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  • Title: Preferred order random kinetic mechanism for homoserine dehydrogenase of Escherichia coli (Thr-sensitive) aspartokinase/homoserine dehydrogenase-I: equilibrium isotope exchange kinetics.
    Author: Wedler FC, Ley BW, Shames SL, Rembish SJ, Kushmaul DL.
    Journal: Biochim Biophys Acta; 1992 Mar 12; 1119(3):247-9. PubMed ID: 1547269.
    Abstract:
    Isotope exchange kinetics at chemical equilibrium have been used to investigate the kinetic mechanism of homoserine dehydrogenase (EC 1.1.1.3) of the (Thr-sensitive) aspartokinase/homoserine dehydrogenase-I multifunctional enzyme from E. coli. For the reaction (L-ASA + NADPH + H+ = L-Hse + NADP+), at pH 9.0, 37 degrees C, Keq = 100 (+/- 20). Under these conditions, the rate for exchange of [14C]-L-homoserine (Hse) in equilibrium L-aspartate-beta-semialdehyde (ASA) is nearly twice that for the [3H]-NADP+ in equilibrium NADPH exchange. This indicates that covalent interconversion between reactants and products bound in the active site cannot be rate-limiting. Upon variation of the concentrations of all four substrates in constant ratio at equilibrium (to minimize dead-end complex formation), the Hse in equilibrium ASA exchange increased smoothly toward a maximum. In contrast, the NADP+ in equilibrium NADPH exchange rate increased to a maximum value at partial saturation, then decreased to approximately half the maximum rate. These data are consistent with a preferred-order random kinetic mechanism in which the dominant pathway involves association of NADPH prior to L-ASA and dissociation of L-Hse prior to NADP+.
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