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Step-by-Step Explanation
Step 1: Context of the TCA Cycle
The tricarboxylic acid (TCA) cycle, also known as the citric acid cycle or Krebs cycle, is a central metabolic pathway in aerobic organisms. Most enzymes of the TCA cycle are found in the mitochondrial matrix of eukaryotic cells.
Step 2: Identifying the Exception
While nearly all TCA cycle enzymes reside in the mitochondrial matrix, succinate dehydrogenase is unique. In eukaryotes, this enzyme is located on the inner mitochondrial membrane (specifically as part of Complex II in the electron transport chain), rather than free in the matrix. In prokaryotes, it is found in the cytosol (their version of the respiratory membrane differs from that in eukaryotes).
Step 3: Role of Succinate Dehydrogenase
Succinate dehydrogenase plays a dual role:
It catalyzes the oxidation of succinate to fumarate in the TCA cycle.
It also functions in the electron transport chain (ETC) by transferring electrons from succinate to ubiquinone (coenzyme Q). Hence, it is often called succinate: ubiquinone oxidoreductase or Complex II.
Step 4: Features of the Enzyme
Succinate dehydrogenase contains:
A flavin adenine dinucleotide ($\text{FAD}$) prosthetic group, which accepts and donates electrons.
Three iron-sulfur clusters ($[2Fe-2S]$, $[4Fe-4S]$, and $[3Fe-4S]$) that mediate electron transfer from succinate to ubiquinone.
Step 5: Conclusion
Because succinate dehydrogenase is the only TCA cycle enzyme that is membrane-bound in eukaryotes (inner mitochondrial membrane) and located in the cytosol of prokaryotes, it is the correct answer to the question of which TCA cycle enzyme is not solely located in the mitochondrial matrix.
