A Level Biology Practice Exam

What defines a competitive inhibitor?

1. An inhibitor that promotes substrate binding

2. An inhibitor that binds to an enzyme's allosteric site

3. An inhibitor that competes with the substrate for the active site

4. An inhibitor that speeds up the reaction

The correct answer is: An inhibitor that competes with the substrate for the active site

A competitive inhibitor is defined specifically as a molecule that competes with a substrate for binding to the active site of an enzyme. This means that the competitive inhibitor has a structure similar to the substrate, allowing it to bind to the active site where the substrate typically fits. By occupying this site, the inhibitor effectively blocks the substrate from binding, thereby reducing the rate of the reaction catalyzed by the enzyme. In the context of the other choices, an inhibitor that promotes substrate binding would enhance enzyme activity, which is contrary to the function of competitive inhibitors. The mention of an allosteric site refers to a different type of inhibition known as allosteric inhibition, where the inhibitor binds to a site other than the active site and induces a change in the enzyme's shape, thereby altering its activity without directly competing with the substrate. The option suggesting that an inhibitor speeds up the reaction describes an activator rather than an inhibitor, as inhibitors are substances that decrease the rate of enzymatic reactions. Therefore, the defining characteristic of a competitive inhibitor is its competition with the substrate for the active site, confirming that the correct answer is indeed the one focusing on this competitive interaction.