Which one of the following best indicates why citrate needs to be isomerized?

Question

BlackTom AI · Accepted Answer

Question restatement: Which one of the following best indicates why citrate needs to be isomerized?

Options:
- so that it can be converted to acetyl CoA
- so that it can diffuse through lipid bilayers
- so that it can phosphorylate ADP
- so that it can be oxidatively decarboxylated
- so that it can be reduced

Analysis of each option:
- Option 1: "so that it can be converted to acetyl CoA". In cellular metabolism, citrate is not directly converted to acetyl-CoA by a simple isomerization step. Instead, citrate can be exported from the mitochondrion when levels are high and acetyl-CoA is generated for fatty acid synthesis, but the direct link to isomerization is not about acetyl-CoA formation. This makes the statement misleading as a primary reason for isomerization.
- Option 2: "so that it can diffuse through lipid bilayers". Citrate is a charged molecule and does not freely diffuse through lipid bilayers; transport occurs via specific carriers. Isomerization does not primarily serve to enable diffusion across membranes, so this is incorrect.
- Option 3: "so that it can phosphorylate ADP". Phosphorylation of ADP to ATP is not a direct consequence or requirement of citrate isomerization. The isomerization step does not directly involve phosphorylation reactions, so this option misstates the role of isomerization.
- Option 4: "so that it can be oxidatively decarboxylated". This is the core reason: citrate is isomerized to isocitrate by aconitase to place the molecule in the correct orientation for the subsequent oxidation and decarboxylation by isocitrate dehydrogenase, producing NADH and releasing CO2 in the citric acid cycle. The isomerization step is a preparatory rearrangement for oxidation and decarboxylation, making this option the correct rationale.
- Option 5: "so that it can be reduced". Reduction is not the immediate purpose of citrate isomerization in the TCA cycle; the next key step after isomerization is oxidative decarboxylation, not reduction of citrate. Therefore this option is inaccurate.

Summary: Among the choices, the only statement that correctly captures the functional purpose of isomerizing citrate in the metabolic pathway is that the isomerization prepares citrate for the subsequent oxidative decarboxylation to generate energy-rich substrates, aligning with the mechanism of the citric acid cycle.

Which one of the following best indicates why citrate needs to be isomerized?

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