Protein Classification Based on Shape

There are three types of proteins on the basis of shape - Fibrous, Globular, Intermediate

1.       FIBROUS PROTEINS

1. They are thread-like elongated proteins which have little or no tertiary structure.
2. Secondary structure in the final structure in fibrous proteins.
3. All the three types of secondary structure occur in fibrous proteins.
4. Fibrous proteins occur singley or join together in coils and bundles to form fibres.
5. They are nonenzymatic proteins which are insoluble in water, produce structural components, provide rigidity and elasticity.
6. Common fibrous proteins are collagen (connective tissue, tendon, cartilage, bone teeth), keratin (skin, hair, beaks, feathers, claws, nails, hoofs, horns), fibroin (silk, spider webs), elastin, tropomyosin, paramyosin, fibrin (blood clot), etc.

2.       GLOBULAR PROTEINS

1. They are rounded, over or oblong in outline.
2. The shape is achieved through tertiary and quaternary folding of polypeptides.
3. Most of the proteins are globular in nature.
4. They are not contractile but otherwise from all other types of proteinaceous structures, both enzymatic and non-enzymatic (including antibodies and hormones).
5. Their solubility in water depends upon size.
6. Smaller globular proteins are soluble in water.
7. They are also not coagulated by heat, e.g., histones.
8. With increase in size the globular proteins become sparingly soluble in water and develop the tendency to get coagulated by heat.
9. Albumins are soluble in water but are coagulated by heat, e.g., egg albumin. Globulins (e.g., Serum globulins) and glutenins (e.g., Wheat, Maize) are both insoluble in water and coagulate with application of heat.

3.       INTERMEDIATE PROTEINS

1. They are neither fibrous nor globular but of intermediate from and characteristics.
2. Myosin with some 1700 amino acid residues in its polypeptide is not fibrous but assumes an extended from in solution.
3. Fibrinogen is a highly soluble extended but somewhat asymmetric protein that forms insoluble fibrin in coagulated blood.