Succinylcholine is structurally almost identical to acetylcholine. If succinylcholine is added to a mixture that contains acetylcholine and the enzyme that hydrolyzes acetylcholine (but not succinylcholine), the rate of acetylcholine hydrolysis is decreased. Subsequent addition of more acetylcholine restores the original rate of acetylcholine hydrolysis. Which of the following correctly explains this observation?

• A. Succinylcholine must be an allosteric regulator for this enzyme.
• B. The active site must have the wrong configuration to permit succinylcholine binding.
• C. Succinylcholine must be a noncompetitive inhibitor.
• D. The presence of succinylcholine changes the conditions in the solution, resulting in a denaturation of the enzyme.
• E. Succinylcholine must be a competitive inhibitor with acetylcholine.

Answer: (E)

Verified step by step guidance

1. Begin by understanding the concept of enzyme inhibition. Enzyme inhibitors are molecules that bind to enzymes and decrease their activity. There are different types of inhibitors, including competitive and noncompetitive inhibitors.
2. Recognize that competitive inhibitors resemble the substrate and compete for binding at the enzyme's active site. In this case, succinylcholine is structurally similar to acetylcholine, suggesting it could compete for the same active site.
3. Consider the effect of succinylcholine on the rate of acetylcholine hydrolysis. The decrease in hydrolysis rate indicates that succinylcholine is occupying the active site, preventing acetylcholine from binding and being hydrolyzed.
4. Note that the addition of more acetylcholine restores the original rate of hydrolysis. This is characteristic of competitive inhibition, where increasing substrate concentration can overcome the inhibitor's effect by outcompeting it for the active site.
5. Conclude that succinylcholine acts as a competitive inhibitor because it competes with acetylcholine for the active site, and its effect can be reversed by increasing the concentration of acetylcholine.