Q.What are enzymes? Explain, using the lock-and-key model, why a given enzyme usually acts only on one particular substrate (or a small family of closely related substrates).

Enzymes are biological catalysts, almost all of them proteins, that speed up specific biochemical reactions in living cells by providing a lower-activation-energy pathway, without themselves being consumed by the reaction they catalyse.

Enzymes typically show high specificity, acting on only one particular substrate or a small family of closely related substrates, rather than catalysing reactions of any molecule indiscriminately. The elementary explanation for this is the lock-and-key model: the enzyme's folded, three-dimensional structure creates a small pocket or cleft on its surface, called the active site, whose precise shape (and the specific chemical groups lining it) fits only one substrate's own shape closely -- in the same way that a particular key's pattern of notches fits only the one lock it was cut to open, and no other lock. The matching substrate binds into the active site to form a temporary enzyme-substrate complex; the reaction is catalysed while it is bound; and the product, having a different shape from the original substrate, is released, leaving the …

Connect specificity directly to shape-matching: only a substrate whose shape complements the active site's shape can bind there …

Describing enzymes as reacting with 'whatever substrate is available' -- their defining feature is exactly the opposite: a strong shape-based preference for …