Citrate synthase is the enzyme you are looking for. Because it regulates the first step of the Krebs cycle, this enzyme is a key regulator of rate of respiration in the mitochondria of cells. The reaction it modulates is a condensation reaction. It is believed that the acetyl Co-A attaches to the citrate synthase molecule first. This attachment changes the enzyme and increases its affinity for oxaloacetate by a factor of about 20. Once the oxaloacetate binds to the enzyme, the citrate synthase molecule changes shape and becomes nearly round, bringing the acetyl Co-A and the oxaloacetate into the correct position to bond, producing a new six-carbon molecule of citrate. The citrate molecule is then released into the Krebs cycle. Citrate synthase activity is inhibited by high levels of ATP, and also by the presence of its own product, citrate.