UBIQUINONE REDUCTION AND PROTONATION IN PHOTOSYNTHETIC REACTION CENTERS FROM RHODOPSEUDOMONAS-VIRIDIS - X-RAY STRUCTURES AND THEIR FUNCTIONAL IMPLICATIONS

In the photosynthetic bacterial reaction centre (RC), light-induced el ectron transfer is coupled to the uptake of protons from the cytoplasm at the binding site of the secondary quinone (Q(B)). Recent crystal s tructures of RCs from Rhodopseudomonas (Rp.) viridis modified at the Q (B) site (Lancaster and Michel, Structure 5 (1997) 1339-1359) can prov ide models for intermediates in the reaction cycle of ubiquinone reduc tion to ubiquinol. The structures of these intermediates provide expla nations for their relative binding affinities as required for proper e nzymatic function of the Q(B) site. The structure of the RC complex wi th ubiquinone-2 refined at 2.45 Angstrom resolution (Protein Data Bank (PDB) entry code 2PRC), constitutes the first crystallographically re liably defined binding site for quinones from the bioenergetically imp ortant quinone pool of biological, energy-transducing membranes. In th e RC structure with a Q(B)-depleted Q(B) site (PDB entry code 3PRC), r efined at 2.4 Angstrom resolution, apparently five, possibly six, wate r molecules are bound instead of the ubiquinone head group, and a dete rgent molecule binds in the region of the isoprenoid tail. Using the s tructures 2PRC and 3PRC as a reference, the original data set (1PRC, D eisenhofer et al., J. Mel. Biol. 246 (1995) 429-457) was re-examined, resulting in the suggestion of a modified dominant Q(B)-binding positi on for the native ubiquinone-9, which differs from that determined for ubiquinone-2. The RC-complex with the inhibitor stigmatellin (PDB ent ry 4PRC), refined at 2.4 Angstrom resolution, indicates that additiona l hydrogen bonds stabilise the binding of stigmatellin over that of ub iquinone-2. The binding pattern observed for the stigmatellin complex can be viewed as a model for the stabilisation of a monoprotonated red uced intermediate (Q(B)H or Q(B)H(-)). This indicates that the Q(B) si te is not optimised for Q(B) binding, but for Q(B) reduction to the qu inol. (C) 1998 Elsevier Science B.V.