The Krebs cycle (see image below) begins with the combination of the acetyl group of acetyl-CoA from the Krebs cycle preparation with the four-carbon compound oxaloacetate to form the six-carbon citric acid compound. Then the citrate is oxidized along the cycle by different reactions catalyzed by enzymes producing several intermediate molecules. During the cycle, 2 additional CO2 molecules, 1 ATP, 3 NADH, and 1 FADH2 molecule are formed per pyruvate molecule. In the end of the cycle, oxaloacetate is regenerated to return as an acetyl acceptor to re-start the cycle.
Figure 1: Steps in the Krebs cycle.