In the citric acid cycle, succinate dehydrogenase catalyzes the conversion of succinate to fumarate. This reaction is inhibited by malonate, which resembles succinate but cannot be acted upon by succinate dehydrogenase. Increasing the ratio of succinate molecules to those of malonate in the reaction reduces the inhibitory effect of malonate. What role does malonate play with respect to succinate dehydrogenase?

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BlackTom AI · Accepted Answer

To tackle this question, first identify what the inhibitor resembles and how it interferes with the enzyme's normal function.
Option 1: 'Malonate blocks the binding of fumarate.' This would imply malonate prevents the product fumarate from binding, which is not how inhibition works here; fumarate is the product of the reaction, and malonate competes with the substrate, not with the product.
Option 2: 'Malonate is a noncompetitive inhibitor.' Noncompetitive inhibition typically lowers Vmax without changing Km, as the inhibitor binds an allosteric site. The description in the stem—where increasing the ratio of succinate relative to malonate lessens inhibition—fits competitive inhibition, where higher substrate concentration can outcompete the inhibitor for the active site.
Option 3: 'Malonate is a competitive inhibitor.' This aligns with the classic description: malonate resembles succinate and competes for the same active site on succinate dehydrogenase. When more succinate is present, it competes more effectively against malonate, reducing the inhibitor’s effect, which matches the given scenario.
Option 4: 'Malonate is an allosteric regulator.' An allosteric regulator binds at a site other than the active site and modulates activity; however, the evidence here shows competition at the active site, not regulation from a distant site. Thus this label would be inaccurate for this situation.
Overall, the observation that increasing succinate concentration diminishes malonate’s inhibition supports competitive inhibition and argues against noncompetitive inhibition or allosteric regulation. The claim that malonate blocks fumarate binding is inconsistent with the stoichiometry of the cycle and the nature of competitive inhibitors, which act at the substrate-binding site rather than on the product.

W25 BIOL D006B 01Y Cell & Molecular Biology Lecture Exam 1

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