MCAT Biochemistry 2

Question 1

Which enzyme catalyzes the rate-limiting step of the citric acid cycle?

Accepted Answer

Isocitrate dehydrogenase

Answer

Isocitrate dehydrogenase is the primary regulatory enzyme of the TCA cycle, inhibited by ATP and NADH and activated by ADP and Ca²⁺.

Question 2

A patient presents with elevated homocysteine levels. Which vitamin deficiency is most likely responsible?

Accepted Answer

Vitamin B12 (cobalamin)

Answer

Vitamin B12 is required for the conversion of homocysteine to methionine; its deficiency causes homocysteine accumulation.

Question 3

Which glycolytic intermediate is also a precursor for the synthesis of serine?

Accepted Answer

3-Phosphoglycerate

Answer

3-Phosphoglycerate is diverted from glycolysis to form serine via the phosphoserine pathway.

Question 4

In fatty acid beta-oxidation, which coenzyme directly accepts electrons in the first oxidation step?

Accepted Answer

FAD

Answer

The first oxidation step of beta-oxidation is catalyzed by acyl-CoA dehydrogenase, which uses FAD as the electron acceptor.

Question 5

Which of the following correctly describes a competitive inhibitor's effect on enzyme kinetics?

Accepted Answer

Increases Km, no change in Vmax

Answer

Competitive inhibitors increase the apparent Km (reducing substrate affinity) but do not alter Vmax because excess substrate can outcompete the inhibitor.

MCAT - Medical College Admission Test Practice Test

MCAT - Medical College Admission Test Practice Test

MCAT Biochemistry 2