Q.Define the active site of an enzyme, in one or two sentences.

An enzyme recognises and acts on its substrate at a specific, often cleft-like active site, formed by amino acid residues brought together by the enzyme's tertiary or quaternary folding, where a transient enzyme-substrate (ES) complex forms. Two classical models describe this binding: the lock-and-key model, proposed by Emil Fischer in 1894, pictures the active site as a rigid structure permanently and precisely complementary to the substrate's shape, like a key fitting a lock, explaining enzyme specificity well but not the small conformational changes actually observed during binding; the induced-fit model, proposed by Daniel Koshland in 1958, pictures the active site as flexible, reshaping itself slightly as it binds the substrate for a closer fit, better accounting for both specificity and the observed structural changes. Once the ES complex forms, the reaction proceeds through a high-energy transition st …