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Hydrophobic mismatch influencing the structure-function relationship of the reaction center from photosynthetic bacterium Rhodobacter sphaeroides

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  • Biology
  • Chemistry
  • Physics

Abstract

The photosynthetic organisms consist a cluster of membrane bound protein-pigment complexes, which utilize absorbed photons to drive electron transfer reactions to convert light energy into chemical energy. In the photosynthetic bacterial reaction center from Rhodobacter sphaeroides , light induces a transfer of an electron from the primary electron donor, a special pair of two bacteriochorophylls forming a dimer, through a series of intermediate electron acceptors and a primary quinone Q A , to a reversibly-bound quinone Q B . Reaction centers from Rhodobacter sphaeroides have been reconstituted into various liposomes, with varying fatty acid chain lengths from C 12 to C 18 , resulting in different hydrophobic thicknesses of the lipid bilayer. Compensations are expected both from the lipid and the protein if the hydrophobic thicknesses are not matched. Lipid-protein interactions were explored and identified due to this hydrophobic mismatch by studying the phase behavior of the lipid and probing the function of the protein. The optimal thickness for the membrane, using saturated phospholipids to incorporate the bacterial reaction center, was found to be equivalent to a carbon length of C 14 . Prolonged illumination induced conformational rearrangements in the protein structure. Lipid environment, acidic pH, long illumination, and low temperature favored the formation of the long-lived charge separated state. This light-adapted conformation had a lifetime up to 8.9 hours, which is three million times as much as the lifetime of the dark-adapted charge separated state induced by flash excitations.

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