Final answer:
The first reaction of the citric acid cycle is the condensation of acetyl-CoA and oxaloacetate to form citrate.
Step-by-step explanation:
The first reaction of the citric acid cycle is the condensation of acetyl-CoA and oxaloacetate to form citrate. This step is catalyzed by the enzyme citrate synthase. During this reaction, the two-carbon acetyl group from acetyl-CoA combines with the four-carbon oxaloacetate to produce a six-carbon molecule of citrate. The reaction is irreversible, highly exergonic, and controlled by negative feedback and the availability of ATP. If ATP levels are high, the rate of this reaction decreases, and if ATP is in short supply, the rate increases.