The coupling between protonmotive force and the NAD(P)+transhydrogenase in chromatophores from photosynthetic bacteria

Abstract

1. The activity of NAD(P)+ transhydrogenase in chromatophores of Rhodobacter capsulatus relaxed from a high rate during illumination to a lower rate after darkening with a half-time of approximately 100 ms. 2. The dissipative ionic current flowing across the chromatophore membrane was increased in the presence of transhydrogenase substrates. This is attributed to proton current through the transhydrogenase enzyme. Subject to the assumption that transhydrogenase does not conduct in the absence of nucleotide substrates, the ratio of protons translocated across the membrane per hydride ion transferred was 0.4 .+-. 0.5. Within the error and uncertainties in the calibration procedure, this ratio may be consistent with a stoichiometry of one but higher values seem unlikely. The ratio of hydride ion transferred in the transhydrogenase to electrons transferred through the cyclic electron transport system was approximately 0.2. 3. The Kmapp values for the transhydrogenase substrates were determined for chromatophores in illuminated and darkened suspensions over a range of pH. These values are discussed in relation to the equivalent parameters reported for mitochondria transhydrogenase [Rydstrom, J. (1977) Biochim, Biophys. Acta 255, 9641-9646] and were used to calculate the concentrations of substrates which effectively saturate the enzyme. 4. At substrate concentrations which were in excess of 8 .times. Kmapp the dependence of transhydrogenase rate on the value of the membrane potential (zero pH gradient) was determined at pH 6.3, 6.9, 7.6 and 9.0. The relation was similar at pH 6.9 and 7.6. At alkaline pH the apparent threshold in the relation became more prominent as it was shifted to slightly higher values of membrane potential. At acid pH a shift in the opposite direction diminished the apparent threshold and saturation at high membrane potential became more dominant. We use these data in an attempt to discriminate between two models of energy transduction: (a) the driving force exerted by the membrane potential is mediated by a pH gradient formed through the operation of a proton well in the transhydrogenase; (b) the membrane potential increases a rate constant for charge translocation through transhydrogenase by decreasing the effective height of the Eyring barrier for charge transfer across the membrane through the enzyme. The second model leads to a more simple description than the first of the pH dependence of transhydrogenase rate on membrane potential. 5. Divalent cations at very low concentrations (.apprxeq. 0.1mM) or monovalent cations at higher concentrations (.apprxeq. 50 mM) stimulate considerably the transhydrogenase activity in chromatophores. The effect of monovalent and divalent cations on the relationship between the rate of transhydrogenase and membrane potential suggests that they do not influence the reaction by altering the membrane-potential-dependent rate constant.