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Biochemistry Flashcards

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  1. A competitive inhibitor is added to an enzyme with Km = 2 mM and Vmax = 100 μmol/min. Which effect best describes the result?

    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.

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

    Answer: Thiamine (B1)

    Thiamine pyrophosphate (TPP) is an essential cofactor for oxidative decarboxylation complexes including pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase.

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

    Answer: Phenylalanine hydroxylase

    Phenylalanine hydroxylase (requiring BH4 as cofactor) converts phenylalanine to tyrosine; its deficiency causes toxic phenylalanine accumulation.

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

    Answer: Aspartate

    The two urea nitrogens originate from ammonium (via carbamoyl phosphate synthetase I) and from aspartate (incorporated at the argininosuccinate synthetase step).

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

    Answer: Pyridoxine (B6)

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

  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?

    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.

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

    Answer: Glucose-6-phosphate dehydrogenase

    G6PD is the rate-limiting enzyme of the hexose monophosphate shunt, producing NADPH essential for glutathione reduction in RBCs.