Stopped-flow reaction kinetics of recombinant components of proton-translocating transhydrogenase with physiological nucleotides

New information on the high resolution structure of the membrane proton pump, transhydrogenase, now provides a framework for understanding kinetic descriptions of the enzyme. Here, we have studied redox reactions catalyzed by mixtures of the recombinant NAD(H)-binding component (dI) of Rhodospirillum rubrum transhydrogenase, and the recombinant NADP(H)-binding component (dIII) of either the R. rubrum enzyme or the human enzyme. By recording changes in the fluorescence emission of native and engineered Trp residues, the rates of the redox reaction with physiological nucleotides have been measured under stopped-flow conditions, for the first time. Rate constants for the binding reaction between NAD(+)/NADH and the R. rubrum dI dIII complex are much greater than those between nucleotide and isolated dl. For the redox step between the physiological nucleotides on the R rubrum dI dIII complex, the rate constant in the forward direction, k(f) approximate to 2900 s(-1), and that for the reverse reaction, k(r) approximate to 110 s(-1). Comparisons with reactions involving an analogue of NAD(H) indicate that the rate constants at this step are strongly affected by the redox driving force.