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

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?

Accepted Answer

Thiamine (B1)

Answer

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?

Accepted Answer

Phenylalanine hydroxylase

Answer

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?

Accepted Answer

Aspartate

Answer

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?

Accepted Answer

Pyridoxine (B6)

Answer

Pyridoxal phosphate (PLP), derived from vitamin B6, is the prosthetic group required for all transamination reactions.

NBME Practice Test

NBME Practice Test

NBME Biochemistry 2