TCB Publications - Abstract

TCB Publications - Abstract

Michaël L. Cartron, John D. Olsen, Melih Sener, Philip J. Jackson, Amanda A. Brindley, Pu Qian, Mark J. Dickman, Graham J. Leggett, Klaus Schulten, and C. Neil Hunter. Integration of energy and electron transfer processes in the photosynthetic membrane of Rhodobacter sphaeroides. Biochimica et Biophysica Acta - Bioenergetics, 1837:1769-1780, 2014. (PMC: PMC4143486)

CART2014 Photosynthesis converts absorbed solar energy to a protonmotive force, which drives ATP synthesis. The membrane network of chlorophyll–protein complexes responsible for light absorption, photochemistry and quinol (QH$_{2}$) production has been mapped in the purple phototrophic bacterium Rhodobacter (Rba.) sphaeroides using atomic force microscopy (AFM), but the membrane location of the cytochrome bc$_{1}$ (cyt bc$_{1}$) complexes that oxidise QH$_{2}$ to quinone (Q) to generate a protonmotive force is unknown. We labelled cytbc$_{1}$ complexes with gold nanobeads, each attached by a Histidine$_{10}$ (His$_{10}$)-tag to the C-terminus of cytc$_{1}$. Electron microscopy (EM) of negatively stained chromatophore vesicles showed that the majority of the cytbc$_{1}$ complexes occur as dimers in the membrane. The cyt bc$_{1}$ complexes appeared to be adjacent to reaction centre light-harvesting 1-PufX (RC-LH1-PufX) complexes, consistent with AFM topographs of a gold-labelled membrane. His-tagged cytbc$_{1}$ complexes were retrieved from chromatophores partially solubilised by detergent; RC-LH1-PufX complexes tended to co-purify with cyt bc$_{1}$, whereas LH2 complexes became detached, consistent with clusters of cyt bc$_{1}$ complexes close to RC-LH1-PufX arrays, but not with a fixed, stoichiometric cytc$_{1}$-RC-LH1-PufX supercomplex. This information was combined with a quantitative mass spectrometry (MS) analysis of the RC, cytbc$_{1}$, ATP synthase, cytaa$_{3}$ and cytbb$_{3}$ membrane protein complexes, to construct an atomic-level model of a chromatophore vesicle comprising 67 LH2 complexes, 11 LH1- RC-PufX dimers & 2 RC-LH1-PufX monomers, 4 cytbc$_{1}$ dimers and 2 ATP synthases. Simulation of the interconnected energy, electron and proton transfer processes showed a half-maximal ATP turnover rate for a light intensity equivalent to only 1% of bright sunlight. Thus, the photosystem architecture of the chromatophore is optimised for growth at low light intensities.

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