Proteins (or proteins, are high-molecular-weight)

Proteins, or proteins, are high-molecular-weight colloidal organic substances built from amino acid residues. Proteins, by their quantitative content in animal organisms, occupy one of the first places among the solid components of the body, and by their biological significance belong to the most important components of the organism, being a constant, absolutely necessary for life and irreplaceable component of the cells of both animals and plants; animal body fluids also contain proteins in greater or lesser quantities. Due to their importance for the body, these substances received the name proteins (from Greek protos - first, main); they are called proteins because of their presence in egg white. Proteins are mostly obtained in the form of colorless or yellowish powders, but some of them are intensely colored (red hemoglobin, blue oxyhemocyanin). In nature, proteins are found in solutions, sometimes syrupy (egg white), in the form of colloidal masses of various consistencies: from semi-liquid to solid (connective tissue, nails, hair), usually in an amorphous form, although in rare cases they are also found in crystals (aleurone crystals in plant seeds, yolk plates, very rarely in epithelial cells, in myeloma). Oxyhemoglobin can be easily obtained in crystals. Hofmeister succeeded, for the first time in 1889, in crystallizing some other proteins (especially albumins); for this purpose, protein solutions are saturated with half ammonium sulfate and diluted H₂SO₄ is added to the filtrate until turbidity appears. Proteins have neither odor nor taste. In water, proteins swell, which is of great importance for the morphological structure and vital processes of the body; from the addition of acids or alkalis, the swelling of proteins increases to a certain limit, then decreases again. Proteins are important water accumulators in the body. After swelling, some proteins (albumins) dissolve in water, but, according to their colloidal nature, they form not true, but false colloidal solutions; other proteins (globulins) are insoluble in water, but dissolve in solutions of medium salts of 5-15% concentration and in diluted acids and alkalis; there are proteins (acid and alkaline albuminates) insoluble in water or medium salts, but soluble in diluted acids and alkalis; there are proteins (elastin) that are insoluble in any solvents and if they dissolve in strong acids, they decompose in the process. In alcohol (with the exception of some plant proteins), in ether, chloroform, benzene, etc., proteins are insoluble. Proteins diffuse and dialyze very slowly, so practically they can be considered incapable of dialysis, which is used to free proteins from crystalloid impurities. Proteins are retained during ultrafiltration. Protein solutions opalesce and exhibit Brownian movement and the Tyndall phenomenon (see Tyndall phenomenon). Proteins significantly reduce the surface tension (see) of water; their solutions foam strongly when shaken. Solutions of the same protein under different conditions and even different particles of protein in the same solution show different degrees of dispersion. In solutions, protein particles have a negative or positive electric charge depending on the reaction of the solution and therefore exhibit electrophoresis phenomena. Such a reaction, in which protein particles do not move in an electric field, is called the isoelectric point (see); for most animal proteins it lies in the acidic region (pH = approximately 5), but for globin it corresponds to pH = 8.1, for many plant proteins pH = 7 to 10. Like-charged protein particles repel each other, while removing their electrical charges with oppositely charged particles or ions of other substances causes the merging of dispersed particles into larger ones, precipitating as aggregates - the theory of Hardy and Bredig. Such precipitation of proteins from their solutions can be caused by the addition of electrolytes with their electrically charged ions or by the addition of colloids whose particles carry electrical charges opposite to those of the precipitated protein: mutual precipitation of colloids, which has importance in biology and technology. Proteins can serve as protective colloids for suspensions. Being amphoteric substances (see), proteins in their solutions are ionized, but very weakly. The degree of ionization increases with the addition of acids, which increases the concentration of protein cations, or alkalis, which causes an increase in the concentration of its anions; therefore, the properties of protein solutions can change very strongly depending on the reaction of the solution. At the isoelectric point, the ionization of proteins and, along with it, the viscosity of their solutions reach a minimum, while precipitability by alcohol or heating reaches a maximum. Protein ions, present in greater or lesser quantities in a highly hydrated state, significantly increase the viscosity of water. In aqueous solutions of gelatin, the viscosity is so great that these solutions turn into jelly when cooled, again liquefying when heated. Surface tension phenomena are strongly developed in protein solutions due to the enormous surface formed by the sum of the surfaces of a huge number of extremely finely dispersed particles. The strong development of surface tension phenomena also determines the pronounced ability of proteins to adsorb various, especially colloidal, substances present in solution along with proteins, and in turn to be adsorbed on the surface of other colloids. This property of proteins significantly complicates obtaining them in a pure state. The separation of protein mixtures is based on their crystallization, fractional precipitation with salts, alcohol or acetone, precipitation of some proteins during dialysis, and ultrafiltration. When calcined, proteins char, give off the smell of burnt hair, and form scorched oily products. When burned in a calorimeter, 1 g of protein gives 4.16-5.99 calories; when 1 g of protein is burned in an animal organism on a mixed diet, 4.1 calories are formed. Reactions to proteins are very numerous, but there is not one of them that would be specific only to proteins. Precipitation reactions: 1. Protein solutions coagulate when boiled, if their reaction is weakly acidic or neutral; alkaline or strongly acidic solutions (with the exception of nitric, trichloroacetic, and sulfosalicylic acids) do not coagulate due to the formation of alkaline or acidic albuminates. The process of protein coagulation from heating is irreversible, the protein clot does not go back into solution upon cooling. The coagulation temperature of different proteins is not the same (40-85°), but even for the same protein it changes depending on the content of other substances in the solution. Protein solutions completely freed from mineral salts do not coagulate when boiled. 2. Proteins are precipitated by concentrated mineral acids, with the exception of orthophosphoric acid, and an excess of acid, with the exception of nitric acid, dissolves the precipitate again. 3. Proteins, with the exception of peptones, are precipitated when their solutions are saturated with ammonium sulfate. When saturated with other medium salts, globulins precipitate, while albumins do not precipitate, but albumins also settle when saturated with a medium salt with the addition of acetic or hydrochloric acid or when saturated with some mixtures of two medium salts. Precipitation by medium salts is called salting out; this process is reversible, and the resulting precipitates dissolve again upon the addition of water. During salting out of proteins, cations of the salt act as a precipitant, while anions prevent precipitation, and the strength of action is different for different cations and anions (Hofmeister series); therefore, different salts have very different precipitating effects. 4. Proteins are precipitated by salts of heavy metals, in which case the cation of the salt is of almost exclusive importance; for precipitation, a small amount of salt is sufficient, but an excess of it often dissolves the precipitate again; the precipitation process is irreversible, and the composition of the resulting precipitate is not constant. 5. Alcohol, ether, chloroform, and acetone precipitate protein solutions. The resulting precipitates are initially soluble in water, but upon prolonged standing with the precipitant, proteins denature, losing their solubility in water; this is the basis of one of the histological methods of fixation. 6. Proteins give alkaloid reactions, i.e., they are precipitated by the same reagents as alkaloids: potassium ferrocyanide with the addition of acetic acid, tannin, picric acid, phosphotungstic and phosphomolybdic acids, solutions of mercury iodide acidified with hydrochloric acid, or bismuth iodide in potassium iodide. These reactions depend on the presence of heterocyclic and diamino groups. Color reactions: 1. Iodine colors proteins yellow-brown. 2. Millon's reagent colors proteins, better when heated, purplish-red. These two reactions serve as microchemical reactions for proteins. 3. The xanthoprotein reaction: yellow coloring when proteins are heated with nitric acid, which turns orange upon alkalization. 4. The biuret reaction: violet coloring from the addition of caustic alkali and a small amount of copper sulfate. Peptones give a pink-red coloring in this case. 5.

Adamkievich reaction: violet-red coloring upon boiling with glacial acetic acid and a few drops of sulfuric acid. 6. Liebermann reaction: blue-violet coloring upon heating with fuming HCl. 7. Raspail reaction: cherry-red coloring upon careful mixing of a protein solution containing a very small amount of cane sugar with concentrated sulfuric acid. 8. Molisch reaction: violet coloring from adding concentrated sulfuric acid to a solution of P. mixed with a small amount of alpha-naphthol. If naphthol is replaced with thymol, the coloring becomes carmine-red. 9. Abderhalden reaction (ninhydrin): blue coloring upon boiling with ninhydrin. In addition to those mentioned, proteins give many other color reactions. Chemical structure. The elementary composition of various P. varies within the following limits: C-49.5-55.0%, H-6.4-7.4%, N-9.6-18.8%, O-19.7-34.2%, S-0.3%-5.0%. In some P. there is also P (phosphoproteins), Fe (hemoglobin), I (thyroglobulin), Cu (hemocyanin). The molecular weight of proteins is very high and is usually estimated at 14,000-16,000, even up to 34,500. Corresponding to the high molecular weight of proteins, the osmotic pressure of their solutions is very small, constituting only a few mm of mercury column. To clarify the chemical structure of proteins, all methods commonly used in organic chemistry have been tried; the most valuable results were obtained from the hydrolysis of P. by acids or enzymes of digestive juices. During hydrolysis, P. pass successively through the stages of albumoses, peptones, and then break down into the final cleavage products, mainly amino acids. P. are subjected to the same cleavage during digestion in the digestive tract and during breakdown in the cells of animals and plants (on the putrefactive breakdown of P. see Putrefaction). For the first time, one of the final products of protein hydrolysis was obtained in 1820 by Braconnot in the form of crystalline glue sugar, which later received the name glycine. Through the research of a number of authors: Ritthausen, Schutzenberger, Drechsel, Hedin, Kossel, Emil Fischer, Abderhalden, Hopkins (Ritthausen, Schutzenberger, Drechsel, Hedin, Kossel, Fischer, Abderhalden, Hopkins) and others, it was established that the main mass of protein hydrolysis products consists precisely of amino acids (see); in total, 25 different amino acids have been isolated from the hydrolysis of various proteins, among which there are representatives of all three series of organic chemistry: alicyclic, carbocyclic, and heterocyclic. In the separation of amino acid mixtures, the method of E. Fischer (1901), based on fractional distillation in vacuum of the complex esters of amino acids, and the method of Drechsel (1889) - the isolation of hexonic bases (see) by means of precipitation with phosphotungstic acid, are of great importance. In addition to amino acids, glucosamine (see) and its stereoisomers, ammonia, and melanoidin substances have been found in the hydrolysis products of P. Those amino acids of the protein molecule that have an asymmetric carbon atom (see Asymmetric carbon) are not racemic but optically active. Accordingly, all proteins are optically active, with almost all rotating the plane of polarization of light rays to the left and only a few of them (H, nucleoproteins) having right-handed rotation. Artificial racemization of proteins, as found by Dakin and Dudley, significantly reduces their biological value as a nutritional material not only for animals but also for microbes. All amino acids participating in the structure of P. are alpha-isomers, and similarly glucosamine contains the amide group in the alpha-position to the aldehyde. At present, it is known in what form the ³/« of the nitrogen contained in it is present in the protein molecule; whether the remaining ¹/_t also contains as yet unknown protein breakdown products cannot yet be said. Individual types of proteins differ from each other not only in the qualitative composition of the breakdown products formed from them, but to an even greater extent in the quantitative content of individual amino acids. The general scheme of the very complex structure of P. has been largely clarified thanks to the classical works of E. Fischer, begun in 1900, but much in this question still remains unknown. The basis of the structure of P. is the polypeptide grouping of the general type: R.CH(NH₂). .CO-HN.CH(R').COOH, where the number of residues of individual amino acids can be significantly more than two. Fischer developed general methods for the synthesis of peptides and showed that from P., with their careful hydrolysis, it is indeed possible to obtain peptides identical to the corresponding artificially synthesized peptides. Like peptones, peptides have a bitter taste, give the biuret reaction, alkaloid reactions, and upon boiling with acids, they break down with the formation of amino acids. The fact that artificially synthesized peptides from residues of naturally occurring amino acids can be digested by trypsin and erepsin, just as proteins, albumoses, and peptones are digested by these enzymes, is especially important for proving the presence of peptide bonds in P., since each enzyme can break only bonds of a strictly specific nature. In the cleavage of peptide bonds, both artificial and biological, some amino acids (for example, tryptophan, tyrosine) are easily cleaved off, others (for example, glycine) with much greater difficulty. The presence of peptide groupings in P. is also supported by the presence in organs of enzymes capable of breaking down polypeptides. The recognition of peptide bonds in P. is well consistent with the fact that proteins are mostly amphoteric; some P. are rich in residues of acid-reacting dibasic amino acids and then have a more pronounced acidic character; conversely, in the case of a predominance of residues of base-reacting hexonic bases, the basic properties become more pronounced in the amphoteric character of P. However, peptide groupings are not the only way of linking N in proteins. Part of N, which is in a weakly bound state, i.e., easily, even at ordinary temperature, cleaved off as ammonia under the action of acids or alkalis, is contained in the form of acid amides, for example, glutamine HOOO.CH(NH₂).CH₂.CH₂.CO.NH₂. Similarly to N, part of S is in P. in a weakly bound state; the ways of its bonding are not yet sufficiently clarified. A third way of linking N in P. is the guanidine-type grouping HN;C(NH₂)„ in the form of arginine residues (see) HN:C(NH₂).NH. .CH₂.CH₂.CH₂.CH(NH₂).COOH, where the H atoms of the amide groups can be replaced by residues of amino acids, forming peptide groupings. In recent years, the study of the question of the chemical structure of P. has taken a new direction, consisting in the assumption of the presence in P., in addition to open peptide chains, also their anhydride cyclic groupings, so-called diketopiperazines; for example, the anhydride of glycyl-alanine NH₂.CH₂.CO-NH.CH(CH₃).COOH is methyl-diketopiperazine CH₂-CO >NH . CO-CH.CH₃. The formation of diketopiperazines during protein hydrolysis has been proven, but the question of whether such rings are present in P. in a pre-formed form or are formed as artificial products of processing cannot yet be considered finally clarified. Systematic and comprehensive research by Abderhalden with great probability speaks for the pre-formed presence of diketopiperazine rings in P., which is consistent with X-ray photographs, for example, of silk. According to the new view, the P. molecule is built not only from long polypeptide chains but also from relatively small cycles, which are associated with each other by means of affinity side units into very large complexes and connected with peptides and amino acids. This view is analogous to the modern concept of the structure of polysaccharides and allows for the possibility of changes in the physicochemical properties of P. already with the simple opening or closing of diketopiperazine rings or their isomerization. The biological transition of one type of P. into another and the initial stages of enzymatic breakdown of P. are possible with this view without deep breakdown of the protein molecule, simply by cleaving off one or another associated complex of atoms. It is possible that digestive enzymes find in P. different places for their action: one enzyme can break the association bonds of individual complexes, another - break diketopiperazine rings, a third - hydrolyze peptide bonds. If the general scheme of the structure of P. or M. has been clarified, the details of this structure of P. remain unknown because the same amino acids can give various combinations with each other depending on the order of connection of amino acids to each other. If one imagines that the P. molecule contains residues of only 12 amino acids, each taken only in one molecule, then there can be about 1/2 billion different combinations, i.e., about 7a billion different proteins of polypeptide type structure.

It is therefore not at all surprising that not only animal proteins differ from plant proteins, but even, for example, blood proteins differ in various animals, as shown by biological reactions; these subtle differences can be inherited, participating in the creation of specific biological hereditary traits. Classification of Proteins. Since the differences in chemical structure of individual types of proteins are still very little studied, the classification of proteins cannot be based on the principle of classification of organic chemistry, i.e., on chemical structure. Various classifications of proteins are based almost exclusively on their external characteristics and are not entirely satisfactory. Proteins can be divided into the following major groups: I. Simple (as opposed to complex), primary (as opposed to modified) proteins, or proteins. II. Proteins modified by the action of various agents. III. Complex proteins, or proteids, which break down into a simple protein and some non-protein body, so-called prosthetic (attached). IV. Albuminoids, which have only some similarity to proteins but differ sharply from them in many ways. Group I is divided into albumins and globulins (see). Both globulins and albumins coagulate when their solutions are boiled. Group II contains acid and alkaline albuminates, albumoses, peptones (see corresponding words), proteins modified by heat, alcohol, etc., by coagulating protein enzymes (fibrin, paracasein, coagulases, plastins), globin formed during the breakdown of hemoglobin, histone obtained from the breakdown of nucleohistone. Globin and histone are clearly alkaline proteins. Group III, whose representatives are acidic proteins, is divided into subgroups: a) phosphoproteins, upon the breakdown of which phosphoric acid is obtained and which are divided into nucleoproteins (e.g., nucleohistone), giving purine bases upon breakdown, and nucleoalbumins (casein, vitellin), not giving these bases; proteins of this subgroup have a close relationship to the processes of cell division and nutrition of young animal and plant organisms; b) chromoproteins (hemoglobin, hemocyanin), which carry the respiratory function in animal organisms and upon breakdown give a protein and a colored complex of atoms; c) glycoproteins, upon the breakdown of which a large amount of amide derivatives of one or another carbohydrate is obtained and which differ significantly from proteins in their properties (mucin, chondromucoid). Group IV includes proteins that are naturally denatured and serve in the organism as an insoluble protective sheath or framework (keratin of the skin and epidermal formations, neurokeratin, collagen and elastin of connective tissue, spongein, fibroin of silk). Synthesis of Proteins. Given the extreme complexity of the chemical structure of proteins, which is far from being clarified, attempts at artificial synthesis of proteins are still premature. In principle, such a task should be considered solvable, and a profitable industrial synthesis of proteins would have enormous practical significance for humanity. In nature, the synthesis of proteins from inorganic compounds is carried out only by plants and occurs on a vast scale in the plant world. For the synthesis of proteins, most plants need to be supplied with combined compounds (see Nitrogen, cycle), mainly nitrate and ammonium salts, which are absorbed by plant roots from the soil and enter the leaves. In the leaves, from carbohydrates and N compounds, protein synthesis occurs, which, unlike the synthesis of carbohydrates by plants, can take place in the dark. The intermediate stages of protein synthesis in plants have not yet been clarified. Some authors admit the formation of formohydroxamic acid salt NO.CH:OH from nitrate salt and formic aldehyde, an intermediate product in the synthesis of carbohydrates in plants; others assume that the reduction of nitrates goes to hydrocyanic acid HCN, which is then consumed by the plant for protein synthesis; there is a view according to which hydrocyanic acid and ammonia give with plant aldehydes amino acids: CH₂:O + HCN + NH₃ → CH₂(NH₂).CN → CH₂.(NH₂).COOH, which would explain the presence in proteins of residues exclusively of α-isomers of amino acids. From the amino acids and their amides (asparagine and glutamine) synthesized in plants, the most complex protein molecule is then built (for the assimilation of free atmospheric N by some plants, see Nitrogen cycle). Animals need to be supplied with ready-made proteins in their food, which undergo gradual hydrolysis in the gastrointestinal tract as a result of the sequential action of digestive juice enzymes and are absorbed, mainly, in the form of products of deep breakdown (simple peptides and amino acids). This ensures the constancy of the chemical structure and physicochemical properties of the proteins of a given animal species, regardless of differences in the structure and properties of food proteins. With sufficiently deep breakdown, fragments of food protein molecules are formed, which have already lost the imprint of their origin that would make these proteins foreign to this organism; for example, thanks to this, the organism of a breast-fed child can synthesize its own proteins rather than deposit casein of milk in its cells. The possibility of synthesizing proteins from absorbed amino acids, the content of which in the blood actually increases during digestion, has been proven by the research of Loewi and other authors, especially Abderhalden, who showed that the food proteins necessary for the life of animals can be replaced by a mixture of products of the final breakdown of proteins, their amino acids, and for a long time (for example, for over 100 days). In the mixture of amino acids prescribed to replace food proteins, some amino acids (e.g., glycine, alanine) may be absent and therefore can be formed in the animal organism from other substances; however, the absence of other amino acids adversely affects the general condition of the animal, causing, for example, a cessation of growth or a loss of weight. Such amino acids, which include, for example, tryptophan, tyrosine, cystine, lysine, cannot be formed in the animal organism and need to be supplied with food in ready form or as proteins. If proteins contain residues of all amino acids necessary for the proper course of life processes and, therefore, a mixture of all necessary amino acids is obtained upon digestion, such proteins are called complete. Proteins in which residues of some necessary amino acids are absent or are present in insufficient quantities are called incomplete; gelatin, some plant proteins belong to the category of incomplete proteins. The former assumption that the reverse synthesis of proteins from amino acids takes place already in the intestinal wall, similar to the synthesis of fats from products of their digestion, or in the liver, similar to the synthesis of glycogen by it, is not sufficiently substantiated, and it is more probable to assume that the synthesis of proteins occurs in all cells of the organism from products of the deep breakdown of food proteins brought to them by the blood. Each organ's cells take from the mixture of these products delivered to them the amino acids they need and in the necessary relative quantities for building their proteins, specific to each organ. The synthesis of proteins is probably carried out with the assistance of enzymes; with respect to digestive enzymes, the research of Taylor and Robertson speaks in favor of the possibility of the reverse enzymatic synthesis of proteins from products of their hydrolytic breakdown. Not all the amount of these products formed is utilized for protein synthesis,-part of them, without being converted into proteins, quickly undergo further breakdown with the elimination of ammonia and the formation of urea and other nitrogenous substances excreted in the urine. The elimination of ammonia from amino acids apparently occurs already in the intestinal wall, and ammonia with the blood of the portal vein enters the liver, where it is used for the synthesis of urea. The resulting nitrogen-free part of the amino acid molecule undergoes complete oxidation to water and carbonic anhydride or is used for the formation of carbohydrates and fats in the organism. Breakdown of Proteins. in cells occurs under the influence of enzymes, as does the postmortem breakdown of proteins in the process of autolysis (see). The products of the intracellular breakdown of proteins during life are, in general, the same as the products of artificial hydrolysis of proteins. If only traces of amino acids can be found in the blood and organs of animals, far from corresponding to the amount of proteins broken down per day, this is because the amino acids, as they are formed in the organism, undergo further breakdown and oxidation. If the amino acids formed in the organism are protected from oxidation by synthesizing them with a hardly oxidizable substance introduced into the organism, then large amounts of the resulting synthesis product are excreted in the urine, as, for example, hippuric acid C₆H₅.CO-NH.CH₂.COOH, when fed with benzoic acid, which synthesizes in the organism with glycine; in germinating plant seeds, where oxidation processes proceed less intensively than in animals, there is a significant amount of amino acids formed as a result of the hydrolytic breakdown of protein reserves.

Amino acids formed in cells from proteins undergo various further transformations. A large part of them undergo deamination (see), at which time their N is excreted in the urine in the form of urea and other nitrogenous products of protein metabolism, and the nitrogen-free part is completely burned or utilized by the organism for the synthesis of carbohydrates and fats. Intermediate products in this type of breakdown of amino acids are keto acids, for example, pyruvic acid CH₃.CO.COOH from alanine CH₃.CH(NH₂).COOH. As research by Knoop, Embden and others has shown, from keto acids and ammonia in the body, amino acids can be formed again, so that the deamination reaction of amino acids is reversible [for example, CH₃.CO.COOH+NH₃→CH₃.CH(NH₂).COOH], and from the resulting amino acids, proteins can be synthesized. On what scale this reverse process takes place in the body is unknown, but in any case, it must be taken into account when assessing the amount of protein broken down in the body by the amount of nitrogenous breakdown products excreted from the body, since part of the N of broken down proteins may not be excreted, but be retained in the body and utilized for the new synthesis of proteins. The long-known fact that the introduction of carbohydrates conserves body proteins can find partial explanation in the fact that pyruvic acid, an intermediate product of the biological breakdown of grape sugar, by capturing ammonia formed during protein breakdown, can utilize it for the synthesis of proteins. Amino acids cleaved off during the intracellular breakdown of proteins not only go to the final breakdown products (urea, creatine, hippuric acid, etc.) excreted in urine and feces, but also serve as material for the synthesis of hormones (adrenaline, thyroxine, etc.). By simple cleavage of carbon dioxide from amino acids, bases are formed, often having a very strong effect on the body, such as histamine C₃H₃N₂.CH₂.CH₂.NH₂, formed from histidine C₃H₃N₂.CH₂.CH(NH₂).COOH, or tyramine HO.C₆H₄.CH₂.CH₂.NH₂ from tyrosine HO.C₆H₄.CH₂.CH(NH₂).COOH. Also in plants, amino acids released during the biological breakdown of proteins serve as material for the synthesis of alkaloids. Some amino acids of breaking down proteins (tyrosine, tryptophan) provide material for the formation of pigments. From glycine, glycocholic acid of bile is synthesized, from cystine, taurocholic acid. From nucleoproteins, during their vital breakdown as a result of a series of enzymatic processes, uric acid is formed. The colored complex of atoms of Hb breaking down in the body serves as material for the formation of bile and some other pigments. Under pathological conditions, the course of protein breakdown can undergo significant deviations from normal.-Ultimately, proteins in the body break down with the formation of carbon dioxide, water, nitrogenous products (mainly urea), sulfuric acid, phosphoric acid. The breakdown products of proteins are excreted into the atmosphere and soil, from where they enter plants and go into them for the new synthesis of complex constituent parts, which animals feed on.