In the first step of the cycle, acetyl CoA combines with a four-carbon acceptor molecule, oxaloacetate, to form a six-carbon molecule called citrate. After a quick rearrangement, this six-carbon molecule releases two of its carbons as carbon dioxide molecules in a pair of similar reactions, producing a molecule of NADH each time.
The enzymes that catalyze these reactions are key regulators of the citric acid cycle, speeding it up or slowing it down based on the cell’s energy needs.
The remaining four-carbon molecule undergoes a series of additional reactions, first making an ATP molecule—or, in some cells, a similar molecule called GTP—then reducing the electron carrier FAD to FADH2,
and finally generating another NADH. This set of reactions regenerates the starting molecule, oxaloacetate, so the cycle can repeat.
Overall, one turn of the citric acid cycle releases two carbon dioxide molecules and produces three NADH, one FADH2, and one ATP or GTP. The citric acid cycle goes around twice for each molecule of glucose that enters cellular respiration because there are two pyruvates—and thus, two acetyl CoAs—made per glucose.
