Symmetry Breaking Structures Involved in the Docking of Cytochrome c and Primary Electron Transfer in Reaction Centers of Rhodobacter sphaeroides
A striking feature of Rb sphaeroides (1–4) and Rps viridis (5,6) reaction center structures is the approximate two-fold symmetry which relates the L and M protein subunits and the positions of the chromophores. Two sets of bacteriochlorophyll, B, bacteriopheophytin, H, and quinone, Q, molecules are positioned at nearly equivalent positions about the bacteriochlorophyll dimer, P. The chromophores are labelled L or M, according to whether they belong to the set predominately associated with the L or M subunit. Each set of chromophores potentially functions as an electron transfer pathway leading away from P. However the two-fold symmetry is only approximate. Significant differences have been noted in the protein environment surrounding the chromophores (2,6,7). These differences arise from changes in the type of amino acid residues at specific, symmetry equivalent positions on the L and M subunits. Small differences in the distances between chromophores on the L and M pathways were found in the Rps viridis structure after refinement using high resolution data (8).
KeywordsReaction Center Charged Residue Rhodobacter Sphaeroides Photosynthetic Reaction Centre Paracoccus Denitrificans
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