Determination of the binding sites of the proton transfer inhibitors Cd2+ and Zn2+ in bacterial reaction centers

The reaction center (RC) from Rhodobacter sphaeroides couples light-driven electron transfer to protonation of a bound quinone acceptor molecule, Q(B) , within the RC. The binding of Cd2+ or Zn2+ has been previously shown to i nhibit the rate of reduction and protonation of Q(B). We report here on the metal binding site, determined by x-ray diffraction at 2.5-Angstrom resolu tion, obtained from RC crystals that were soaked in the presence of the met al. The structures were refined to R factors of 23% and 24% for the Cd2+ an d Zn2+ complexes, respectively. Both metals bind to the same location, coor dinating to Asp-H124, His-H126, and His-H128. The rate of electron transfer from Q(A)(-) to Q(B) was measured in the Cd2+-soaked crystal and found to be the same as in solution in the presence of Cd2+. In addition to the chan ges in the kinetics, a structural effect of Cd2+ on Glu-H173 was observed. This residue was well resolved in the x-ray structure-i.e., ordered-with Cd 2+ bound to the RC, in contrast to its disordered state in the absence of C d2+, which suggests that the mobility of Glu-H173 plays an important role i n the rate of reduction of QB. The position of the Cd2+ and Zn2+ localizes the proton entry into the RC near Asp-H124, His-H126, and His-H128. Based o n the location of the metal, likely pathways of proton transfer from the aq ueous surface to Q(B)(radical anion) are proposed.