Principles of Biochemistry

5.1.3.1 Classification based on biological functions

Based on their biologic functions, proteins might be classified as (i) enzymes (ii)transport proteins (iii) storage proteins (iv) regulatory proteins (v) contractile or motile protein(vi) structural proteins (vii) defense proteins.

i) Enzymes: Enzymes are most specialized proteins with catalytic activity. All chemical reactions of organic molecules in cells are catalysed by enzymes. More than 2000 different enzymes have been discovered in different forms of life.

 E.g. Trypsin, Ribonuclease, pepsin, Trypsin etc.

ii) Transport proteins: Transport proteins are present in blood plasma. They bind and carry specific molecules from one organ to another.

 E.g Hemoglobin of Red blood cells, serum albumin, lipoproteins

iii) Storage proteins: The seeds of many plants and eggs of birds and milk of animals store nutrient proteins required for the growth of the embryo and young ones

 E.g Gliadin of wheat; Ovalbumin of egg; Casein of milk,ferritin, Calsequestrinetc.

iv) Regulatory protein: These proteins help regulate cellular or physiological activity

 E.g. Hormone-insulin; growth hormone of the pituitary

v) Contractile or motile proteins: These proteins endow cells and organisms with the ability to contract, to change shape, or to move about.

 E.g. Actin and myosin of muscle proteins; Tubulin of microtubules of flagella and cilia.

vi) Structural proteins: These proteins serve as supporting filaments, cables and sheets of biological structures

 E.g. Collagen of tendons and cartilages, elastin of ligaments; keratin of hair, feathers, hoof and nails

vii) Defense proteins: These proteins defend organisms against invasion by other species or protect them from injury.

 E.g. Immunoglobulin or antibodies of vertebrates; fibrinogen and thrombin of blood clotting proteins; snake venoms; bacterial toxins,complements.

There are numerous other proteins whose functions are rather exotic and not easily classified. The blood plasma of some Antarctic fish contains “antifreeze” proteins, which protect their blood from freezing.

Specialized systems of classification distinguish certain complex proteins. Thus, plasma lipoproteins are termed, α₁-, α₂-, α₁ or β- lipoproteins based on their electrophoretic mobility at pH 8.6; or as chylomicrons, VLDL, LDL, HDL, or VHDL based on their sedimentation behavior in an ultracentrifuge.