A 28-year-old male with a family history of mitochondrial myopathy presents with progressive muscle weakness and fatigue. Genetic testing reveals a mutation in the gene encoding the enzyme marked **C** in the Krebs cycle diagram. Which of the following best explains why a defect in this enzyme is particularly severe compared to other Krebs cycle enzyme deficiencies?

Explanation

## Why option 1 is correct The enzyme marked **C** is succinate dehydrogenase, the only Krebs cycle enzyme that is embedded in the inner mitochondrial membrane. This unique positioning allows it to function as both a TCA cycle enzyme AND Complex II of the electron transport chain. Critically, succinate dehydrogenase uses FAD as its electron acceptor (producing FADH₂), not NAD+, making it the direct bridge between oxidative phosphorylation and the citric acid cycle. A mutation here disrupts not only TCA cycle function but also impairs electron transport chain activity, explaining the severe mitochondrial phenotype. This dual role is why defects in this enzyme cause particularly severe energy metabolism dysfunction (Harper 32e, Ch 17). ## Why each distractor is wrong - **Option 2**: Citrate synthase (enzyme A) catalyzes the first committed step and is the primary regulatory point, not succinate dehydrogenase. While succinate dehydrogenase is regulated, it is not the rate-limiting step of the cycle. - **Option 3**: Succinate dehydrogenase is NOT the rate-limiting enzyme of the Krebs cycle (that is isocitrate dehydrogenase, enzyme B), and it uses FAD as the electron acceptor, not NAD+. This is a common student confusion. - **Option 4**: Malate dehydrogenase (enzyme D) catalyzes the final step and regenerates oxaloacetate (not acetyl-CoA). This describes the wrong enzyme entirely. **High-Yield:** Succinate dehydrogenase = only membrane-bound TCA enzyme = Complex II of ETC = uses FAD, not NAD+ = bridges TCA and ETC. [cite: Harper 32e Ch 17]