in Cell Structure and Function by

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Mechanism of enzyme function

Enzymes are protein substances which act as biological catalysts with a high degree of specificity produced within the cell having enormous ability to catalyze all metabolic reactions in a highly effective manner. Enzymes may be simple protein or conjugated protein formed of apoenzyme (protein part) and cofactor (prosthetic group or coenzymes or metallic cofactor).

Edward Buchner (1987) showed that juice of groundnut and preserved yeast cells ferment sugar.

J. B. Sumner purified and crystallized urease enzymes from Jack beans. He also suggested that enzymes are proteins.

Functions of Enzymes:

Enzymes act catalysts, are required in small amount, highly specific, act best at a temperature between 25 - 40°C and at low temperature action slows down and at high temperature get denatured. Enzymes require optimum pH specific for each enzyme. The relative concentration of enzyme and substrate affect the velocity of reaction. Certain inhibitors check the enzymatic action. Extracellular enzymes are produced in the inactive forms called proenzymes are slightly different in molecular structure but perform identical activities, they are known as isoenzyme or isozymes (e.g., Lactic dehydrogenase which catalyses conversion of pyruvate to lactate has five more isomeric form).

Mechanism of action:

Fisher 1894 Suggested the Lock and Key hypothesis.

E + S




E + Product

(Enzyme substrate)


(Enzymes substrate complex)



Koshland Induced Fit Theory:

Lock and Key Theory (Contemplate theory) has been modified and accepted as induced fit theory. This suggests existence of two groups i.e. site of enzyme. As the substrate gets associated with buttressing region, the active site changes configurations so that catalytic group comes to lie opposite to the area where substrate bonds where are to be changed.