In this article we will discuss about the methods used for classification of proteins.
1. Classification on the Basis of Chemical Composition of the Proteins:
1. Simple proteins:
The proteins which contain only amino acids in their molecules are known as simple proteins, e.g., albumins, globulins, histones, prolamines and protamines.
2. Conjugated proteins:
The proteins which contain amino acids and other chemical compounds in their molecules are known as conjugated proteins.
(i) Chromo proteins contain amino acids and coloured pigments in their molecules, e.g., haemocyanin, haemoglobin, flavoproteins and cytochromes.
(ii) Glycoproteins and mucoproteins containing carbohydrates and proteins in their molecules, e.g., mucin of saliva.
(iii) Phosphoproteins contain proteins and phosphoric acid in their molecules, e.g., casein of the milk.
(iv) Lipoproteins contain amino acids and lipids in their molecules, e.g., lipovitellin of egg yolk, serum, protein of brain and nerve tissues.
(v) Nucleoproteins contain amino acids and nucleic acids in their molecules, e.g., the viruses and nucleic acid proteins.
(vi) Metalloproteins contain amino acids and metal ions in their molecules, e.g., the enzyme carbonic anhydrase contains zinc and amino acids in its molecules.
2. Classification on the Basis of the Molecular Shapes of the Proteins:
1. Globular proteins:
These proteins contain spherical or oval molecules and are generally soluble in water.
2. Fibrous proteins:
These proteins are composed of elongated molecules of coiled polypeptide chains and are fibrous in shape. These are insoluble in water, e.g., keratin of skin, hair and feathers, the collagens of cartilage and tendons, the elastin’s of ligaments and silk fibroin of silk fibres.
3. Classification on the Basis of the Solubility of the Proteins:
1. Albumins are simple proteins which are soluble in the distilled water, dilute salt solutions, dilute acids and bases, e.g., egg albumin, serum albumin and milk albumin.
2. Globulins are simple proteins which are insoluble in distilled water but soluble in neutral salt solutions, e.g., globulins of egg white, serum and globulins of many seeds as squash, soya bean, hemp and others.
3. Globins are simple proteins which are insoluble in distilled water but soluble in ammonium hydroxide, e.g., hemoglobin’s.
4. Histones are simple proteins and are insoluble in ammonium hydroxide, e.g., nucleoproteins.
5. Prolamines are simple proteins which are insoluble in water but soluble in 70% ethyl alcohol, e.g., gliadin of wheat, zein of corn.
6. Protamines are simple proteins which are soluble in ammonium hydroxide, e.g., proteins of fish sperms.
7. Glutelins are simple proteins which are insoluble in distilled water and alcohol but soluble in dilute acids, e.g., glutelins occur in plants.
8. Scleroproteins are the conjugated proteins which contain fibrous molecules. These proteins are insoluble in water as well as other solvents, e.g., fibrous proteins and conjugated proteins.
4. Classification of the Proteins According to the Arrangement of the Peptide Chains in the Molecules:
1. Primary proteins:
In primary proteins many amino acids occur but all remain arranged in a single linear chain by the peptide bonds, e.g., insulin, ribonuclease.
2. Secondary proteins:
The secondary proteins contain spirally or helically arranged chains of hundreds of the amino acids in their molecules. These proteins contain H bonds, e.g., fibrous or scleroproteins as keratin, collagen, etc.
3. Tertiary proteins:
The tertiary proteins are composed of many polypeptide chains which remain held together by -S-S- bonds in their molecules. These molecules contain compactly arranged helix which assumes ellipsoid or other form, e.g., globular proteins.
4. Quaternary proteins:
The quaternary proteins contain two to many polypeptidic chains of similar or different nature which remain bound together by weak covalent bonds, e.g., haemoglobin.