3prc
PHOTOSYNTHETIC REACTION CENTER FROM RHODOPSEUDOMONAS VIRIDIS (QB-DEPLETED)
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OverviewOverview
BACKGROUND: In a reaction of central importance to the energetics of, photosynthetic bacteria, light-induced electron transfer in the reaction, centre (RC) is coupled to the uptake of protons from the cytoplasm at the, binding site of the secondary quinone (QB). In the original structure of, the RC from Rhodopseudomonas viridis (PDB entry code 1PRC), the QB site, was poorly defined because in the standard RC crystals it was only, approximately 30% occupied with ubiquinone-9 (UQ9). We report here the, structural characterization of the QB site by crystallographic refinement, of UQ9-depleted RCs and of complexes of the RC either with ubiquinone-2, (UQ2) or the electron-transfer inhibitor stigmatellin in the QB site., RESULTS: The structure of the RC complex with UQ2, refined at 2.45 A, resolution, constitutes the first crystallographically reliably defined, binding site for quinones from the bioenergetically important quinone pool, of biological, energy-transducing membranes. In the UQ9-depleted QB site, of the RC structure, refined at 2.4 A resolution, apparently five (and, possibly six) water molecules are bound instead of the ubiquinone head, group, and a detergent molecule binds in the region of the isoprenoid, tail. All of the protein-cofactor interactions implicated in the binding, of the ubiquinone head group are also implicated in the binding of the, stigmatellin head group. In the structure of the stigmatellin-RC complex, refined at 2.4 A resolution, additional hydrogen bonds stabilize the, binding of stigmatellin over that of ubiquinone. The tentative position of, UQ9 in the QB site in the original data set (1PRC) was re-examined using, the structure of the UQ9-depleted RC as a reference. A modified QB site, model, which exhibits greater similarity to the distal ubiquinone-10, (UQ10) positioning in the structure of the RC from Rhodobacter sphaeroides, (PDB entry code 1PCR), is suggested as the dominant binding site for, native UQ9. CONCLUSIONS: The structures reported here can provide models, of quinone reduction cycle intermediates. The binding pattern observed for, the stigmatellin complex, where the ligand donates a hydrogen bond to Ser, L223 (where 'L' represents the L subunit of the RC), can be viewed as a, model for the stabilization of a monoprotonated reduced intermediate (QBH, or QBH-). The presence of Ser L223 in the QB site indicates that the QB, site is not optimized for QB binding, but for QB reduction to the quinol.
About this StructureAbout this Structure
3PRC is a Protein complex structure of sequences from Blastochloris viridis with FE2, SO4, BCB, BPB, MQ7, HEM, NS5 and LDA as ligands. Full crystallographic information is available from OCA.
ReferenceReference
The coupling of light-induced electron transfer and proton uptake as derived from crystal structures of reaction centres from Rhodopseudomonas viridis modified at the binding site of the secondary quinone, QB., Lancaster CR, Michel H, Structure. 1997 Oct 15;5(10):1339-59. PMID:9351808
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