In photosynthetic reaction centers, the free energy difference for electron transfer between quinones bound at the primary and secondary quinone-binding sites governs the observed secondary site specificity.
The secondary quinone-binding site (QB site) of bacterial reaction centers from Rhodobacter sphaeroides is generally regarded to be highly specific for its native ubiquinone-10 molecule. We demonstrate here that this is a misconception rooted in the kinetic methods used to assay for occupancy of a quinone in the QB site. We show that observance of occupancy of the QB site, revealed by kinetic assay, is sensitive to the free-energy difference for electron transfer between the quinone at the primary quinone-binding site (QA site) and the QB site (-delta G0e-). For many of the compounds previously tested for binding at the QB site, the -delta G0e- between QA and QB is too small to permit detection of the functional quinone in the QB site. With an increased -delta G0e- achieved by replacing the native ubiquinone-10 at the QA site with lower-potential quinones or by testing higher-potential QB candidates, it is shown that the QB site binds and functions with the unsubstituted 1,4-benzoquinone, 1,4-naphthoquinone, and 9,10-phenanthraquinone, as well as with their various substituted forms. Moreover, quinones with the ortho-carbonyl configuration appear to function in a similar manner to quinones with the para-carbonyl configuration.
Giangiacomo
K M
KM
Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia 19104.
Dutton
P L
PL
D001426:Bacterial Proteins / Q000502:physiology*; D001429:Bacteriochlorophylls / Q000502:physiology*; D002734:Chlorophyll / Q000031:analogs & derivatives*; D004579:Electron Transport; D045342:Light-Harvesting Protein Complexes; D010084:Oxidation-Reduction; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins; D011809:Quinones / Q000502:physiology*; D012241:Rhodopseudomonas / Q000502:physiology*; D013057:Spectrum Analysis; D013816:Thermodynamics
D001426:Bacterial Proteins; D001429:Bacteriochlorophylls; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D011809:Quinones; D002734:Chlorophyll