A 28-year-old male presents with progressive muscle weakness and exercise intolerance. Genetic testing reveals a mutation in the gene encoding the structure marked **C** in the Krebs cycle diagram. Which of the following best explains why this enzyme is uniquely positioned to bridge the TCA cycle and the electron transport chain?

Explanation

## Why option 1 is correct The structure marked **C** is succinate dehydrogenase, the only Krebs cycle enzyme embedded in the inner mitochondrial membrane. Unlike other TCA cycle enzymes (citrate synthase, isocitrate dehydrogenase, malate dehydrogenase) which use NAD+ as the electron acceptor, succinate dehydrogenase uniquely uses FAD as the electron acceptor, producing FADH₂. This direct membrane localization and FAD-dependent mechanism allow it to function as Complex II of the electron transport chain, creating the critical link between the TCA cycle and oxidative phosphorylation. This dual role is why mutations in succinate dehydrogenase cause mitochondrial myopathy and exercise intolerance (Harper 32e, Ch 17). ## Why each distractor is wrong - **Option 2**: While other Krebs cycle enzymes are located in the mitochondrial matrix and use NAD+, succinate dehydrogenase is distinctly embedded in the inner membrane and uses FAD, not NAD+. This is the defining feature that makes it unique. - **Option 3**: Succinate dehydrogenase does not use cytochrome c as an electron acceptor; it uses FAD. Cytochrome c is part of the ETC downstream of Complex II, not a direct substrate for this enzyme. - **Option 4**: While allosteric regulation by ADP/AMP is important for other Krebs cycle enzymes (particularly isocitrate dehydrogenase and citrate synthase), the unique bridging function of succinate dehydrogenase is defined by its membrane location and FAD-dependent mechanism, not by allosteric regulation. **High-Yield:** Succinate dehydrogenase = only membrane-bound TCA enzyme = uses FAD (not NAD+) = Complex II of ETC = the TCA-ETC bridge. [cite: Harper 32e, Ch 17]