Krebs cycle step 1

First step: Citrate Synthase

Figure: Mechanism of the citrate formation

In the first reaction (see Figure) of the Krebs cycle, acetyl-CoA reacts with oxal acetate. Acetyl-CoA is coupled (yellow background) with the C2 atom of the oxal acetate and the C6 body citryl-CoA is formed. This is a high-energy thioester bound in the enzyme, and it is not released. In the second step of this reaction, the energy-rich thioester is hydrolyzed, and citrate and the coenzyme A (CoA-SH) were formed.

The mechanism of the citrate synthase prevents premature and undesirable hydrolysis of acetyl-CoA, and, consequently, a waste of energy.