Structural basis of the drastically increased initial electron transfer rate in the reaction center from a Rhodopseudomonas viridis mutant described at 2.00-Å resolution

It has previously been shown that replacement of the residue His L168 with Phe (HL168F) in the Rhodopseudomonas viridis reaction center (RC) leads to an unprecedented drastic acceleration of the initial electron transfer rate. Here we describe the determination of the x-ray crystal structure at 2.00-Angstrom resolution of the HL168F RC. The electron density maps confirm that a hydrogen bond from the protein to the special pair is removed by this mutation. Compared with the wild-type RC, the acceptor of this hydrogen bond, the ring I acetyl group of the special pair bacteriochlorophyll, D-L, is rotated, and its acetyl oxygen is found 1.1 Angstrom closer to the bacteriochlorophyll-Mg2+ Of the other special pair bacteriochlorophyll, D-M. The rotation of this acetyl group and the increased interaction between the D-L ring I acetyl oxygen and the D-M-Mg2+ provide the structural basis for the previously observed 80-mV decrease in the D+/D redox potential and the drastically increased rate of initial electron transfer to the accessory bacteriochlorophyll, B-A The high quality of the electron density maps also allowed a reliable discussion of the mode of binding of the triazine herbicide terbutryn at the binding site of the secondary quinone, Q(B).