Determination of the binding sites of the proton transfer inhibitors Cd2+ and Zn2+ in bacterial reaction centers
AUTOR(ES)
Axelrod, H. L.
FONTE
The National Academy of Sciences
RESUMO
The reaction center (RC) from Rhodobacter 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 to R 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.
ACESSO AO ARTIGO
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=26471Documentos Relacionados
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