Proteins

Classification of Proteins.

The system of classification adopted by the American Society of Biochemists is in general use and is closely followed in Great Britain ; where the British Physio logical Society uses a different name this is indicated by (B). The three main groups are those of the simple, conjugated and derived proteins, as follows : I. Simple Proteins: naturally occurring proteins formed from amino-acids only.

Albumins. Coagulable by heat, soluble in water and dilute salt solutions ; e.g., serum albumin, egg albumin and lactalbumin.

Globulins. Coagulable by heat, soluble in dilute salt solutions and dilute solutions of acids and alkalis. Generally insoluble in water, but a few ("pseudo-globulins") are soluble in water; e.g., serum globulin of the blood and myosin, the chief protein of meat.

Glutelins. Found in cereals. Heat coagulable, insoluble in neutral solvents, but soluble in dilute acids and alkalis.

Prolamines (Gliadins [B] ). Also found in cereals, but dis tinguished from the glutelins by their solubility in 75% alcohol; e.g., zein from Indian corn (Zea inais), and gliadin from wheat.

Albuminoids (Scleroproteins [B] ). Found in the skeletal and connective tissues of animals. They are characterized by their insolubility in most reagents; e.g., keratin of hoofs and hair, elastin of yellow elastic tissue, collagen (the anhydride of gelatin) from tendons and white connective tissue.

Protamines. Basic proteins which are formed from a few amino acids only and these mainly the basic amino-acids. They are found in the heads of ripe spermatozoa and in ova.

Histones. Similar to the protamines, but less rich in the basic amino-acids. They are not so basic as the protamines. They are present in unripe spermatozoa, in the red blood corpuscles and in the lymphoid tissues of the body.

II. Conjugated Proteins: proteins joined to a non-protein or prosthetic group.

Cliromo-proteins. The prosthetic group is coloured, e.g., haemoglobins of vertebrate blood, and haemocyanin of inverte brate blood, both of which are connected with respiration.

Glycoproteins (Glucoproteins [B]). The prosthetic group con tains a carbohydrate radical; e.g., mucin and mucoids.

Nucleoproteins. The prosthetic group is nucleic acid, which con tains phosphorus, purine bases, pyrimidine bases and a sugar group.

They are widely distributed in animal and vegetable cells, es pecially in nuclei.

Phosphoproteins. The prosthetic group is phosphoric acid. They are distinguished from the nucleoproteins in that they do not contain purine or pyrimidine bases; e.g., casein of milk and vitellin of egg-yolk.

III. Derived Proteins : these are the decomposition products of any of the above, produced by boiling in water or by hydrolysis with dilute acids, alkalis or by the action of enzymes; the arti ficially synthesized polypeptides are also included.

Metaproteins. Insoluble in water or dilute salt solutions. Soluble in dilute acids or alkalis. Coagulated by heat if they are in suspension.

Protcoses. Soluble in water, not coagulated by heat. Precipi tated by saturation with ammonium sulphate.

Peptones. Similar to the proteoses, but not precipitated by saturation with ammonium sulphate. They are relatively simple proteins and are much more diffusible.

Peptides. Simple peptones, generally of known constitution.

Reactions and Properties.

In the foregoing scheme of classification physical properties are mainly used for the char acterization of the proteins. Certain colour reactions can be obtained arising from the chemical structure of the protein mole cule and due either to the presence of certain amino-groupings in the protein molecule or to the special linkages formed by their union with one another. The more important of these colour re actions are, (I) the xanthoproteic, indicating such aromatic groups as tyrosine and tryptophane; (2) the glyoxylic, due to the pres ence of tryptophane; (3) Millon's, due to tyrosine; (4) the sul phur test, due to cystine ; and (5) Molisch's test for a carbohydrate complex. It follows that a protein like gelatin, which does not contain either tryptophane, tyrosine or cystine, fails to give the glyoxylic, Millon's or sulphur tests. Zein does not contain trypto phane, but does contain tyrosine ; hence it gives Millon's test but not the glyoxylic reaction. The biuret colour reaction, however, is of a different character, in that it is not given by any amino-acid but is given by all proteins, owing to the particular way in which the amino-acids are linked together.