Quick questions on Enzyme action and rate of reaction: VCE Biology Unit 3

Every enzyme has an active site: a pocket or cleft formed by the tertiary fold of the polypeptide. The R groups lining this site give it a specific shape and chemistry that match one substrate (or a small family of related substrates). This is the basis of enzyme specificity.

Temperature. As temperature rises, kinetic energy and successful collisions increase, so rate rises. Above the optimum (around 37 degrees Celsius for human enzymes), the weak bonds that hold tertiary structure break, the enzyme denatures, and rate falls sharply. Denaturation is usually irreversible.

A competitive inhibitor has a shape similar to the substrate and binds the active site. While it occupies the site, the real substrate cannot bind, so rate falls. Adding more substrate displaces the inhibitor and restores Vmax. The apparent Km (substrate concentration for half Vmax) rises.

Many enzymes need a non-protein partner to be active.

As temperature rises, kinetic energy and successful collisions increase, so rate rises. Above the optimum (around 37 degrees Celsius for human enzymes), the weak bonds that hold tertiary structure break, the enzyme denatures, and rate falls sharply. Denaturation is usually irreversible.

Each enzyme has an optimum pH at which its R groups carry the charges needed for substrate binding and catalysis. Pepsin works near pH 2 (stomach); trypsin near pH 8 (small intestine); most cytosolic enzymes near pH 7. Changes in pH alter ionic and hydrogen bonding within the enzyme, distort the active site, and reduce rate.

At low substrate concentrations, rate rises linearly with substrate because most active sites are empty. As more substrate is added, more active sites are occupied, and the rate approaches a maximum (Vmax) when all active sites are saturated. Beyond this point, adding more substrate has no further effect.

Provided substrate is in excess, rate rises linearly with enzyme concentration because more active sites are available.

The optimum is a single temperature or pH at which rate is highest, not the range over which the enzyme works.

VCAA expects induced fit. The lock and key model is only mentioned as a historical contrast.

Coenzymes are organic (NAD+, FAD, NADP+, coenzyme A). Cofactors are inorganic ions.

Competitive: same site as substrate, overcome by more substrate, Vmax unchanged. Non-competitive: allosteric site, not overcome by more substrate, Vmax lowered.