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 can be competitive, noncompetitive, or allosteric, each affecting the enzyme's activity differently.
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 competes for the same active site.
3. Consider the scenario: succinylcholine is added to a mixture containing acetylcholine and its hydrolyzing enzyme. The rate of acetylcholine hydrolysis decreases, indicating that succinylcholine is occupying the active site, preventing acetylcholine from binding.
4. Note that when more acetylcholine is added, the original rate of hydrolysis is restored. This implies that acetylcholine can outcompete succinylcholine for the active site when its concentration is increased, a hallmark of competitive inhibition.
5. Conclude that succinylcholine acts as a competitive inhibitor because it competes with acetylcholine for the enzyme's active site, temporarily reducing the rate of acetylcholine hydrolysis until more acetylcholine is added.