The thioester Acetyl Coenzyme A not only can undergo acyl transfer mechanisms initiated by nucleophilic attack upon the carbonyl carbon, Acetyl Coenzyme A is also reactive at the α carbon. In other words, in addition to serving as an acyl transfer substrate in biosynthesis, the thioester Acetyl Coenzyme A can also undergo reactions at the α carbon of the acetyl group. The condensation of oxaloacetate and acetyl CoA to form citryl CoA in the citric acid cycle occurs by means of nucleophilic attack by the α carbon of the enolate anion of acetyl CoA upon the carbonyl carbon of oxaloacetate to form a C-C bond. This is another reason why aldol condensation is such an important reaction for the MCAT.