NBME Biochemistry 2 — Questions and Answers
Question 1: A competitive inhibitor is added to an enzyme with Km = 2 mM and Vmax = 100 μmol/min. Which effect best describes the result?
- Km increases, Vmax unchanged (Correct answer)
- Km unchanged, Vmax decreases
- Both Km and Vmax increase
- Both Km and Vmax decrease
Correct answer: Km increases, Vmax unchanged
Competitive inhibitors bind the active site reversibly, raising apparent Km but leaving Vmax unchanged since excess substrate can outcompete the inhibitor.
Question 2: A chronic alcoholic presents with peripheral neuropathy and Wernicke encephalopathy. Which vitamin cofactor is deficient in pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase?
- Riboflavin (B2)
- Thiamine (B1) (Correct answer)
- Pyridoxine (B6)
- Cobalamin (B12)
Correct answer: Thiamine (B1)
Thiamine pyrophosphate (TPP) is an essential cofactor for oxidative decarboxylation complexes including pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase.
Question 3: A newborn screening reveals elevated phenylalanine. Classic phenylketonuria results from deficiency of which enzyme?
- Tyrosinase
- Homogentisate oxidase
- Phenylalanine hydroxylase (Correct answer)
- Fumarylacetoacetate hydrolase
Correct answer: Phenylalanine hydroxylase
Phenylalanine hydroxylase (requiring BH4 as cofactor) converts phenylalanine to tyrosine; its deficiency causes toxic phenylalanine accumulation.
Question 4: In the urea cycle, one nitrogen is derived from free ammonia and the other is donated by which compound?
- Glutamine
- Aspartate (Correct answer)
- Alanine
- Glycine
Correct answer: Aspartate
The two urea nitrogens originate from ammonium (via carbamoyl phosphate synthetase I) and from aspartate (incorporated at the argininosuccinate synthetase step).
Question 5: Which vitamin serves as a coenzyme for all aminotransferase (transaminase) reactions, linking amino acid catabolism to the TCA cycle?
- Biotin
- Niacin (B3)
- Pantothenic acid
- Pyridoxine (B6) (Correct answer)
Correct answer: Pyridoxine (B6)
Pyridoxal phosphate (PLP), derived from vitamin B6, is the prosthetic group required for all transamination reactions.
Question 6: A patient has no rise in venous lactate after forearm ischemic exercise test, but shows a normal ammonia rise. Which enzyme deficiency is most likely?
- Pyruvate kinase
- Glucose-6-phosphatase
- Myophosphorylase (McArdle disease) (Correct answer)
- Aldolase B
Correct answer: Myophosphorylase (McArdle disease)
McArdle disease (GSD type V) is caused by muscle phosphorylase deficiency, preventing glycogenolysis in muscle, so lactate cannot rise during ischemic exercise.
Question 7: Which enzyme catalyzes the rate-limiting step of the pentose phosphate pathway and is important for NADPH production in red blood cells?
- Transketolase
- Ribose-5-phosphate isomerase
- 6-Phosphogluconate dehydrogenase
- Glucose-6-phosphate dehydrogenase (Correct answer)
Correct answer: Glucose-6-phosphate dehydrogenase
G6PD is the rate-limiting enzyme of the hexose monophosphate shunt, producing NADPH essential for glutathione reduction in RBCs.
NBME Biochemistry 2
A competitive inhibitor is added to an enzyme with Km = 2 mM and Vmax = 100 μmol/min.
Which effect best describes the result?