SSIFICATION 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 nucle