Determination of the binding sites of the proton transfer inhibitors Cd 2+ and Zn 2+ in bacterial reaction centers

The reaction center (RC) fromRhodobacter sphaeroides couples light-driven electron transfer to protonation of a bound quinone acceptor molecule, QB , within the RC. The binding of Cd2+ or Zn2+ has been previously shown to inhibit the rate of reduction and protonation of QB . We report here on the metal binding site, determined by x-ray diffraction at 2.5-Å resolution, obtained from RC crystals that were soaked in the presence of the metal. The structures were refined toR factors of 23% and 24% for the Cd2+ and Zn2+ complexes, respectively. Both metals bind to the same location, coordinating to Asp-H124, His-H126, and His-H128. The rate of electron transfer from QA − to QB 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 changes 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 Cd2+ bound to the RC, in contrast to its disordered state in the absence of Cd2+ , which suggests that the mobility of Glu-H173 plays an important role in 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 on the location of the metal, likely pathways of proton transfer from the aqueous surface to QB ⨪ are proposed.