Hydrolysis of proteins determination of amino acid composition. Amino acid composition of proteins
Protein synthesis occurs on ribosomes in the form of a primary structure, i.e. located in a certain number and in a certain sequence of amino acids connected by peptide bonds formed by carboxyl and α-amino groups of neighboring amino acid residues Peptide bond is rigid, covalent, genetically determined. In structural formulas, it is depicted as a single bond: and nitrogen has the character of a partially double bond:
Rotation around it is impossible and all four atoms lie in the same plane, i.e. coplanar. The rotation of other bonds around the polypeptide backbone is quite free.
The primary structure was discovered in 1898 by Danilevsky, a professor at the Kazan University. In 1913, Emil Fischer synthesized the first peptides.
This amino acid sequence is unique for each protein and is genetically fixed. In violation of the process of synthesis of the primary structure of the protein on the ribosome, various genetic diseases can develop. For example, when two amino acids in hemoglobin are disturbed, sickle cell anemia develops.
To study the amino acid composition of proteins, a combination (or one of them) of acidic (HCl), alkaline (Ba(OH)2) and less often enzymatic hydrolysis is used. It has been established that during the hydrolysis of a pure protein that does not contain impurities, 20 different a-amino acids are released. All other amino acids discovered in the tissues of animals, plants and microorganisms (more than 300) exist in nature in a free state or in the form of short peptides or complexes with other organic substances.
α-amino acids are derivatives of carboxylic acids, in which one hydrogen atom, at the α-carbon, is replaced by an amino group (-NH2), for example: it should be emphasized that all amino acids that make up natural proteins are a-amino acids, although the amino group in free aminocarboxylic acids can be, as we will see below, in β, γ, δ, ε positions.
9. Secondary structure of proteins - α-helices and β-structures. Structure and functional role of domains.
The secondary structure is the spatial arrangement of the polypeptide chain in the form of an α-helix or β-folding, regardless of the types of side radicals and their conformation. It is stabilized by hydrogen bonds, which are closed between peptide, amide (-N-H) and carbonide (-C=O) groups, i.e. are included in the peptide unit, and disulfide bridges between cysteine residues
Pauling and Corey proposed a model of the protein secondary structure in the form of a left-handed α-helix, in which hydrogen bonds are closed between each first and fourth amino acid, which makes it possible to preserve the native structure of the protein, perform its simplest functions, and protect it from destruction. There are 3.6 amino acid residues per turn of the helix, the helix pitch is 0.54 nm. All peptide groups take part in the formation of hydrogen bonds, which ensures maximum stability, reduces hydrophilicity and increases the hydrophobicity of the protein molecule. The alpha helix forms spontaneously and is the most stable conformation corresponding to the minimum free energy
Pauling and Corey also proposed another ordered structure - a folded β-layer. In contrast to the condensed α-helix, β-layers are almost completely elongated and can be arranged both in parallel and antiparallel
Disulfide bridges and hydrogen bonds also take part in the stabilization of these structures.
A supersecondary structure is a higher level of organization of a protein molecule, represented by an ensemble of secondary structures interacting with each other: α-helix - two antiparallel sections, interacting with hydrophobic complementary surfaces (according to the hollow-protrusion principle) αсα, supercoiling of α-helix, (βхβ)-elements in globular proteins, represented by two parallel β-chains connected by an x segment, βαβαβ-elements, represented by two α-helix segments inserted between three parallel β-chains.
Contents Introduction 1. The main components of milk 2. Methods for the analysis of amino acids 1. Chromatographic method of analysis 2. Spectrophotometric method of analysis 3. Titrimetric method of analysis 4. Electrochemical method of analysis 3. Methods for determining the amino acid composition 1. Determination of amino acids by thin layer chromatography 3.2. Determination of amino acids by spectrophotometric method 4. Review of abstract journals References Introduction The problem of nutrition is one of the most important social problems.
Human life, health and work are impossible without good food. According to the theory of balanced nutrition, the human diet should contain not only proteins, fats and carbohydrates in the required amount, but also substances such as essential amino acids, vitamins, minerals in certain proportions that are beneficial to humans.
In the organization of proper nutrition, the primary role is given to dairy products. This fully applies to milk, the nutritional value of which is due to the high concentration of milk protein and fat in it, the presence of essential amino acids, calcium and phosphorus salts, which are so necessary for the normal development of the human body. Easy digestibility is one of the most important properties of milk as a food product. Moreover, milk stimulates the absorption of nutrients from other foods.
Milk adds variety to nutrition, improves the taste of other products, and has therapeutic and prophylactic properties. Milk contains more than 120 different components, including 20 amino acids, 64 fatty acids, 40 minerals, 15 vitamins, dozens of enzymes, etc. The energy value of 1 liter of raw milk is 2797 kJ. One liter of milk satisfies the daily requirement of an adult for fat, calcium, phosphorus, 53% for protein, 35% for vitamins A, C and thiamine, and 26% for energy. The main purpose of this course work is to identify the amino acid composition of milk. one.
Main components of milk
From the physicochemical standpoint, milk is a complex polydisp... 5.1). The largest specific gravity in milk is water (more than 85%, the rest ... The dry residue includes all the nutrients of milk. It determines the yield of finished products in the production of dairy products...
Chromatographic method of analysis
One of the most promising methods is the highly efficient method... But the advantages of the method greatly outweigh its disadvantages. In addition, it can be used to complete chemical analysis.... On modern gas chromatographic capillary columns in one ex... The method is characterized by high sensitivity and allows the amount of...
Titrimetric method of analysis
Titrimetric method of analysis. Of the titrimetric methods of quantitative determination, the widest ... Titration can be carried out with an indicator (crystal violet ... However, this method has a number of significant drawbacks: use ... For the quantitative analysis of individual amino acids, met ...
Electrochemical method of analysis
In recent decades, el... 3.. under optimized conditions, they have become more and more widespread, they allow you to determine only individual am... So, a method has been developed for the polarographic determination of tryptophan, based on... An electrochemical method of analysis.
Methods for determining the amino acid composition
Methods for determining the amino acid composition 3.1.
Determination of amino acids by thin layer chromatography
In 1 liter of distilled water dissolve 84 g of citric acid monohydrate... 3.2. samples and standard amino acids are applied to the starting line on the plate...
Determination of amino acids by spectrophotometric method
Amino acids, primary amines, polypeptides and peptones when heated with ... 0.2 - 3% solution of ninhydrin are prepared in different solvents (isobu ... 2007. K. 2. Tsvetkova N.D.
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First of all, peptide bonds are destroyed by hydrolysis. Since peptide bonds are stable at neutral pH, acid or alkaline catalysis is used. Enzymatic catalysis is less suitable for complete hydrolysis. Complete hydrolysis of a protein into its constituent amino acids is inevitably accompanied by a partial loss of some amino acid residues. Best to carry out
Rice. 4.7. Gel filtration of cyanogen bromide (CB) fragments of human fetal globin. Chromatography was carried out on a Sephadex column balanced with -HCOOH, followed by elution with the same solution. The numbering of fragments of a- and y-chains is arbitrary. (Courtesy of J. D. Pearson et al., Department of Biochemistry, Purdue University.)
hydrolysis in 6 n. at 110°C in an evacuated ampoule. Under these conditions, tryptophan is completely destroyed to cysteine and partially cystine. In the presence of metals, a partial loss of methionine and tyrosine is observed. Glutamine and asparagine are quantitatively deamidated to glutamate and aspartate. The content of serine and threonine also turns out to be underestimated, and to a greater extent, the longer the hydrolysis is carried out. Finally, some bonds between neutral residues are only 50% hydrolyzed even after 20 h. Typically, replicate samples are hydrolyzed for 24, 48, 72, and 96 hours. The serine and threonine data are then plotted on a semi-log scale and extrapolated to time zero. For valine and isoleucine, the results obtained from the 96-hour hydrolysis are used. Dicarboxylic acids and their amides are defined collectively and reported as “Glx” or “Asx”. Cysteine and cystine are converted into acid-stable derivatives (for example, cysteic acid) before hydrolysis.
Rice. 4.8. High pressure reverse phase liquid chromatography: elution profile of cyanogen bromide (CB) fragments of human fetal globin. The numbering of fragments of a- and y-chains is arbitrary. (Courtesy of J. D. Pearson et al.. Department of Biochemistry, Purdue University.)
For the analysis of tryptophan, alkaline hydrolysis is carried out; in this case, the destruction of serine, threonine, arginine and cysteine and the racemization of all amino acids occur. After hydrolysis is complete, the amino acid composition can be determined using automatic ion exchange chromatography (see Figure 3.12) or pressure liquid chromatography.
The following steps can be distinguished in elucidating the primary structure of proteins and peptides:
1. Protein isolation in pure form and determination of its molecular weight
2. Determination of amino acid composition
3. Determination of the N-terminal amino acid
4. Determination of the C-terminal amino acid
5. Determination of the amino acid sequence
Isolation of pure protein. Typically, the starting material contains many different proteins. This raises the problem of isolating the pure protein of interest from this mixture. When purifying proteins, methods are used that are based on the difference:
1. Surface charge of proteins
2. Molecular size of proteins (depending on their molecular weight)
3. Biological activity due to binding to substrates or inhibitors
Separation of proteins according to the difference in the magnitude of the surface charge. The total surface electrical charge of a protein at a given pH value can be negative, neutral, or positive. To separate proteins with different charges, as was the case with amino acids, the method of ion exchange chromatography (see above) can be used. The protein concentration in the test tubes with the eluate is determined using a spectrophotometer by the intensity of absorption of ultraviolet light and its graphical dependence on the volume of liquid flowing out of the chromatographic column is plotted.
Separation of proteins by molecular weight. If we imagine protein molecules as balls of various sizes, the size of which depends on their molecular weight, then it turns out that large balls will have a larger molecular weight or molecular size. This means that proteins can be separated like particles in a sieve - a molecular sieve formed by a gel. This method is often called gel filtration or size exclusion chromatography. Below is an illustration of how gel filtration can separate a mixture of proteins of different sizes (Fig. 1.12).
The chromatographic column is filled with swollen gel. The gel particles are made from a cross-linked polysaccharide material and contain a large number of micropores. The size of the micropores is selected in such a way that the smaller of the separated molecules penetrate into them, while the larger ones cannot do this. The mixture of proteins to be separated is applied to the top of the column and eluted with a buffer solution. Entrained by the current of the descending liquid, large molecules, unable to penetrate into the pores of the gel particles, will move faster. Smaller molecules penetrate the pores and stay there. If the solution flowing from the column is collected in equal portions in test tubes, it will turn out that the earlier portions of the flowing liquid will contain proteins of large sizes, and later - of smaller sizes. By adjusting the pore size, separation of a wide variety of protein mixtures can be achieved.
Fig.1.12. Schematic representation of protein separation by gel filtration
If we take into account that the size of a molecule depends on the molecular weight, it turns out that by separating proteins by gel filtration, one can simultaneously determine its molecular weight.
Fig.1.13. Graph of the dependence of the molecular weight of proteins on the volume of their exit from the chromatographic column during gel filtration
The volume of eluate flowing out of the column is inversely proportional to the logarithm of the molecular weight of the protein. Thus, it is sufficient to know the volume of liquid in which the protein of interest left the column so that, using a similar graph, its molecular weight can be determined (Fig. 1.13).
Another method that allows you to separate proteins depending on their molecular weight is gel electrophoresis(see above).
Ultracentrifugation. If you shake a vessel filled with sand and water, and then put it on a flat surface, the sand will quickly settle to the bottom due to the force of gravity. With high-molecular substances in solution, this will not happen, since thermal (Brownian) motion preserves their uniform distribution in solution. The sedimentation of macromolecules, like grains of sand, will occur only if they are subjected to significant acceleration.
Determination of the amino acid composition of proteins can be carried out by various methods: chemical, chromatographic, microbiological and isotopic. Chromatographic methods are more commonly used.
Paper chromatography. Paper chromatography is used to identify the components of a mixture of amino acids with di- and tri-peptides obtained by partial hydrolysis of proteins and polypeptides.
Hydrolysis can be carried out by acid, alkali or enzymatic methods. The acid method is used more frequently (6 N HCl, 8 N H 2 SO 4). Hydrolysis is carried out with heating, sometimes at elevated pressure. Indicators of the end of hydrolysis can be: the cessation of the growth of carboxyl or amine groups in the hydrolyzate, or a negative biuret reaction. The excess hydrolyzing agent is removed: sulfuric acid is precipitated with Ca(OH) 2 , hydrochloric acid is distilled off in vacuo, and the acid residue is precipitated with silver nitrate.
The components of the hydrolyzate are distributed between the water adsorbed on the cellulose, which is the stationary phase, and the organic solvent, the mobile phase, which moves up or down along the sheet. As a mobile phase, a mixture of butanol-acetic acid-water (4:1:5) is used. The more lipophilic amino acids are more strongly carried away by the organic solvent, the more hydrophilic ones show a greater tendency to bind to the stationary phase. Homologous compounds that differ even by one methylene unit move at different speeds and can be easily separated. At the end of the chromatography, the paper is dried and treated with a developer (0.5% solution of ninhydrin in a mixture of acetone-glacial acetic acid-water) and heated for several minutes. Amino acids appear as colored spots. Mobility - a constant value characteristic of each compound increases with increasing molecular weight. For straight chain amino acids, the mobility is somewhat greater than for the corresponding isomers. The introduction of polar groups into the molecule reduces the mobility of the compound. Amino acids with bulky non-polar side chains (leucine, isoleucine, phenylalanine, tryptophan, etc.) move faster than amino acids with shorter non-polar side chains (proline, alanine, glycine) or with polar side chains (threonine, arginine, cysteine, histidine, lysine). This is due to the greater solubility of polar molecules in the hydrophilic stationary phase and nonpolar ones in organic solvents.
Paper chromatography can be used to quantify amino acid content. Each spot is excised and eluted with a suitable solvent; then carry out a quantitative colorimetric (ninhydrin) analysis. In another embodiment, the paper is sprayed with ninhydrin and the color intensity of the spot is measured using a photometer in reflected or transmitted light. In a semi-quantitative assessment, the content of amino acids is estimated by the area of spots on the chromatogram, which are proportional to the concentrations of amino acids in the mixture being separated.
Thin layer chromatography. Thin layer chromatography can also be used to separate and determine amino acids. TLC, as is known, exists in two versions. Partition TLC is similar to paper TLC, and adsorption TLC is based on completely different principles.
When performing RTLC on cellulose powder or other relatively inert media, the same solvent systems and developing reagents can be used as in paper chromatography.
Separation using ATC is determined by the ability of the solvent (this solvent is not necessarily a binary or more complex mixture) to elute the components of the sample from the place of its adsorption on the activated sorbent. For example, on heated silica gel. ATLC is useful for separating non-polar compounds such as lipids, but not for separating amino acids and most peptides. To separate amino acids, PTLC is used, which allows you to quickly separate and determine 22 amino acids of protein hydrolysates.
Amino acids in a protein hydrolyzate can also be determined by gas chromatography, but amino acids are usually converted to volatile compounds before chromatographic analysis.
Interaction with ninhydrin. The corresponding aldehydes are formed.
Thus, a mixture of aldehydes is obtained and analyzed. This is the simplest case, suitable only for some amino acids.
They convert amino acids into volatile esters (alkyl esters, methyl esters of hydroxy acids, methyl esters of chlorosubstituted acids, etc.).
The choice of derivatives depends on the studied mixture of amino acids.
Ion exchange chromatography. Currently, the amino acid composition of food products is determined exclusively by automatic ion exchange chromatography.
| Ion exchange chromatography is based on the reversible stoichiometric exchange of ions in solution for ions that are part of the ion exchanger (cation exchanger, anion exchanger) and on the different ability of the ions to be separated to ion exchange with fixed sorbent ions formed as a result of the dissociation of ionogenic groups. For organic ions, the electrostatic interaction with fixed charges of the ion exchanger is superimposed by the hydrophobic interaction of the organic part of the ion with the ion exchanger matrix. To reduce its contribution to the retention of organic ions and to achieve the optimal selectivity of their separation, an organic component (1–25% methanol, isopropanol, acetonitrile) is added to the aqueous eluent. |
The Moore and Stein method uses short and long columns filled with sulfonated polystyrene resin in the Na+ form. When an acid hydrolyzate at pH=2 is applied to the column, the amino acids are bound by cation exchange with sodium ions. Next, the column is eluted with sodium citrate solution at preprogrammed pH and temperature. A short column is eluted with one buffer, a long column with two. The eluate is treated with ninhydrin, measuring the color intensity using a flow colorimeter. The data is automatically recorded on the tape recorder and can be transferred to a computer to calculate the area under the peak.
High-voltage electrophoresis on inert carriers. In biochemistry, the separation of amino acids, polypeptides, and other ampholytes (molecules whose total charge depends on the pH of the medium) under the action of a superimposed constant electric field has found wide application. This is a method of high-voltage electrophoresis on inert media. When separating amino acids, strips of paper or thin layers of cellulose powder are most often used as inert carriers. Separation is carried out for 0.5–2 h at a voltage of 2000–5000 V, depending on the total charges of the ampholytes and their molecular weights. Among molecules that carry the same charge, the lighter ones migrate faster. But a more important parameter in the separation is the total charge. The method is used to separate amino acids, low molecular weight peptides, some proteins, nucleotides. The sample is placed on the carrier, moistened with buffer at the appropriate pH and connected to the buffer reservoir with a strip of filter paper. The paper is covered with a glass plate or immersed in a hydrocarbon solvent to cool. In an electric field, molecules that carry a negative charge at a given pH migrate to the anode, and those that carry a positive charge migrate to the cathode. Next, the dried electrophoregram is “developed” with ninhydrin (when working with amino acids, peptides) or absorbance is measured in UV light (when working with nucleotides).
The choice of pH is determined by the pK values of the dissociating groups that make up the molecules of the mixture. At pH 6.4, glutamate and aspartate carry a charge of -1 and move towards the anode; their separation is carried out due to the difference in molecular weight. Lysine, arginine and histidine move in the opposite direction, while all other amino acids that make up the protein remain close to the site of application. When separating peptides resulting from enzymatic cleavage, lowering the pH to 3.5 leads to an increase in the charge of the cationic groups and provides better separation.
| Amino acids carry at least two weakly ionized groups: -COOH and -NH 3 + . In solution, these groups are in two forms, charged and uncharged, between which proton equilibrium is maintained: R-COOH « R-COO - + H + R-NH 3 + « R-NH 2 + H + (conjugated acids and bases) R -COOH and R-NH 3 + are weak acids, but the former is several orders of magnitude stronger. Therefore, most often (blood plasma, intercellular fluid pH 7.1–7.4) carboxyl groups are in the form of carboxylate ions, amino groups are protonated. Amino acids in molecular (non-dissociated) form do not exist at any pH. Approximate pK values for an a-amino acid and an a-amino group in an a-amino acid are 2 and 10, respectively. The total (total) charge (the algebraic sum of all positive and negative charges) of an amino acid depends on pH, i.e. on the concentration of protons in solution. The charge of an amino acid can be changed by varying the pH. This facilitates the physical separation of amino acids, peptides and proteins. The pH value at which the total charge of an amino acid is zero and therefore does not move in a constant electric field is called the isoelectric point (pI). The isoelectric point is in the middle between the closest pK values of the dissociating groups. |
Methods of paper, thin-layer chromatography, microbiological, gas chromatographic and a number of others are practically not used at present due to poorer reproducibility and long duration. Modern chromatographs make it possible to determine the amino acid composition of a mixture containing only 10–7–10–9 mol of each component with a reproducibility of up to 5% in 2–4 hours.
Analysis of the amino acid composition includes the complete hydrolysis of the protein or peptide under study and the quantitative determination of all amino acids in the hydrolyzate. Since peptide bonds are stable at neutral pH, acid or alkaline catalysis is used. Enzymatic catalysis is less suitable for complete hydrolysis. Complete hydrolysis of a protein into its constituent amino acids is inevitably accompanied by a partial loss of some amino acid residues. For hydrolysis, 6N is usually used. aqueous solution of hydrochloric acid (110ºС), in an evacuated ampoule. The quantitative determination of amino acids in the hydrolyzate is carried out using an amino acid analyzer. In most of these analyzers, a mixture of amino acids is separated on sulfonic cation exchangers, and detection is carried out spectrophotometrically by reaction with ninhydrin or fluorimetrically with about-phthalic dialdehyde.
However, data on the amino acid composition of the same type of products, obtained in different laboratories for individual amino acids, sometimes differ by up to 50%.
These differences are due not only to varietal, species or technological differences, but mainly to the condition for the hydrolysis of the food product. Standard acid hydrolysis (6 N HCl, 110–120ºС, 22–24 hours) results in partial destruction of some amino acids, including threonine, serine (by 10–15% and the more the longer the hydrolysis is carried out) and especially methionine ( 30–60%) and cystine 56–60%, as well as the almost complete destruction of tryptophan and cysteine. This process is enhanced in the presence of large amounts of carbohydrates in the product. For the quantitative determination of methionine and cystine, it is recommended to pre-oxidize them with performic acid. In this case, cystine is converted into cysteic acid, and methionine into methionine sulfone, which are very stable during subsequent acid hydrolysis.
cystine cysteic acid
A difficult task in amino acid analysis is the determination of tryptophan. As already mentioned, acid hydrolysis almost completely destroys it (up to 90%). Therefore, to determine tryptophan, one of the variants of alkaline hydrolysis of 2 N is carried out. NaOH, 100°C, 16–18 hours in the presence of 5% stannous chloride or 2N barium hydroxide, at which it is destroyed slightly (up to 10%). Minimal degradation occurs in the presence of thioglycolic acid and pre-hydrolysed starch. (Alkaline hydrolysis destroys serine, threonine, arginine, and cysteine). The hydrolyzate after neutralization with a mixture of citric and hydrochloric acids is immediately analyzed (in order to avoid gel formation) on an amino acid analyzer. As for the numerous chemical methods for the determination of tryptophan, they are usually poorly reproducible in food products and therefore their use is not recommended.
For meat products, an additional necessary amino acid is hydroxyproline, which characterizes the amount of connective tissue proteins in meat. It can be determined by ion exchange chromatography using automatic analyzers or by chemical colorimetry. The method is based on the neutralization of the acid hydrolyzate to pH 6.0, the subsequent oxidation of hydroxyproline with a 1.4% solution of chloramine T (or chloramine B) in a mixture of propyl alcohol and buffer, colorimetric determination at 533 nm of the oxidation products of hydroxyproline after reaction with 10% - para-dimethylaminobenzaldehyde solution in a mixture of perchloric acid and propyl alcohol (1:2).
Due to the fact that tyrosine, phenylalanine and proline can be partially oxidized in the presence of oxygen, standard acid hydrolysis is recommended to be carried out in a nitrogen atmosphere. A number of amino acids, including leucine, isoleucine, and valine, require longer acid hydrolysis, up to 72 hours, for their complete isolation from proteins. In biochemistry, when analyzing proteins, parallel samples are hydrolyzed for 24, 48, 72, and 96 hours.
For accurate quantitative determination of all amino acids, 5 different hydrolysis is required, which greatly lengthens the determination. Usually, 1–2 hydrolysis is carried out (standard with hydrochloric acid and with preliminary oxidation with performic acid).
To avoid loss of amino acids, the removal of excess acid during acid hydrolysis should be carried out immediately by repeated evaporation in a vacuum desiccator with the addition of distilled water.
When the analyzer is operating correctly, ion exchange columns operate without changing the resin for quite a long time. However, if the samples contain appreciable amounts of dyes and lipids, the column will quickly become clogged, and multiple regenerations, sometimes with repacking of the column, are required to restore its separating abilities. Therefore, for products containing more than 5% fat, it is recommended to remove lipids by extraction beforehand. Table 2.3 shows the conditions for sample preparation of basic food products in the analysis of the amino acid composition.
Table 2.3. – Conditions for preparing food samples for analysis
| Product | Lipid removal method | Weight ratio of protein: HCl (6M) |
| Protein concentrates (isolates) | Not required | 1:200 |
| Meat, fish, canned meat and fish, by-products) | Extraction with 10 times the amount of diethyl ether 3-4 times or with a mixture of ethanol-chloroform (1:2) 10 times the amount 2 times | 1:250 |
| Milk and dairy products | Extraction 10-fold to a sample amount with a mixture of ethanol-chloroform (1:2) 2 times | 1:1000 |
| Grain and grain products | Not required | 1:1000 |
| herbal products | Not required | 1:500 |
| Meat and vegetable and fish and vegetable products | Extraction with 10 times the amount of diethyl ether 3-4 times; with a mixture of ethanol-chloroform (1:2) 10-fold amount to a sample 2 times | 1:1000 |
| Egg, egg products | Extraction with a mixture of ethanol-chloroform (1:2), 10-fold amount to a sample 2 times | 1:200 |
Test questions:
1. Define the concept of "proteins".
2. What groups are proteins divided into according to their functions in the body?
3. What is the role of proteins in human nutrition?
6. What essential amino acids do you know and which amino acids can become essential?
7. How is the content of total nitrogen in food products determined?
8. How is the amino acid composition of proteins determined?
9. What methods for determining amino acids do you know?
§ 2.4. Carbohydrates
Carbohydrates are widely present in plants and animals, where they perform both structural and metabolic functions. In plants, in the process of photosynthesis, glucose is synthesized from carbon dioxide and water, which is then stored in the form of starch or converted into cellulose, the structural basis of plants. Animals are able to synthesize a range of carbohydrates from fats and proteins, but most carbohydrates come from plant foods.
