NBME Biochemistry 2

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?

Accepted Answer

Km increases, Vmax unchanged

Answer

Km unchanged, Vmax decreases

Answer

Both Km and Vmax increase

Answer

Both Km and Vmax decrease

Question 2

A chronic alcoholic presents with peripheral neuropathy and Wernicke encephalopathy. Which vitamin cofactor is deficient in pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase?

Accepted Answer

Thiamine (B1)

Answer

Riboflavin (B2)

Answer

Pyridoxine (B6)

Answer

Cobalamin (B12)

Question 3

A newborn screening reveals elevated phenylalanine. Classic phenylketonuria results from deficiency of which enzyme?

Accepted Answer

Phenylalanine hydroxylase

Answer

Tyrosinase

Answer

Homogentisate oxidase

Answer

Fumarylacetoacetate hydrolase

Question 4

In the urea cycle, one nitrogen is derived from free ammonia and the other is donated by which compound?

Accepted Answer

Aspartate

Answer

Glutamine

Answer

Alanine

Answer

Glycine

Question 5

Which vitamin serves as a coenzyme for all aminotransferase (transaminase) reactions, linking amino acid catabolism to the TCA cycle?

Accepted Answer

Pyridoxine (B6)

Answer

Biotin

Answer

Niacin (B3)

Answer

Pantothenic acid

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?

Accepted Answer

Myophosphorylase (McArdle disease)

Answer

Pyruvate kinase

Answer

Glucose-6-phosphatase

Answer

Aldolase B

Question 7

Which enzyme catalyzes the rate-limiting step of the pentose phosphate pathway and is important for NADPH production in red blood cells?

Accepted Answer

Glucose-6-phosphate dehydrogenase

Answer

Transketolase

Answer

Ribose-5-phosphate isomerase

Answer

6-Phosphogluconate dehydrogenase