Protein Classification on Chemical Properties

Protein classification according to their chemical properties as well as their physical properties are done into three main groups—simple proteins, conjugated proteins and derived proteins; each of these groups is subdivided into a number of classes.

Protein Classification on Chemical Properties: Simple proteins

Proteins made up of amino acid only, are called simple proteins. These are further classified into the following seven types:—

a) Albumins : These are soluble in water, coagulated by heat and found in plants as well as animals. Examples—egg white (ovalbumin), serum albumin, lactalbumin of milk, legumelin of peas and leucosin of wheat.
b) Globulins: These are insoluble in pure water but soluble in dilute solution of salts and coagulated by heat. Examples—ovoglobulin of egg yolk, serum globulin, myosin of muscle, legtimin of peas, phaseolin of beans etc:
c) Glutelins: They are plant proteins, soluble in very dilute acid or alkali but insoluble in neutral solvents. e.g.—glutenin of wheat, oryzenin of rice etc
d) Prolamins: These are plant proteins containing much proline and hence so named. They are soluble in 70-80% alcohol but insoluble in absolute alcohol or water. e.g., zein of corn, hordein of barley, gliaclin of wheat etc.
e) Albuminoids or Scleroproteins: These are the least soluble proteins being insoluble in water, salt solutions, acids or alkalies and alcohol. This type is found in animals only. Examples are keratin of exoskeletal structures (e.g., horn, hoof, nails and hair), elastin and collagen of connective tissues etc.
f) Histones: The histones are basic proteins, soluble in water, dilute acids and alkalies but insoluble in dilute ammonia and not glutinous by heat. Being basic, they usually occur in tissues as salt combinations of acidic substances such as nucleic acids, porphyrins etc,, to form conjugated proteins. Examples are globin of hemoglobin, nucleohistones of nucleoproteins etc:
g) Protamins: The protamins are similar to histones in that they are also basic-proteins, soluble in water, acids or alkalies, and not coagulated by heat. But unlike histones, the protamins are soluble in dilute ammonia as well. They also occur in comination with acids, particularly with nucleic acid of sperm. e.g., salmine of salmon sperm, clupeine of herring sperm etc.

Protein Classification on Chemical Properties: Conjugated proteins

Conjugated proteins are composed of a simple protein combined with some non-protein substance which is referred to as the prosthetic group. They are classified on the basis of the non-protein part present in the molecule. Conjugated proteins may be of following types:—
a) Nucicoproteins: These are made up of nucleic acids (DNA or RNA) and a basic protein i.histone or protamin). According to the prosthetic group (i.e., nucleic acid), the nudeoproteins may be of two types— DNA-proteins of nuclear chromatin and genes and RNA-proteins of ribosomes.
b) VIucoproteins or glycoproteins: They are carbohydrate (mucopolysaccharide) containing proteins. Examples— mucin, tropic hormones like FSH, LM and TSH, ground substance of connective tissues etc.
c) Phosphoproteins: Proteins containing phosphoric acid as prosthetic group are of this type. Examples- casein of milk and vitellin of egg-yolk.
d) Lipoprotclns: These are lipid containing proteins. The lipid prosthetic group in them may be fats, phospholipids, fatty acids etc. Examples- protein of biological membranes, thromboplastin etc.
e) Chromoproteins: They contain some pigments like porphyrins, carotenoids and flavin compounds as the prosthetic group. Examples are hemoglobin and chlorophyll (containing a porphyrin heine), rhodopsin (containing retinene) of retinal rods ere..
f) Metalloprotcins: These contain metals like Cu, Fe, Zn, Co, Mn, Mg, etc., as the prosthetic group. Various enzymes belong to this group e.g., carbonic anhydrase containing /n, cytochromes containing Cu or Fe, ceruloplasmin containing Cu, etc. Hemoglobin and other heme containing Chromoproteins are also metalloproteins.

Protein Classification on Chemical Properties: Derived proteins

Derived proteins include those substances formed from simple or conjugated proteins. These are of two types— primary derived proteins and secondary derived proteins.
a) Primary derived proteins: These are formed by only slight change in the protein molecule without hydrolytic cleavage of peptide linkages. These are also called denatured proteins. Although primary derived protein has almost same molecular weight as the original protein, they differ in physical properties like solubility, precipitation etc. Primary derived proteins may be of three types—

i) Proteans: -These are insoluble products formed by the action of water, dilute acids or enzymes. e.g., myosan from myosin, fibrin from fibrinogen etc.
ii) Metaproteins: -These are soluble in very dilute acids or alkalies and formed by the action of strong acids or alkalies. Examples- acid metaproteins (formed in stomach by the action of HC1) and alkali metaproteins (formed in duodenum by the action of bicarbonates).
iii) Coagulated proteins: -Heat, alcohol, U-V rays, X-rays, high pressure etc., convert soluble native proteins into insoluble coagulated proteins. Examples include cooked egg albumin or meat, alcohol precipitated proteins etc.
b) Secondary derived proteins: These are formed by hydrolytic cleavage of peptidu linkages of protein molecules. Examples are proteoses, peptones and peptides that are formed in the gastrointestinal tract during digestion of proteins.