Variant properties of bovine liver 2-keto-4-hydroxyglutarate aldolase; its [beta]-decarboxylase activity, lack of substrate stereospecificity, and structural requirements for binding substrate analogs

Abstract

1. 1. 2-Keto-4-hydroxyglutarate aldolase (2-oxo-4-hydroxyglutarate glyoxylatelyase; reaction: 2-oxo-4-hydroxyglutarate [left harpoon over right] pyruvate + glyoxylate) catalyzes the cleavage and the formation of both optical isomers of 2-keto-4-hydroxyglutarate at the same rate and to the same extent.2. 2. The specificity of azomethine (Schiff base) formation with this enzyme was studied. Of some forty compounds tested, inactivation in the presence of NaBH4 occurs (in order of decreasing effectiveness) with 2-keto-4-hydroxy-4-methylglutarate, 2-ketoglutarate, 2-keto-4-hydroxybutyrate, 2-keto-3-deoxy-6-phosphogluconate, fructose 1,6-diphosphate, 2-keto-4,5-dihydroxyvalerate, 2-keto-3-deoxygluconate, and 5-keto-4-deoxyglucarate (among 2-keto-4-hydroxyglutarate analogs); only with bromopyruvate and 2-ketobutyrate (among pyruvate analogs); and also with glyoxal, formaldehyde, acetaldehyde, and glycolaldehyde (glyoxylate analogs). In this regard, therefore, a high degree of specificity is shown for pyruvate but not for glyoxylate; this aldolase is also quite specific for analogs of 2-keto-4-hydroxyglutarate having a pyruvate-like structure on one end of the molecule.3. 3. 2-Keto-4-hydroxyglutarate aldolase actually catalyzes the cleavage of only 2-keto-4,5-dihydroxyvalerate, 2-keto-4-hydroxy-4-methylglutarate, 5-keto-4-deoxyglucarate, 2-keto-3-deoxy-6-phosphogluconate, and 2-keto-4-hydroxybutyrate at 33%, 8%, 3%, 2% and 1%, respectively, the rate of 2-keto-4-hydroxyglutarate cleavage. For certain substrate analogs, therefore, there is a dissociation of azomatheine formation from a concurrent cleavage of that compound.4. 4. In addition, this purified aldolase was found to catalyze the [beta]-decarboxylation of oxaloacetate at 50% the rate of 2-keto-4-hydroxyglutarate cleavage.