Effect of metal binding on electrogenic proton transfer associated with reduction of the secondary electron acceptor (QB) in Rhodobacter sphaeroides chromatophores.
The influence of metal ion (Cd(2+), Zn(2+), Ni(2+)) binding on the electrogenic phases of proton transfer connected with reduction of quinone Q(B) in chromatophores from Rhodobacter sphaeroides was studied by time-resolved electric potential changes. In the presence of metals, the electrogenic transients associated with proton transfer on first and second flash at pH 8 were found to be slower by factors of 3-6. This is essentially the same effect of metal binding that was observed on optical transients in isolated reaction centers (RC), where the metal ion was shown to inhibit proton transfer [Paddock, M. L., Graige, M. S., Feher, G., and Okamura, M. Y. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 6183-6188]. The effect of metal binding on the kinetics in chromatophores is, therefore, similarly attributed to inhibition of proton uptake, which becomes rate-limiting. A striking observation was an increase in the amplitude of the electrogenic proton-uptake phase after the first flash with bound metal ion. We attribute this to a loss of internal proton rearrangement, requiring that the protons that stabilize Q(B)(-) come from solution. In mutant RCs, in which His-H126 and His-H128 are replaced with Ala, the apparent binding of Cd(2+) and Ni(2+) was decreased, showing that the binding site of these metal ions is the same as found in RC crystals [Axelrod, H. L., Abresch, E. C., Paddock, M. L., Okamura, M. Y., and Feher, G. (2000) Proc. Natl. Acad. Sci. U.S.A. 97, 1542-1547]. Therefore, the unique proton entry point near His-H126, His-H128, and Asp-M17 that was identified in isolated RCs is also the entry point in chromatophores.
Keller
S
S
CEA Saclay, Section de Bioénergétique, 91191 Gif-sur-Yvette, France.
Beatty
J T
JT
Paddock
M
M
Breton
J
J
Leibl
W
W
D000409:Alanine / Q000235:genetics; D020130:Bacterial Chromatophores / Q000737:chemistry* / Q000235:genetics; D001665:Binding Sites / Q000235:genetics; D002104:Cadmium / Q000737:chemistry; D006639:Histidine / Q000235:genetics; D019216:Metals, Heavy / Q000737:chemistry*; D016297:Mutagenesis, Site-Directed; D009532:Nickel / Q000737:chemistry; D010084:Oxidation-Reduction; D010777:Photochemistry / Q000295:instrumentation / Q000379:methods; D010782:Photolysis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000235:genetics; D011522:Protons; D011809:Quinones / Q000737:chemistry*; D012242:Rhodobacter sphaeroides / Q000737:chemistry* / Q000235:genetics; D015032:Zinc / Q000737:chemistry
D019216:Metals, Heavy; D045322:Photosynthetic Reaction Center Complex Proteins; D011522:Protons; D011809:Quinones; D002104:Cadmium; D006639:Histidine; D009532:Nickel; D015032:Zinc; D000409:Alanine