Aldol Cleavage in Water and the Power of Citrate Lyase as a Catalyst

Citrate lyase allows Klebsiella aerogenes to grow anaerobically on citrate as the sole carbon source. Arrhenius analysis of experiments at high temperatures indicates that citrate is cleaved nonenzymatically to acetate and oxaloacetate with a t 1/2 of 6.9 million years in neutral solution at 25 °C, while malate cleavage occurs even more slowly (t 1/2 = 280 million years). However, t 1/2 is only 10 days for the nonenzymatic cleavage of 4-hydroxy-2-ketoglutarate, indicating that the introduction of an α-keto group enhances the rate of aldol cleavage of malate by a factor of 1010. The aldol cleavages of citrate and malate, like the decarboxylation of malonate (t 1/2 = 180 years), are associated with a near-zero entropy of activation, and their extreme differences in rate reflect differences between their heats of activation. Citrate lyase enhances the rate of substrate cleavage 6 × 1015-fold, comparable in magnitude with the rate enhancement produced by OMP decarboxylase, although these enzymes are strikingly different in their mechanisms of action.