Enzyme stereoselectivity: The reversible reaction catalyzed by 2-keto-4-hydroxyglutarate aldolase of Escherichia coli

Abstract

SummaryThe condensation of [3-3H3]-pyruvate with glyoxylate in 3HOH to form [3-3H2]-ketohydroxyglutarate was catalyzed by ketohydroxyglutarate aldolase of Escherichia coli to study the stereochemical behavior of the enzyme. It was found that, a) very early in the reaction the (4S) predominates over the (4R) isomer in a ratio of about 10:1, b) during the course of the reaction the ratio of isomers decrease in parallel with the increase in product formation, c) at equilibrium the ratio of isomer distribution approaches 1:1. These results are consistent with the aldolase being stereo-selective and catalyzing a reversible reaction. Consequently, although kinetic restrictions demand that initially one isomer be turned-over with greater facility than its enantiomer, with the approach to thermodynamic equilibrium the rate ratio of isomer turnover approaches unity. The separation of kinetic from thermodynamic control in this case may enable further tests to be undertaken as to how Nature directs the stereochemistry of product synthesis in an aldolase catalyzed reaction.