Enzyme inhibitors, Vmax and Km

Last time

Before we start: last time, in Enzyme action, the factors and progress curves, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Switching an enzyme off

Many drugs and poisons work by switching off one enzyme. Cyanide blocks an enzyme in respiration. Some medicines block an enzyme in a bacterium. There are two main ways to block an enzyme, and Cambridge expects you to tell them apart from a graph.

Competitive inhibitors

A competitive inhibitor has a shape similar to the substrate. It fits into the active site of the enzyme and blocks it. Substrate and inhibitor compete for the same active site. While the inhibitor is bound, the substrate cannot enter. The binding is usually reversible. If the substrate concentration is raised, more substrate molecules win the competition, and the effect of the inhibitor is overcome. So with a competitive inhibitor, the maximum rate can still be reached, but it takes more substrate.

Non-competitive inhibitors

A non-competitive inhibitor binds to a different place on the enzyme, away from the active site. This is called an allosteric site. Binding changes the shape of the whole enzyme, including the active site, so the substrate no longer fits well. Adding more substrate does not help, because the substrate is not competing for the same place. The maximum rate stays lower. The number of working enzyme molecules is effectively reduced, as if there was less enzyme.