title: Determination of the binding sites of the proton transfer inhibitors Cd2+ and Zn2+ in bacterial reaction centers.
abstract: 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 Cd(2+) or Zn(2+) has been previously shown to inhibit 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-A resolution, obtained from RC crystals that were soaked in the presence of the metal. The structures were refined to R factors of 23% and 24% for the Cd(2+) and Zn(2+) complexes, respectively. Both metals bind to the same location, coordinating to Asp-H124, His-H126, and His-H128. The rate of electron transfer from Q(A)(-) to Q(B) was measured in the Cd(2+)-soaked crystal and found to be the same as in solution in the presence of Cd(2+). In addition to the changes in the kinetics, a structural effect of Cd(2+) 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 Cd(2+), which suggests that the mobility of Glu-H173 plays an important role in the rate of reduction of Q(B). The position of the Cd(2+) and Zn(2+) 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 Q(B) are proposed.
authors.affiliation: Department of Physics 0319, 9500 Gilman Drive, University of California at San Diego, La Jolla, CA 92093, USA.
authors.forename: H L
authors.initials: HL
authors.lastname: Axelrod
authors.forename: E C
authors.initials: EC
authors.lastname: Abresch
authors.forename: M L
authors.initials: ML
authors.lastname: Paddock
authors.forename: M Y
authors.initials: MY
authors.lastname: Okamura
authors.forename: G
authors.initials: G
authors.lastname: Feher
mesh_terms: D001665:Binding Sites; D002104:Cadmium / Q000494:pharmacology; D004579:Electron Transport; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D045322:Photosynthetic Reaction Center Complex Proteins / Q000037:antagonists & inhibitors* / Q000737:chemistry; D011522:Protons; D011809:Quinones / Q000378:metabolism; D012242:Rhodobacter sphaeroides; D014961:X-Ray Diffraction; D015032:Zinc / Q000494:pharmacology
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D011522:Protons; D011809:Quinones; D002104:Cadmium; D015032:Zinc