Q.Explain

[!TLDR] (a) Rate rises with substrate until all active sites are occupied, then plateaus (saturation). (b) Above the optimum temperature the enzyme's protein structure denatures, destroying the active site. [!ANSWER] (a) At low substrate concentration, more active sites become occupied as substrate rises, raising rate; once all sites are saturated, adding more substrate cannot increas …

(a) At low substrate concentration, most of the enzyme's fixed population of active sites is empty, so as substrate concentration rises, more sites become occupied per unit time and the rate of product formation rises roughly in step with substrate concentration, much like an ordinary first order dependence. But the total number of active sites present (set by the fixed amount of enzyme) is limited; as substrate concentration keeps rising, an ever-larger fraction of sites is continuously occupied, and once essentially all sites are occupied at all times (saturation), adding still more substrate cannot make the enzyme molecules process substrate any faster than they already are -- the rate levels off at a maximum, becoming effectively independent of (zero order in) substrate concentration. (b) An enzyme's catalytic ability depends entirely on the precise three-dimensional folded shape of its protein structure, which creates the exact geometry of the active site. Raising temperature initially speeds the reaction, exactly as the Arrhenius equation predicts for any reaction. But beyond a certain optimum temperature, the added thermal energy begins to break the weak (hydrogen-bonding, ionic, and other non-covalent) interactions that hold the protein's folded shape together, causing the enzyme to denature -- unfold and lose its specific three-dimensional structure. Once denatured, the active site's precise shape is destroyed and the enzyme can no longer bind its substra …

For (a), reason from a fixed number of active sites becoming progressively saturated; for (b), connect activity loss to denaturation (unfolding) o …

For (a), a common incomplete answer just says 'enzymes get saturated' without explaining WHY (the fixed, limited number of active sites); for (b), a common error is to treat enzyme temperature-sensitivity as if it followed the same ever-increasing Arrhenius tren …