Question

Below are described two enzyme inhibitors, CH₃OH and heavy metals. What type of inhibitor is each of these substances in the situations described below: a competitive inhibitor or a noncompetitive inhibitor? Explain your answer.

A. Methanol, CH₃OH, is a toxic alcohol and competes with ethanol for the active site on the enzyme alcohol dehydrogenase. One of the products of methanol dehydration is the primary toxin. Therefore if methanol can be prevented from binding to the enzyme, it cannot be oxidized, and because it is water soluble, in time it will be excreted without doing any damage. A patient who has ingested methanol can be treated by drinking large quantities of ethanol, which successfully competes for the enzyme's active site and displaces methanol. The same treatment is used for accidental ingestion of ethylene glycol, HO-CH₂-CH₂-OH.

B. Some enzymes possess sulfhydryl (-SH) groups that are distant from the active sites of enzymes, but are important for maintaining the integrity of their conformational structures. Heavy metals, such as Ag⁺ and Hg²⁺, will bind to those sites and modify the conformational structure so that enzyme activity is reduced.

Answer 1

The first case is an example of competitive inhibition as ethanol is competing with methanol to get attached to the enzyme active site. If methanol gets attached then it would create toxin which is harmful for the body so in presence of ethanol, ethanol gets preferentially attached to the active site, thereby blocking the active site. Methanol can no longer bind and is excreted out. This is competitive inhibition.

In the second case the presence of thiol groups by binding to soft metal centres causes conformation modification of the enzymes which disturbs the attachment to the active site. But it is not competitive inhibition as for competitive inhibition we need a competing metal and the race is for only one active site. But here there are two attachment sites and the incoming metal does not compete with the Ag+ or Hg+.