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
    1. Albumins—soluble in water and dilute salt solutions, coagulated by heat.
    2. Globulins—insoluble in water, soluble in dilute salt solutions, coagulated by heat.
    3. Glutelins—insoluble in water or dilute salt solutions, soluble in dilute acids or alkalies, coagulated by heat.
    4. Prolamins—insoluble in water, etc., soluble in 80 per cent alcohol.
    5. Histones—soluble in water, insoluble in ammonia, not coagulated by heat.
    6. Protamines—soluble in water and ammonia, not coagulated by heat, yielding large proportions of diamino-acids on hydrolysis.
    7. Albuminoids—insoluble in water, salt solutions, acids, or alkalies.

    1. Chromoproteins—compounds of proteins with pigments.
    2. Glucoproteins—compounds of proteins with carbohydrates.
    3. Phosphoproteins—proteins of the cytoplasm, containing phosphoric acid.
    4. Nucleoproteins—proteins of the nucleus, containing nucleic acids.
    5. Lecithoproteins—compounds of proteins with phospholipins.
    6. Lipoproteins—compounds of proteins with fats, existence in nature doubtful, artificial forms easily prepared.
  3. Conjugated Proteins
    1. Primary protein derivatives.
      1. Proteans—first products of hydrolysis, insoluble in water.
      2. Metaproteins—result from further action of acids or alkalies, soluble in weak acids and alkalies, but insoluble in dilute salt solutions.
      3. Coagulated proteins—insoluble forms produced by the action of heat or alcohol.
    2. Secondary protein derivatives.
      1. Proteoses—products of hydrolysis, soluble in water, not coagulated by heat, precipitated by saturation of solution with ammonium sulfate.
      2. Peptones—products of further hydrolysis soluble in water, not coagulated by heat, not precipitated by ammonium sulfate, give biuret reaction.
      3. Peptides—individual amino-acids, or poly-peptides, may or may not give biuret reaction.

  5. Derived Proteins

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," seeds of European chestnut.
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.