Enzyme Kinetics MCAT Practice Question
A 34-year-old man with a family history of metabolic disease presents with elevated lactate levels (4.2 mmol/L; normal <2) and recurrent muscle cramps after exercise. Genetic testing identifies a heterozygous mutation in the LDHA gene encoding lactate dehydrogenase A. Biochemical analysis of the mutant enzyme shows: Wild-type LDHA: kcat = 1000 sā»Ā¹, Km = 50 Ī¼M
Mutant LDHA: kcat = 1000 sā»Ā¹, Km = 200 Ī¼M The mutation affects the substrate-binding pocket but not the catalytic residues. Which of the following best explains why this patient's lactate clearance is impaired despite normal catalytic turnover?
Answer choices
- BThe mutation increases kcat, causing the enzyme to work too quickly and deplete NADāŗ
- DThe mutation increases Km selectively in the mitochondria, while cytoplasmic enzyme remains unchanged
- AThe mutation decreases Vmax, reducing the maximum velocity at which lactate can be cleared
- CThe mutation decreases catalytic efficiency (kcat/Km), reducing enzyme activity at physiologic substrate concentrationsCorrect answer
- EThe mutation prevents allosteric activation by AMP, impairing metabolic responsiveness
- FThe mutation eliminates the enzyme entirely, with residual activity from compensatory isoforms (LDHB)
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