Extended x-ray absorption fine structure (EXAFS) studies of cytochromes c: structural aspects of oxidation-reduction

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Abstract
EXAFS [extended X-ray absorption fine structure] fluorescence spectra were recorded for high-potential c-type cytochromes [from tuna heart, Rhodospirillum rubrum, Rhodomicrobacterium vanielii, Euglena gracilis, Chlorobium thiosulfatophilum] which range in oxidation-reduction potential from +145 to +365 mV. No average Fe-ligand bond length differences greater than 0.03 .ANG. were observed, for any cytochrome source or oxidation state. Least-squares analysis in combination with model calculations allowed limits to be set on the average Fe-N bond length (1.97-1.99 .ANG.) and the Fe-S bond length (2.29-2.32 .ANG.). A change of 0.05 .ANG. in either the average Fe-N or the Fe-S bond length is readily detectable with the range and quality of the data presented here. Two major conclusions are drawn from this study: In octahedrally coordinated Fe porphyrin systems, Fe.sbd.N and Fe.sbd.S bond lengths are independent of oxidation-reduction potential, and they are also independent of oxidation state. A model for the regulation of oxidation-reduction potential in cytochromes c is proposed.