Pumping capacity of bacterial reaction centers and backpressure regulationof energy transduction

Transduction of free-energy by Rhodobacter sphaeroides reaction-center-ligh t-harvesting-complex-1 (RCLH1) was quantified. RCLH1 complexes were reconst ituted into liposomal membranes. The capacity of the RCLH1 complex to build up a proton motive force was examined at a range of incident light intensi ties, and induced proton permeabilities, in the presence of artificial elec tron donors and accepters. Experiments were also performed with RCLH1 compl exes in which the midpoint potential of the reaction center primary donor w as modified over an 85-mV range by replacement of the tyrosine residue at t he M210 position of the reaction center protein by histidine, phenylalanine , leucine or tryptophan. The intrinsic driving force with which the reactio n center pumped protons tended to decrease as the midpoint potential of the primary donor was increased. This observation is discussed in terms of the control of the energetics of the first steps in light-driven electron tran sfer on the thermodynamic efficiency of the bacterial photosynthetic proces s. The light intensity at which half of the maximal proton motive force was generated, increased with increasing proton permeability of the membrane. This presents the first direct evidence for so-called backpressure control exerted by the proton motive force on steady-state cyclic electron transfer through and coupled proton pumping by the bacterial reaction center.