Chapter XV
, he return to this chapter dealing with the proteins for an illustrative study of the applications of the principles presented there.
THE CLASSIFICATION OF THE PROTEINS
Formerly, the classification of proteins was based almost wholly upon their solubility and coagulation reactions. More recently, since their products of hydrolysis have been extensively studied, their classification has been modified, in attempts to make it correspond as closely as possible to their chemical constitution and physical properties. As knowledge of these matters progresses, the schemes of classification change. On that account, no one definite scheme is universally used. For example, the English system varies considerably from the one commonly used by American biochemists, which is the one presented below.
The proteins are divided into three main classes, as follows:
(1) Simple proteins, which yield only amino-acids when hydrolyzed.
(2) Conjugated proteins, compounds of proteins with some other non-protein group.
(3) Derived proteins, decomposition products of simple proteins.
The first two of these classes comprise all the natural proteins; while the third includes the artificial polypeptides and proteins which have been modified by reagents.
These major classes are further subdivided into the following sub-classes, which depend in part upon the solubilities of the individual proteins, and in part upon the nature of their products of hydrolysis:
1. _The Simple Proteins_
A. Albumins--soluble in water and dilute salt solutions, coagulated by heat.
B. Globulins--insoluble in water, soluble in dilute salt solutions, coagulated by heat.
C. Glutelins--insoluble in water or dilute salt solutions, soluble in dilute acids or alkalies, coagulated by heat.
D. Prolamins--insoluble in water, etc., soluble in 80 per cent alcohol.
E. Histones--soluble in water, insoluble in ammonia, not coagulated by heat.
F. Protamines--soluble in water and ammonia, not coagulated by heat, yielding large proportions of diamino-acids on hydrolysis.
G. Albuminoids--insoluble in water, salt solutions, acids, or alkalies.
2. _Conjugated Proteins_
A. Chromoproteins--compounds of proteins with pigments.
B. Glucoproteins--compounds of proteins with carbohydrates.
C. Phosphoproteins--proteins of the cytoplasm, containing phosphoric acid.
D. Nucleoproteins--proteins of the nucleus, containing nucleic acids.
E. Lecithoproteins--compounds of proteins with phospholipins.
F. Lipoproteins--compounds of proteins with fats, existence in nature doubtful, artificial forms easily prepared.
3. _Derived Proteins_
A. Primary protein derivatives.
_a._ Proteans--first products of hydrolysis, insoluble in water.
_b._ Metaproteins--result from further action of acids or alkalies, soluble in weak acids and alkalies, but insoluble in dilute salt solutions.
_c._ Coagulated proteins--insoluble forms produced by the
## action of heat or alcohol.
B. Secondary protein derivatives.
_a._ Proteoses--products of hydrolysis, soluble in water, not coagulated by heat, precipitated by saturation of solution with ammonium sulfate.
_b._ Peptones--products of further hydrolysis soluble in water, not coagulated by heat, not precipitated by ammonium sulfate, give biuret reaction.
_c._ Peptides--individual amino-acids, or poly-peptides, may or may not give biuret reaction.
The plant proteins which have been investigated, thus far, fall into these groups as follows:
1A. _Albumins_
Leucosin, found in the seeds of wheat, rye, and barley.
Legumelin, " " pea, horse-bean, vetch, soy-bean, lentil, cowpea, adzuki-bean.
Phaselin, " " kidney-bean.
Ricin, " " castor-bean.
1B. _Globulins_
Legumin, found in the seeds of pea, horse-bean, lentil and vetch.
Vignin, " " cowpea.
Glycinin, " " soy-bean.
Phaseolin, " " beans (_Phaseolus spp._)
Conglutin, " " lupines.
Vicilin, " " pea, horse-bean, lentil.
Corylin, " " hazel nut.
Amandin, " nuts of almond and peach.
Juglansin, " seeds of walnut and butternut.
Excelsin, " " Brazil nut.
Edestin, " hemp seed.
Avenalin, " oats.
Maysin, " corn.
Castanin, " the seeds of European chestnut.
Tuberin, " potato tubers.
And, crystalline globulins found in the seeds of flax, squash, castor-bean, sesame, cotton, sunflower, radish, rape, mustard, and in cocoanuts, candlenuts, and peanuts.
1C. _Glutelins_
Glutenin, found in the seeds of wheat.
Oryzenin, " " rice.
1D. _Prolamins_
Gliadin, found in the seeds of rye, wheat, with glutenin forms "gluten."
Hordein, " " barley.
Zein, " " corn.
1E-1G. _Histones, Protamines and Albuminoids._--So far as is now known, no representatives of these classes are found in plants.
2. Conjugated Proteins.--There is no conclusive evidence of the existence in plants of any of the conjugated proteins, other than the nucleoproteins and the chromoproteins, the composition and properties of which have been discussed in previous chapters. The nucleoproteins undoubtedly occur in the embryos of many, if not all, seeds.
3. Derived Proteins.--Representatives of the various types of derived proteins are undoubtedly found as temporary intermediate products in plants, both as products of hydrolysis produced during the germination of seeds and as intermediate forms in the synthesis of proteins. So far as is known, however, they do not occur as permanent forms in any plant tissues. They have been prepared in large numbers and quantities, by the hydrolysis of the natural proteins and the artificial synthesis of polypeptides.
In the present state of our knowledge concerning the functioning of the proteins, no significance in the physiology of plant life, or metabolism, is to be attached to the particular type of protein material which it contains, at least so far as the simple proteins of the cytoplasm are concerned.
DIFFERENCES BETWEEN PLANT AND ANIMAL PROTEINS
A much larger variety of protein materials is found in animal tissues than in plants. This is undoubtedly because different animal organs perform so much more varied physiological functions than do those of plants. Three groups of simple proteins, the histones, the protamines, and the albuminoids, which are quite common in animal tissues, are entirely unknown in plants. Further, conjugated proteins of greater complexity and more varied structure are found in animal tissues, especially in the brain, nerve-cells, etc., than in plants.
Plant proteins, in general, usually contain larger proportions of proline and of glutamic acid than are found in animal proteins; also more arginine than is found in any of the animal proteins except the protamines, which contain as high as 85 per cent of this amino-acid.
Of the twenty-five plant proteins which have thus far been hydrolyzed and studied from this standpoint, all contained leucine, proline, phenylalanine, aspartic acid, glutamic acid, tyrosine, histidine, and arginine; two gave no glycine; two others, no alanine; four contained no lysine; and one, no tryptophane. Zein, the principal protein of corn contains no glycine, lysine, or tryptophane. It is not sufficient to support animal life and promote growth, if used as an exclusive source for protein for food.
THE EXTRACTION OF PROTEINS FROM PLANT TISSUES
Since proteins are indiffusible, it is essential that the cell-walls of the tissue shall be thoroughly ruptured as the first step in any process for the extraction of these compounds from plant tissues. This is usually accomplished by grinding the material as finely as possible, preferably with the addition of sharp quartz sand, or broken glass, to aid in the tearing of the cell-wall material.
The solvent to be used in extracting the proteins from this finely ground material depends upon the nature and solubility of the proteins which are present, and also upon whether it is desired to separate the proteins which may be present in the plant, during the process of the extraction. A glance at the scheme of classification of the proteins will show the following solubilities which serve as a guide to the procedure to be followed: (_a_) proteoses, albumins, and some globulins may be extracted with water; (_b_) globulins and most of the water-soluble proteins may be extracted by using a 10 per cent solution of common salt; (_c_) prolamines are extracted by 70-90 per cent alcohol; glutelins and prolamins dissolve in dilute acids or dilute alkali.
A common procedure is to extract groups (_a_) and (_b_), using a 10 per cent salt solution as the solvent, and then to separate the albumins, globulins, etc., from this solution by suitable precipitants; then to treat the material with 80 per cent alcohol, to extract the prolamines; and finally with dilute alkali, to extract the glutelins. The dissolved proteins in each extract can be subsequently purified by dialysis, precipitation, etc. The insoluble proteins can be studied only after removing the other materials associated with them in the tissue, by suitable mechanical or chemical means.
THE SYNTHESIS OF PROTEINS IN PLANTS
The synthesis of proteins in plants is not a process of photosynthesis, as it can take place in the dark and in the absence of chlorophyll, or any other energy-absorbing pigment. However, protein-formation normally takes place in conjunction with carbohydrate-formation. The carbon, hydrogen, and oxygen necessary for protein synthesis are undoubtedly obtained from carbohydrates. The nitrogen and sulfur come from the salts absorbed from the soil through the roots and brought to the active cells in the sap. Atmospheric nitrogen cannot be used by plants for this purpose, except in the case of certain bacteria and other low plants, notably the bacteria which live in symbiosis with the legumes in the nodules on the roots of the host plants. In general, the sulfur must come in the form of sulfates and the nitrogen in the form of nitrates; although many plants can make use of ammonia for protein-formation. Presumably, the nitrate nitrogen must be reduced in the plant to nitrites, and then to ammonia form, in order to enter the amino-arrangement required for the greater proportion of the protein nitrogen.
The mechanism by which ammonia nitrogen becomes amino-acids in the plant is not understood. Artificial syntheses of amino-acids, by the action of ammonia upon glyoxylic acid and sorbic acid, both of which occur in plants and may be obtained by the oxidation of simple sugars, have been accomplished, and it seems probable that similar reactions in the plant protoplasm may give rise to the various amino-acids which unite together to form proteins. Nothing is known, however, of the process by which the more complicated closed-ring amino-acid compounds, such as proline, histidine, or tryptophane, are synthetized.
The condensation of amino-acids into proteins, or the reverse decomposition, is very readily accomplished in all living protoplasm, under the influence of special protein-attacking enzymes, which are almost universally present in the cytoplasm. These reactions in connection with the proteins are similar to the easy transformation of sugars to starches, and _vice versa_, under the action of the corresponding carbohydrate-attacking enzymes.
PHYSIOLOGICAL USES OF PROTEINS
There can be no doubt that the all-important rôle of proteins, in either plant or animal tissue, is to furnish the colloidal protoplasmic material in which the vital phenomena take place. Their occurrence in seeds, and other storage organs, is, of course, in order to provide the protoplasm-forming material for the young seedling plant.
They are, moreover, the source for the material which goes into some of the secretion groups of organic compounds; as they are easily broken down by various agents of decomposition into nitrogen-free alcohols, aldehydes, and acids, which produce the essential oils, pigments, etc.
Much, if not all, of their physiological activity is due to their colloidal nature, the importance and effects of which will be more apparent after the chapters dealing with the colloidal condition of matter and with the physical chemistry of protoplasm have been studied.
REFERENCES
ABDERHALDEN, E.--"Neuere Ergebnisse auf dem Gebiete der Speziellen Eiweisschemie," 128 pages, Jena, 1909.
FISCHER, E.--"Untersuchungen über Aminosäuren, Polypeptide, und Proteine, 1899-1906," 770 pages, Berlin, 1906.
MANN, G.--"Chemistry of the Proteids," 606 pages, London, 1906.
OSBORNE, T. B.--"The Vegetable Proteins," 138 pages, _Monographs_ on Biochemistry, London, 1909.
PLIMMER, R. H. A.--"The Chemical Constitution of the Proteins, Part I, Analysis," 188 pages; and "Part II, Synthesis, etc." 107 pages, _Monographs_ on Biochemistry, London, 1917. (3d ed.).
ROBERTSON, T. B.--"The Physical Chemistry of the Proteins," 477 pages, New York, 1918.
SCHRYBER, S. B.--"The General Characters of the Proteins," 86 pages, _Monographs_ on Biochemistry, London, 1909.
UNDERHILL, F. P.--"The Physiology of the Amino-acids," 169 pages, 13 figs. 1 plate. Yale University Press, 1915.
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