Almost every enzyme of the citric acid cycle is soluble. Which single enzyme is the exception, and where does it sit?
APyruvate dehydrogenase, in the inner membrane
BIsocitrate dehydrogenase, in the outer membrane
CSuccinate dehydrogenase, in the inner membrane
DMalate dehydrogenase, in the outer membrane
Answer & Solution
Correct answer: C. Succinate dehydrogenase, in the inner membrane
1. Soluble enzymes float free in the matrix, so an embedded one stands out.
2. Succinate dehydrogenase is the single exception and is embedded in the inner membrane of the mitochondrion.
3. That position lets its reduced carrier hand electrons straight to the electron transport chain.
4. Isocitrate dehydrogenase is a soluble regulatory enzyme of the same cycle, so it is not the exception.
5. Pyruvate dehydrogenase works before the cycle begins, converting pyruvate into an acetyl group.
6. Naming the outer membrane is the trap here, since the working membrane of respiration is the inner one.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 7 "Cellular Respiration", section 7.3 Oxidation of Pyruvate and the Citric Acid Cycle_
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