Pumping capacity of bacterial reaction centers and backpressure regulation of energy transduction

Abstract

Transduction of free-energy by Rhodobacter sphaeroides reaction-center-light-harvesting-complex-1 (RCLH1) was quantified. RCLH1 complexes were reconstituted into liposomal membranes. The capacity of the RCLH1 complex to build up a proton motive force was examined at a range of incident light intensities, and induced proton permeabilities, in the presence of artificial electron donors and acceptors. Experiments were also performed with RCLH1 complexes in which the midpoint potential of the reaction center primary donor was modified over an 85-mV range by replacement of the tyrosine residue at the M210 position of the reaction center protein by histidine, phenylalanine, leucine or tryptophan. The intrinsic driving force with which the reaction 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 transfer on the thermodynamic efficiency of the bacterial photosynthetic process. 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.Keywords: thermodynamics; bacterial photosynthesis; electron transport; reaction center; proton motive force.The fundamental event in photosynthesis is the conversion of light-energy into (Gibbs) free-energy of protons. This process of energy transduction takes place in plants, algae and certain species of bacteria. The transmembrane electron transfer, or charge separation, process occurs in specialized pigment±protein complexes collectively known as photosynthetic reaction centers. The best understood reaction center is that of the purple photosynthetic bacteria, and in particular of the bacterium Rhodobacter sphaeroides. Not only the atomic structure of this reaction center is known [1±3] but it has also been subjected to intensive spectroscopic study. The reaction center (RC) is intimately associated with the LH1 light-harvesting complex, to form the so-called RCLH1`core' complex.In RCLH1 core complexes, the photosynthetic process is initiated by the absorption of a photon, usually by the bacteriochlorophyll (Bchl) or carotenoid molecules of the LH1 antenna that are present in excess over the reaction center pigments. Excitation energy captured by the antenna pigments is transferred to the reaction center on the timescale of a few tens of picoseconds [4], creating the first singlet excited state of the reaction center primary donor, P, (P*) and so transforming P into a powerful reductant that is capable of reducing the adjacent cofactors.Transmembrane electron transfer is a multistep reaction involving the donation of an electron to the (Q A ) ubiquinone-A located within the reaction center protein on the opposite sid...

Membrane growth can generate a transmembrane pH gradient in fatty acid vesicles

“…As predicted, further acidification was observed upon addition of fresh micelles (Table 2). Taken together, these results indicate that vesicle growth was not limited by intrinsic properties of the membrane, but rather that growth stops when the ''back pressure'' of the proton gradient equals the driving force for growth (52,53). In this case, the vesicle interior and exterior were initially buffered at pH 8.0 with 0.2 M arginine-bicine.…”

Analysis of light-induced transmembrane ion gradients and membrane potential in Photosystem I proteoliposomes

“…For instance, the backpressure control exerted by the pH gradient, which limits the rate of proton translocation in these pumps [19], has not been observed in PSI preparations [4]. In addition, methods based on the thermodynamics are less attractive, since the model equations contain a number of phenomenological coefficients which are not easily available [20].…”

Photosynthetic models with maximum entropy production in irreversible charge transfer steps

“…The protonmotive force (output force) is fixed by using experimental set up with constant membrane potential and constant proton gradient across photosynthetic membrane. The black lipid membrane with incorporated photosynthetic proteins is such an experimental arrangement, where both external forces are easily controlled, but good control of external forces can be also achieved in reconstituted liposomal systems (Van Rotterdam et al, 2001).…”

“…In photosynthesis, charge separation is performed, and proton electrochemical gradient is created. For instance, in the purple photosynthetic bacterium Rhodobacter sphaeroides membrane bound electron transfer proteins, the reaction center and cytochrome bc 1 complex, couple electron transfer to proton release into the periplasmic space of the bacterium (Van Rotterdam et al, 2001). In even simpler photosynthesis, performed by the bacterium Halobacterium salinarium, photon free energy is directly converted into the electrochemical proton gradient by integral membrane protein bacteriorhodopsin (Lanyi and Luecke, 2001).…”