The complex organic nitrogenous substances found in living cells (G.J. Milder, 1840) are known as proteins. The term protein (Gr.proteios- holding the first rank) suggested by Berzelius (1938), in itself speaks about pivotal position they occupy in living world. They are primarily concerned in (1) structural architecture and (2) functioning of the living system.
Proteins play a crucial role in virtually all biological processes,viz,
(i) Transport and storage (haemoglobin in blood, myoglobin in muscles, both as oxygen carriers-egg’s white, ovalbumin, glutelin in cereals, iron storing ferritin in animals);
(ii) Coordinated motion (muscle movement, elastic and non-elastic fibres in connective tissues and contractile fibres- actin and myosin in muscles),
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(iii) Mechanical support (tensile strength of skin and bone collagen, a fibroses protein),
(iv) Immune protection (antibodies),
(v) Generation and transmission of nerve impulse (receptor proteins),
(vi) Control of growth and differentiation (sequential expression of genetic information),
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(vii) Enzymatic catalysis (unique role in determining the pattern of chemical transformation in biological systems),
(viii) Stable colloidal behaviour (the protein ions in solution repel one another and attract the water molecules to orient around).
Proteins are the most abundant and most varied of the macromolecules of the living cells. They constitute about 50% of the dry weight of cells. These macromolecules are really large, ranging in molecular weight from several thousand to several million. They are the polymers of amino acid monomers. The proteins are made up of 20 different types of naturally occurring amino acids.