Of the enzymes of the citric acid cycle, which is not a freely soluble enzyme?

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Question restatement: Among the enzymes of the citric acid cycle, identify which one is not a freely soluble enzyme.

Option 1: citrate synthase
- This enzyme is soluble and located in the mitochondrial matrix. It catalyzes the condensation of acetyl-CoA with oxaloacetate to form citrate, and it operates in the matrix. There is no membrane-anchoring feature typical of this protein, so this option is not the non-soluble enzyme.

Option 2: succinate dehydrogenase
- This enzyme is a membrane-associated complex that spans the inner mitochondrial membrane and is part of both the citric acid cycle and the electron transport chain (as Complex II). Its core components are embedded in the membrane and it associates with the ubiquinone pool, making it not freely soluble in the mitochondrial matrix or cytosol. This membrane integration is the key reason this enzyme is not freely soluble.

Option 3: malate dehydrogenase
- Malate dehydrogenase is a soluble enzyme located in the mitochondrial matrix, participating in the malate-aspartate shuttle and NAD+-dependent oxidation of malate to oxaloacetate. It is not membrane-bound, so it is freely soluble.

Option 4: α-ketoglutarate dehydrogenase
- This enzyme is a multi-subunit complex located in the mitochondrial matrix, functioning similarly to other dehydrogenase complexes. It is soluble and not membrane-anchored, so it is freely soluble rather than membrane-bound.

Summary reasoning: The distinguishing feature here is membrane association. Among the listed CAC enzymes, succinate dehydrogenase is the only one that is embedded in the inner mitochondrial membrane as part of Complex II, whereas citrate synthase, malate dehydrogenase, and α-ketoglutarate dehydrogenase reside in the soluble mitochondrial matrix. Therefore, the non-freely soluble enzyme is the membrane-associated succinate dehydrogenase.

Of the enzymes of the citric acid cycle, which is not a freely soluble enzyme?

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