Identify an inhibitor that lowers both Vmax and Km | MCAT Proteins and amino acids Question

Biological and Biochemical Foundations

An enzyme is analyzed in the presence and absence of an inhibitor. The inhibitor decreases both the apparent $V_{max}$ and the apparent $K_m$ to 40% of their values in the absence of the inhibitor. Which of the following best describes the mechanism of inhibition?

Select one answer.

AThe inhibitor competes with the substrate for binding to the active site of the free enzyme.BThe inhibitor binds only to the enzyme-substrate complex at a site distinct from the active site.CThe inhibitor binds the free enzyme and the enzyme-substrate complex with equal affinity.DThe inhibitor covalently inactivates a fraction of the enzyme molecules.

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Answer: B (The inhibitor binds only to the enzyme-substrate complex at a site distinct from the active site.)

The inhibitor decreases the apparent $V_{max}$ and $K_m$ by the same proportion, which is characteristic of uncompetitive inhibition. An uncompetitive inhibitor binds only to the enzyme-substrate complex. This binding reduces the amount of catalytically productive enzyme-substrate complex, lowering $V_{max}$ , and stabilizes substrate binding, lowering the apparent $K_m$ .

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Biological and Biochemical Foundations

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