title: Tritium exchange reactions catalyzed by 2-oxo-4-hydroxyglutarate aldolase from Escherichia coli K-12.
abstract: Tritiated water and tritiated substrates have been used to study exchange reactions catalyzed by Escherichia coli 2-oxo-4-hydroxyglutarate aldolase (4-hydroxy-2-oxoglutarate glyoxylate-lyase, EC 4.1.3.16, 2-oxo-4-hydroxyglutarate in equilibrium pyruvate + glyoxylate). With pyruvate, the enzyme catalyzes a rapid first-order exchange of all three methyl hydrogens in the absence of added acceptor aldehyde (i.e. glyoxylate). This reaction is not rate limiting for aldol condensation or cleavage; quite different pH-activity profiles for the exchange reaction versus aldol cleavage and also comparative effects that pH changes have on Km and V values for the two processes favor this conclusion. The exchange reaction with 2-oxobutyrate, a substrate analog, is stereoselective; one methylene hydrogen is removed at a 6-fold faster rate than the other but eventually both are exchanged. No tritium exchange occurs with glyoxylate.
mesh_terms: D004926:Escherichia coli / Q000201:enzymology*; D007553:Isotope Labeling; D007656:Ketoglutaric Acids; D007700:Kinetics; D007652:Oxo-Acid-Lyases / Q000378:metabolism*; D011773:Pyruvates; D014316:Tritium
chemical_list: D007656:Ketoglutaric Acids; D011773:Pyruvates; D014316:Tritium; D007652:Oxo-Acid-Lyases; C021415:4-hydroxy-2-oxoglutarate aldolase