The graph below shows the effect of an inhibitor on enzyme activity when the enzyme and inhibitor were preincubated prior to addition of substrate, and when no preincubation occurred. Based on the graph, which of the following statements is true?

Question

The graph below shows the effect of an inhibitor on enzyme activity when the enzyme and inhibitor were preincubated prior to addition of substrate, and when no preincubation occurred.  Based on the graph, which of the following statements is true?

Quizplus · Accepted Answer

11ecba2d_0e98_9d4c_a3bf_ddadd998a80e_MD0008_00 Irreversible inhibitors typically react with nucleophilic side chains, forming covalent bonds to the enzyme. Once covalently linked, the inhibitor cannot be displaced by any amount of substrate. Preincubation with inhibitor provides more time for covalent linkages to occur; therefore, it can increase the level of inhibition because more enzymes will be inactivated. On the other hand, reversible inhibitors bind their target enzymes noncovalently. These interactions are generally much faster than the formation of covalent bonds, so additional time is not needed for reversible binding to occur. Therefore, reversible inhibitors tend to have the same effect whether or not they are preincubated with an enzyme. When the inhibitor was preincubated with enzyme prior to the addition of the substrate (empty circles), only ~10 nM was required to induce significant inhibition compared to ~50 nM when inhibitor and substrate were added simultaneously. It is therefore most likely an irreversible inhibitor. (Choice A) Inhibition increases when inhibitor and enzyme are preincubated prior to activity assays. This indicates that binding is irreversible. (Choice B) Less inhibitor is required to inactivate the enzyme when preincubation occurs, so the inhibitor is more effective with preincubation. (Choice D) The inhibitor is most likely irreversible. To be irreversible, an inhibitor must form strong (usually covalent) bonds with active site residues. Educational objective: Irreversible inhibitors form covalent bonds with enzymes and become more potent given sufficient time to react. Reversible inhibitors quickly form noncovalent bonds with target enzymes and do not require much time to achieve their full effect.

The Graph Below Shows the Effect of an Inhibitor on Enzyme