Malonate is a competitive inhibitor of succinate dehydrogenase, a key enzyme in the Krebs tricarboxylic acid cycle. The presence of malonate will affect the kinetic parameters of succinate dehydrogenase in which one of the following ways?
## **Core Concept**
The question tests understanding of enzyme kinetics, specifically the effects of competitive inhibitors on enzyme activity. Competitive inhibitors structurally resemble the substrate and compete with it for binding to the active site of the enzyme. Succinate dehydrogenase is an enzyme in the Krebs cycle that catalyzes the conversion of succinate to fumarate.
## **Why the Correct Answer is Right**
In the presence of a competitive inhibitor like malonate, the maximum velocity (Vmax) of the enzyme-catalyzed reaction remains unchanged. However, the affinity of the enzyme for its substrate appears to decrease, which is reflected by an increase in the Michaelis constant (Km). This is because the inhibitor occupies some of the active sites, requiring more substrate to achieve half of Vmax. Therefore, the correct answer shows an increase in Km with no change in Vmax.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests a decrease in both Km and Vmax, which is characteristic of a mixed inhibitor, not a competitive inhibitor.
- **Option B:** This option shows a decrease in Km and no change in Vmax, which is not consistent with competitive inhibition. A decrease in Km would indicate an increase in affinity, the opposite of what occurs with competitive inhibitors.
- **Option D:** This option indicates a decrease in Vmax with no change in Km, which is characteristic of a non-competitive inhibitor, not a competitive inhibitor.
## **Clinical Pearl / High-Yield Fact**
Competitive inhibitors can be overcome by increasing the concentration of the substrate, which effectively outcompetes the inhibitor for the active site of the enzyme. This is a key concept in pharmacology, as it relates to how some drugs can be counteracted.
## **Correct Answer:** .