Enzyme Kinetics MCAT Practice Question
A 34-year-old man with type 2 diabetes mellitus is enrolled in a study examining the pharmacokinetics of a new antidiabetic drug that inhibits hepatic gluconeogenesis. Researchers measure the initial velocity (V₀) of the rate-limiting enzyme phosphoenolpyruvate carboxykinase (PEPCK) under different substrate and enzyme concentrations. When PEPCK enzyme concentration is doubled while maintaining substrate (oxaloacetate) concentration at 2× Km, the reaction velocity doubles. However, when substrate concentration is doubled at constant enzyme concentration, the reaction velocity increases by only 40%. Which of the following best explains these kinetic observations?
Answer choices
- AThe enzyme is saturated with substrate and operating near Vmax
- BOxaloacetate acts as a non-competitive inhibitor of PEPCK at physiologic concentrations
- CThe enzyme exhibits positive cooperativity and follows Hill kinetics rather than Michaelis-Menten kinetics
- DThe enzyme is operating in the first-order region of the Michaelis-Menten curve where V₀ is proportional to [S]Correct answer
- EThe enzyme requires allosteric activation by a downstream product for full activity
- FThe reaction velocity is independent of enzyme concentration due to substrate depletion
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