At what point in the citric acid cycle do the methyl carbon

At what point in the citric acid cycle do the methyl carbon from acetyl-CoA and the carbonyl carbon from oxaloacetate become chemically equivalent?

Solution

This happens with succinate. The structure of succinate is (CH₂-COO^-)_2. In the structure of succinate both carbon (one from methyl group of acetyl-CoA and one from oxaloacetate) are attached with 2 hydrogen atom and one COO^- group. Hence in the structure of succinate, both carbon are chemically equivalent.