Aspartate aminotransferase catalyzed oxygen exchange with solvent from oxygen-18-enriched alpha-ketoglutarate: evidence for slow exchange of enzyme-bound water.
Partitioning of the ketimine (or ketimine + quinonoid) intermediate(s) in the mitochondrial aspartate aminotransferase reactions was investigated by following the rates of loss of 18O from carbonyl-18O-enriched alpha-ketoglutarate together with the rate of L-glutamate formation. The ratio of these rate constants was found to equal 1 at 10 degrees C, implying that the above intermediate(s) face(s) equal barriers with respect to the forward and reverse reactions. This partition ratio of 1 together with that measured from the alpha-amino acid side of the reaction [Julin, D.A., Wiesinger, H., Toney, M. D., & Kirsch, J.F. (1989) Biochemistry (preceding paper in this issue)] suggests that the rate constant for exchange of alpha-ketoglutarate-derived H2(18)O from the ketimine (or ketimine + quinonoid) form(s) of the enzyme with solvent is comparable with that for kcat.
McLeish
M J
MJ
Department of Biochemistry, University of California, Berkeley 94720.
Julin
D A
DA
Kirsch
J F
JF
D000818:Animals; D001219:Aspartate Aminotransferases / Q000378:metabolism*; D005971:Glutamates / Q000378:metabolism; D018698:Glutamic Acid; D007553:Isotope Labeling / Q000379:methods; D007656:Ketoglutaric Acids / Q000378:metabolism; D007700:Kinetics; D008433:Mathematics; D008929:Mitochondria, Heart / Q000201:enzymology; D008962:Models, Theoretical; D010103:Oxygen Isotopes; D011485:Protein Binding; D012997:Solvents; D013552:Swine; D014316:Tritium
D005971:Glutamates; D007656:Ketoglutaric Acids; D010103:Oxygen Isotopes; D012997:Solvents; D014316:Tritium; D018698:Glutamic Acid; D001219:Aspartate Aminotransferases