MCAT Biochemistry Question 17: Answer and Explanation

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Question: 17

2. Which of the following statements about enzyme kinetics is FALSE?

  • A. An increase in the substrate concentration (at constant enzyme concentration) leads to proportional increases in the rate of the reaction.
  • B. Most enzymes operating in the human body work best at a temperature of 37°C.
  • C. An enzyme–substrate complex can either form a product or dissociate back into the enzyme and substrate.
  • D. Maximal activity of many human enzymes occurs around pH 7.4.

Correct Answer: A

Explanation:

Most enzymes in the human body operate at maximal activity around a temperature of 37°C and a pH of 7.4, which is the pH of most body fluids. In addition, as characterized by the Michaelis–Menten equation, enzymes form an enzyme–substrate complex, which can either dissociate back into the enzyme and substrate or proceed to form a product. So far, we can eliminate choices (B), (C), and (D), so let’s check choice (A). An increase in the substrate concentration, while maintaining a constant enzyme concentration, leads to a proportional increase in the rate of the reaction only initially. However, once most of the active sites are occupied, the reaction rate levels off, regardless of further increases in substrate concentration. At high concentrations of substrate, the reaction rate approaches its maximal velocity and is no longer changed by further increases in substrate concentration.

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