Enzyme Kinetics MCAT Practice Question
A 45-year-old man with a history of gout is being evaluated for recurrent hyperuricemia despite allopurinol therapy. Researchers are investigating the kinetic mechanism of xanthine oxidase, an important enzyme in purine metabolism that catalyzes the sequential oxidation of hypoxanthine to xanthine and then xanthine to uric acid. The enzyme transfers oxygen atoms from molecular oxygen to purine substrates via a covalent molybdenum cofactor intermediateāa mechanism characteristic of ping-pong (double-displacement) kinetics. To confirm this mechanism, kinetic studies are performed using Lineweaver-Burk double reciprocal plots. When the concentration of hypoxanthine is varied (holding oxygen constant) and the concentration of oxygen is varied (holding hypoxanthine constant), two families of lines are generated. Which kinetic pattern is most consistent with a ping-pong enzyme mechanism?
Answer choices
- AIntersecting lines in both plots with the intersection point located in the second quadrant
- BIntersecting lines in both plots with the intersection point on the y-axis
- CIntersecting lines for hypoxanthine but parallel lines for oxygen
- DParallel lines in both the hypoxanthine and oxygen Lineweaver-Burk plotsCorrect answer
- EHyperbolic (non-linear) relationships in both plots rather than straight lines
- FIntersecting lines in both plots with the intersection point on the x-axis
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