Cytochrome c orientation in electron-transfer complexes with photosynthetic reaction centers of Rhodobacter sphaeroides and when bound to the surface of negatively charged membranes: characterization by optical linear dichroism
Heme orientation with respect to the membrane normal has been measured for the cytochromes c and c/sub 2/ bound to photosynthetic reaction centers from Rhodobacter (rhodopseudomonas) sphaeroides R-26 in reconstituted phosphatidylcholine vesicles. Previous kinetic studies have suggested that each cyt...
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Veröffentlicht in: | Biochemistry (Easton) 1987, Vol.26 (2), p.397-410 |
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Zusammenfassung: | Heme orientation with respect to the membrane normal has been measured for the cytochromes c and c/sub 2/ bound to photosynthetic reaction centers from Rhodobacter (rhodopseudomonas) sphaeroides R-26 in reconstituted phosphatidylcholine vesicles. Previous kinetic studies have suggested that each cytochrome may bind in two configurations, which lead to either rapid or slow electron transfer to the flash-oxidized reaction center bacteriochlorophyll dimer. The rapid oxidation of cytochrome c is approx.20-fold slower than that of cytochrome c/sub 2/. Optical linear dichroism measurements reported here show that, for both cytochromes, only the population undergoing rapid oxidation is dichroic. A stoichiometry of 0.5 dichroic cytochrome c or c/sub 2/ is found per reaction center. Prominent differences between the dichroism of the cytochrome c/sub 2/-reaction center complex and that of the cytochrome c-reaction center complex show that heme orientation differs in the two cases. The dichroism of cytochrome c bound to the reaction center can be distinguished from its dichroism when bound to the surface of negatively charged membranes. Analysis of the dichroism spectra suggests that, for cytochrome c/sub 2/, the heme is tilted 7/sup 0/-8/sup 0/ closer to the membrane normal and rotated by 32/sup 0/ compared to the cytochrome c-reaction center complex. The dichroism spectra are consistent with the notion that the site on the cytochrome c surface that binds to the reaction center is the same site that binds to mammalian cytochrome c oxidase and reductase. However, a different locus is implicated on the surface of cytochrome c/sub 2/. These data suggest that although the tertiary structures of the cytochromes are homologous, the bind site is not conserved. |
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ISSN: | 0006-2960 1520-4995 |
DOI: | 10.1021/bi00376a010 |