Proton NMR spectroscopy of cytochrome c-554 from Alcaligenes faecalis

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Abstract
Cytochrome c-554 from the bacterium A. faecalis (ATCC 8750) is a respiratoory electron-transport protein homologous to other members of the cytochrome c family. Its structure was studied by 1H NMR spectroscopy in the ferric and ferrous states. The ferric spectrum is characterized by downfield hyperfine-shifted heme methyl resonances at 46.25, 43.60, 38.40 and 36.73 ppm (25.degree. C, pH 7.1). Chemical shifts of these resonances change with temperature opposite to expectations derived from Curie''s law. The pH behavior of the hyperfine-shifted resonances titrates with a pK of 6.3 that has been interpreted as due to ionization of a heme propionate. In the ferrous state, heme methyl, meso and thioether bridge resonances were observed and assigned. All aromatic proteins were assigned according to the side chain of origin and the structural environment about the sole tryptophan residue was examined. The electron-transfer rate between ferric and ferrous forms was estimated to be on the order or 3 .times. 108 M-1 s-1, which is the largest such self-exchange rate yet observed for a cytochrome.