated with the absorption of amino acids. In this case, some protein substances can cause allergic reactions in the body, including the appearance of problems in the work of the gastrointestinal tract, itching and nausea.
Amino acid assimilation
The speed and completeness of assimilation of amino acids depends on the type of products containing them. Amino acids contained in egg whites, low-fat cottage cheese, lean meat and fish are well absorbed by the body.
Amino acids are also quickly absorbed with the right combination of products: milk is combined with buckwheat porridge and white bread, all kinds of flour products with meat and cottage cheese.
Useful properties of amino acids, their effect on the body
Each amino acid has its own effect on the body. So methionine is especially important for improving fat metabolism in the body, is used as the prevention of atherosclerosis, cirrhosis and fatty degeneration of the liver.
For certain neuropsychiatric diseases, glutamine, aminobutyric acids are used. Glutamic acid is also used in cooking as a flavoring agent. Cysteine \u200b\u200bis indicated for eye diseases.
The three main amino acids, tryptophan, lysine and methionine, are especially needed by our body. Tryptophan is used to accelerate the growth and development of the body, and it also maintains nitrogen balance in the body.
Lysine ensures the normal growth of the body, participates in the processes of blood formation.
The main sources of lysine and methionine are cottage cheese, beef, and some types of fish (cod, pike perch, herring). Tryptophan is found in optimal amounts in organ meats, veal and game infarction.
Amino acids for health, vitality and beauty
To successfully build muscle mass in bodybuilding, amino acid complexes consisting of leucine, isoleucine and valine are often used.
Athletes use methionine, glycine and arginine, or foods containing them, as dietary supplements to maintain energy during exercise.
Anyone who leads an active, healthy lifestyle needs special foods that contain a number of essential amino acids to maintain excellent physical shape, quickly recuperate, burn excess fat or build muscle mass.
LIPIDS
Lipids are water-insoluble oily or fatty substances that can be extracted from cells with non-polar solvents. It is a heterogeneous group of compounds directly or indirectly associated 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 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 bicarbon 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 (after the formula are the number of carbon atoms, name, melting point):
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 about 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 in animals and plants are found twice as often as unsaturated ones,
Saturated fatty acids do not have a rigid linear structure, they are very flexible and can assume 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,
Double bonds of almost all natural fatty acids are found in cis-conformation, 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, and unsaturated fatty acids are liquids,
Sodium and potassium soaps of fatty acids are capable of emulsifying 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 the membrane lipids of bacteria. Many of the bacterial strains containing these lipids (thermophiles, acidophiles, and gallophils) are adapted to extreme conditions.
iso-branched
anti-branched
cyclopropane-containing
ω-cyclohexyl-containing
isopranial
cyclopentane phytanyl
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, the important derivatives of arachidonic acid are the histohormones prostaglandins, thromboxanes, and leukotrienes, which are combined into the eicosanoid group and have an extremely broad biological activity.
prostaglandin H 2
Lipid classification:
1. Triacylglycerides (fats) are esters of an alcohol of glycerol and three fatty acid molecules. They constitute the main component of the fat depots of plant and animal cells. Not contained in membranes. Simple triacylglycerides contain residues of the same fatty acids in all three positions (tristearin, tripalmitin, triolein). Mixed contains different fatty acids. By specific gravity it is lighter than water, well soluble in chloroform, benzene, ether. Hydrolyzed by boiling with acids or bases, or by the action of lipase. In cells, under normal conditions, the self-oxidation 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 fatty drops 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 because they can be stored in very large quantities in an almost pure, unhydrated form and, per unit weight, they store twice as much energy as carbohydrates. In addition, triacylglycerides under the skin form an insulating layer that protects the body from very low temperatures.
neutral fat
The following constants are used to characterize the properties of fat:
Acid number - the amount of mg KOH required for neutralization
free fatty acids contained in 1 g of fat;
Saponification number - the amount 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 act as a protective coating that lubricates and softens the skin and protects it from water. Hair, wool, fur, animal feathers, as well as 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. Necessarily contain in its composition polyhydric alcohol glycerin, phosphoric acid and residues of fatty acids.
Phosphoglycerolipids can be divided into several types by chemical structure:
1) phospholipids - consist of glycerol, two fatty acid residues at the 1st and 2nd positions of glycerol and a phosphoric acid residue, with which the remainder of another alcohol is bound (ethanolamine, choline, serine, inositol). As a rule, the fatty acid in the 1st position is saturated, and in the 2nd it is unsaturated.
phosphatidic acid - a precursor for the synthesis of other phospholipids, it is contained in tissues in small amounts
phosphatidylethanolamine (cephalin)
phosphatidylcholine (lecithin), it is practically absent in bacteria
phosphatidylserine
phosphatidylinositol - a precursor of two important secondary messengers (intermediaries) diacylglycerol and inositol-1,4,5-triphosphate
2) plasmalogens - phosphoglycerolipids, in which one of the hydrocarbon chains is a vinyl ether. Plasmalogens are not found in plants. Ethanolamine plasmalogens are widely present in myelin and in the sarcoplasmic reticulum of the heart.
ethanolamine plasmalogen
3) lysophospholipids - are formed from phospholipids during enzymatic cleavage of one of the acyl residues. The snake venom contains phospholipase A 2, which forms lysophosphatides with hemolytic action;
4) cardiolipins - phospholipids of the inner membranes of bacteria and mitochondria, are formed by interaction with glycerol of two residues of phosphatidic acid:
cardiolipin
4. Phosphingolipids - 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. Phosphosphingolipids are rare in the membranes of plant and bacterial cells. 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 in which a carbohydrate attached via a glycosidic bond is located in position 3 of glycerol does not contain a phosphate group. Glycoglycerolipids are widely found in chloroplast membranes, as well as in blue-green algae and bacteria. Monogalactosyldiacylglycerol is the most common polar lipid in nature, since it accounts for half of all lipids of 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 do not carry an electrical charge. Glycosphingolipids can be divided into two more 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 several 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 impulses from one nerve cell to another.
7. Isoprenoids - isoprene derivatives (active form - 5-isopente-nyldiphosphate), performing a wide variety of functions.
isoprene 5-isopentenyl diphosphate
The ability to synthesize specific isoprenoids is characteristic of only some species of animals and plants.
1) rubber - several types of plants are synthesized, primarily Brazilian Hevea:
rubber fragment
2) fat-soluble vitamins A, D, E, K (due to the structural and functional affinity with steroid hormones, vitamin D is now referred to as hormones):
vitamin A
vitamin E
vitamin K
3) animal growth hormones - retinoic acid in vertebrates and neotenins in insects:
retinoic acid
neotenine
Retinoic acid is a hormonal derivative of vitamin A, stimulates the growth and differentiation of cells, neotenins are insect hormones, stimulate the growth of larvae and inhibit molting, are antagonists of ecdysone;
4) plant hormones - abscisic acid, is a stress phytohormone that triggers the systemic immune response of plants, which manifests itself 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 basically contain cyclopentaneperhydrofenanthrene (in essence, triterpene). The main sterol in animal tissues is 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. Due to the fact that the four condensed rings create a rigid structure, the presence of cholesterol in membranes regulates membrane fluidity at extreme temperatures. Plants and microorganisms contain related compounds - ergosterol, stigmasterol and β-sitosterol.
cholesterol
ergosterol
stigmaster
β-sitosterol
Bile acids are formed from cholesterol in the body. They ensure the solubility of cholesterol in bile and aid in the digestion of lipids in the intestine.
cholic acid
Cholesterol also produces steroid hormones - lipophilic signaling molecules that regulate metabolism, growth and reproduction. In the human body, there are six main steroid hormones:
cortisol aldosterone
testosterone estradiol
progesterone calcitriol
Calcitriol is a hormonal vitamin D that differs from the hormones of vertebrates, but is also based on cholesterol. Ring B is opened by a light-dependent reaction.
A cholesterol derivative is the moulting hormone of insects, spiders and crustaceans - ecdysone. Signaling steroid hormones are also found in plants.
7) lipid anchors that hold protein molecules or other compounds on the membrane:
ubiquinone
As we can see, lipids are not polymers in the literal sense of the word, however, both metabolic and structurally, they are close to the polyoxybutyric acid present in bacteria, an important storage substance. This highly reduced polymer consists exclusively of ester-linked D-β-hydroxybutyric acid units. Each chain contains about 1500 residues. The structure is a compact right-handed helix, about 90 such chains are stacked to form a thin layer in bacterial cells.
poly-D-β-hydroxybutyric acid
Amino acids are carboxylic acids containing an amino group and a carboxyl group. Natural amino acids are 2-amino carboxylic acids, or α-amino acids, although there are amino acids such as β-alanine, taurine, γ-aminobutyric acid. The generalized formula for an α-amino acid looks like this:
The α-amino acids at carbon 2 have four different substituents, that is, all α-amino acids, except glycine, have an asymmetric (chiral) carbon atom and exist as 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 in the form of 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, an amino acid is a zwitter ion, that is, its amine group is protonated, and the carboxyl group is dissociated. In the neutral pH range, 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, acetalization and formation of Schiff bases at amino groups, esterification at carboxyl groups.
The biological role of amino acids:
1) are the structural elements of peptides and proteins, the so-called proteinogenic amino acids. The proteins contain 20 amino acids, which 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 the most important 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, gli, G, Gly
alanine, ala, A, Ala
valine, shaft, V, Val *
Leucine, lei, L, Leu *
isoleucine, silt, 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 range, Y, Tyr
tryptophan, three, W, Trp *
histidine, gis, H, His
Aromatic amino acids contain mesomeric resonance stabilized cycles. In this group, only the amino acid phenylalanine exhibits low polarity, tyrosine and tryptophan are characterized by noticeable, and histidine - even high polarity. Histidine can also be referred to as basic amino acids.
4. Neutral amino acids:
serine, gray, 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 nonionic, the asparagine and glutamine molecules 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 fully ionized over the entire physiological pH range.
6. Essential amino acids:
lysine, l of, K, Lys *
arginine, arg, R, Arg
The side chains of basic amino acids are fully protonated in the neutral pH range. A strongly basic and very polar amino acid is arginine, which contains a guanidine moiety.
7. Imino acid:
proline, about, P, Pro
The proline side chain consists of a five-membered ring containing 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 is not protonated at physiological pH values. Due to its 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 ingested with food. These are essential amino acids marked with asterisks.
As mentioned above, proteinogenic amino acids are 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 decarboxylation of aspartic acid, and cysteamine by decarboxylation of cysteine:
β-alanine cysteamine
The remainder of glutamic acid 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 ones, are formed in the liver and are present in bile in the form of salts.
glycocholic acid
Proteinogenic amino acids are precursors of some antibiotics - biologically active substances synthesized by microorganisms and inhibiting the growth 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 the mold of the genus Penicillium... They are characterized by the presence of a reactive β-lactam ring in the structure, with the help of which they inhibit the synthesis of cell walls of gram-negative microorganisms.
general formula of penicillins
Biogenic amines - neurotransmitters, hormones and histohormones - are obtained from amino acids by decarboxylation.
The amino acids glycine and glutamate are themselves neurotransmitters in the central nervous system.
Alkaloids are also derivatives of amino acids - natural nitrogen-containing compounds of a basic nature, formed in plants. These compounds are extremely active physiological compounds widely used in medicine. Examples of alkaloids include the phenylalanine derivative papaverine, the isoquinoline alkaloid of hypnotic poppy (antispasmodic), and the tryptophan derivative physostigmine, an indole alkaloid from calabar beans (anticholinesterase drug):
papaverine physostigmine
Amino acids are extremely popular biotechnology targets. There are many options for the chemical synthesis of amino acids, but the result is amino acid racemates. Since only L-isomers of amino acids are suitable for the food industry and medicine, racemic mixtures must be separated into enantiomers, which is a serious problem. Therefore, the biotechnological approach is more popular: enzymatic synthesis using immobilized enzymes and microbiological synthesis using whole microbial cells. In both the latter cases, pure L-isomers are obtained.
Amino acids are used as food additives and feed ingredients. Glutamic acid enhances the taste of meat, valine and leucine improve the taste of baked goods, glycine and cysteine \u200b\u200bare 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 plant proteins contain a small amount of the essential amino acid lysine.
Amino acids are widely used in medical practice. These are such amino acids as methionine, histidine, glutamic and aspartic acids, glycine, cysteine, valine.
In the last decade, amino acids have begun to be added to cosmetics for skin and hair care.
Chemically modified amino acids are also widely used in industry as surfactants in polymer synthesis, in the production of detergents, emulsifiers, and fuel additives.