Biochemistry Flashcards

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

Answer: Isocitrate dehydrogenase

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

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

Answer: Vitamin B12 (cobalamin)

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

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

Answer: 3-Phosphoglycerate

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

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

Answer: FAD

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

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

Answer: Increases Km, no change in Vmax

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

Maple syrup urine disease results from deficiency of which enzyme complex?

Answer: Branched-chain alpha-keto acid dehydrogenase

Branched-chain alpha-keto acid dehydrogenase deficiency leads to accumulation of leucine, isoleucine, and valine metabolites, causing characteristic sweet-smelling urine.

Which linkage type is found in cellulose but NOT in starch?

Answer: β-1,4-glycosidic

Cellulose consists of glucose monomers joined by β-1,4-glycosidic bonds, which humans cannot digest, unlike the α-1,4 bonds in starch.