MCAT Biochemistry Question 28: Answer and Explanation

Home > MCAT Test > MCAT biochemistry practice tests

Test Information

Question: 28

13. A certain cooperative enzyme has four subunits, two of which are bound to substrate. Which of the following statements can be made?

  • A. The affinity of the enzyme for the substrate has just increased.
  • B. The affinity of the enzyme for the substrate has just decreased.
  • C. The affinity of the enzyme for the substrate is at the average for this enzyme class.
  • D. The affinity of the enzyme for the substrate is greater than with one substrate bound.

Correct Answer: D

Explanation:

Cooperative enzymes demonstrate a change in affinity for the substrate depending on how many substrate molecules are bound and whether the last change was accomplished because a substrate molecule was bound or left the active site of the enzyme. Because we cannot determine whether the most recent reaction was binding or dissociation, choices (A) and (B) are eliminated. We can make absolute comparisons though. The unbound enzyme has the lowest affinity for substrate, and the enzyme with all but one subunit bound has the highest. The increase in affinity is not linear, and therefore choice (C) is not necessarily true. An enzyme with two subunits occupied must have a higher affinity for the substrate than the same enzyme with only one subunit occupied; thus, choice (D) is correct.

Previous       Next