The enzymes that catalyze lyase reactions on citrate to yield a C2 and a C4 unit are: citrate lyase, citrate synthase, and adenosine triphosphate (ATP) citrate lyase. Citrate lyase has been reported only in certain bacteria, citrate synthase has been found in all cells examined for it, and ATP citrate lyase has been found only in eukaryotic cells. The reaction catalyzed in common by these three enzymes is an aldol type, that is the reversible formation of a C—H bond from a C—C bond. It is possible that there exist some catalytic similarities in the three enzymatic reactions so that comparison of the results obtained with each enzyme might be useful in understanding the others. The citrate enzymes comprise a unique biological system in which three enzymes catalyze the same bond-breaking-making reaction. The fact that the simplest of these reactions, that catalyzed by citrate lyase, occurs in the most primitive organisms and that the most complex, the ATP citrate lyase reaction, occurs in the most recently evolved organisms, suggests a possible evolutionary relation between these enzymes.