PROTEINS are highly complex substances found in all living cells, in blood and in materials elaborated for the development of the young animal or plant, such as milk, eggs and seeds. The word is derived from the Greek 7rpCyros, meaning first, because proteins are apparently of first importance in respect of the phenomena which we call life, which can only exist in association with a material called protoplasm, which consists very largely of proteins. The composition of proteins varies somewhat. They all contain carbon, hydrogen, oxygen and nitrogen ; most of them contain sulphur, and some contain phosphorus. The average composition is, C 51%, 0 25%, N 16%, H 7%, S 0.4%, P 0.4%. They are formed by the union of a certain number of substances called amino-acids, of which about 20 have been discovered.
Amino-adds.—The living cells of the plant are able to form the amino-acids from relatively simple compounds, such as nitrates or ammonium salts, obtained from the soil. These amino-acids are then built up into the characteristic proteins of the particular plant. Animal cells are unable to form amino-acids from simple inorganic substances ; the amino-acids must consequently be sup plied in the food, in the form of plant proteins in the case of herbivorous animals, and animal proteins in the case of the car nivora. By the processes of digestion these food proteins are broken down into their constituent amino-acids, which are ab sorbed into the blood and are then available for building up the proteins of the tissues. It is probable that certain of the amino acids can be formed in the body from other amino-acids, but on the other hand some essential amino-acids cannot be formed thus, i.e., they must pre-exist in the protein of the food. The essential amino-acids are tryptophane, lysine, histidine, cystine, and pos sibly tyrosine and arginine. It follows that the nature of the amino-acids of which a protein is composed determines, to a certain extent, its value as a foodstuff for animals. Protein is disintegrated into its constituent amino-acids by boiling with dilute mineral acid, which converts tryptophane into a black substance called humin. Some proteins can be broken down by the
action of certain digestive ferments, notably trypsin.
The amino-acids isolated by breaking protein down into its constituents are substances which contain both an acid (car boxylic) group and a basic (amino-) group. They can be divided into classes depending on the number of these groups, as f ol lows :—(a) neutral amino-acids, containing one acid and one amino-group, e.g., glycine, alanine, leucine, tyrosine and cystine, (b) acid amino-acids, containing two acid and one amino-group, e.g., aspartic and glutaminic acids; (c) basic amino-acids, contain ing one acid and two amino-groups, e.g., arginine and lysine; (d) heterocyclic compounds, in which a ring compound of carbon and nitrogen is united to an amino-acid group, the two most important members being histidine and tryptophane. The amino-acids are united to one another by a condensation (or subtraction of the elements of water) between the carboxylic group of one compound and the amino-group of its neighbour : The compound formed by such an union (peptide linkage) is called a di-peptide.
Very long chains of amino-acids have been formed synthet ically, and the resulting compounds, called polypeptides, closely resemble the proteins in many ways. These peptides are split up into their constituent amino-acids by prolonged boiling with dilute mineral acids, the elements of water being added. This splitting or hydrolysis is also effected by two of the digestive enzymes found in the body, viz., trypsin of the pancreatic juice and erepsin of the intestinal juice. It is almost certain that there is at least one other type of union between the amino-acids, for pepsin, the enzyme of the gastric juice which digests proteins, is unable to split the peptide linkage, yet can break the highly complex food proteins into relatively simple compounds.