Fig. 1.

Chemical transformations of water by the citric acid cycle. Eight enzymes (green text) catalyze a series of reactions that in total, consume three water molecules, produce one water molecule, protonate three water molecules, and convert an acetyl group to two carbon dioxide molecules. The series of reactions produces three NADH, an FADH₂, and a GTP. Unprotonated water molecules are indicated by red spheres. Protonated water molecules are blue. Water molecules that are mechanistically involved in the reactions are green (Color figure online)