The two carbon atoms that are lost as CO2 in the third and fourth steps of the citric acid cycle are the same as the two carbon atoms of acetyl CoA because of the stereochemistry of the isocitrate dehydrogenase reaction.

Question

Quizplus · Accepted Answer

The two carbon atoms lost as CO2 in the citric acid cycle are not the same two carbon atoms that enter the cycle as part of acetyl CoA. The cycle involves a series of transformations where the acetyl group (a 2-carbon molecule) from acetyl CoA combines with oxaloacetate (a 4-carbon molecule) to form citrate (a 6-carbon molecule). The carbon atoms lost as CO2 come from the original oxaloacetate and other intermediates, not directly from the acetyl group added.

The Two Carbon Atoms That Are Lost as CO2 in the Third