Biochemistry Module 6 Practice Exam

What mechanism transports acetyl-CoA from mitochondria to cytosol for lipid synthesis?

Citrate shuttle exporting citrate from mitochondria to cytosol, where it is cleaved to yield acetyl-CoA. Direct transport of acetyl-CoA across inner mitochondrial membrane. Pyruvate transporter. Malate shuttle.

Correct Answer:

Citrate shuttle exporting citrate from mitochondria to cytosol, where it is cleaved to yield acetyl-CoA.

Explanation:

Acetyl-CoA itself can’t cross the inner mitochondrial membrane, so the cell moves the carbon for lipid synthesis as citrate. In the mitochondrion, acetyl-CoA condenses with oxaloacetate to form citrate. Citrate is then shuttled out of mitochondria into the cytosol, where ATP-citrate lyase cleaves it back to acetyl-CoA and oxaloacetate. The cytosolic acetyl-CoA is then used for fatty acid synthesis. The other routes don’t work for delivering acetyl units to the cytosol: there’s no direct acetyl-CoA transporter across the inner membrane, the malate shuttle mainly transfers reducing equivalents (NADH), and the pyruvate transporter moves pyruvate rather than acetyl groups.

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