1H NMR studies on ferricytochromec 3 fromDesulfovibrio vulgaris Miyazaki F and its interaction with ferredoxin I

Summary The¹H NMR signals of the heme methyl, propionate and related chemical groups of cytochromec₃ fromDesulfovibrio vulgaris Miyazaki F (D.v. MF) were site-specifically assigned by means of ID NOE, 2D DQFCOSY and 2D TOCSY spectra. They were consistent with the site-specific assignments of the hemes with the highest and second-lowest redox potentials reported by Fan et al. (Biochemistry,29 (1990) 2257–2263). The site-specific heme assignments were also supported by NOE between the methyl groups of these hemes and the side chain of Val¹⁸. All the results contradicted the heme assignments forD.v. MF cytochromec₃ made on the basis of electron spin resonance (Gayda et al. (1987)FEBS Lett.,217 57–61). Based on these assignments, the interaction of cytochromec₃ withD.v. MF ferredoxin I was investigated by NMR. The major interaction site of cytochromec₃ was identified as the heme with the highest redox potential, which is surrounded by the highest density of positive charges. The stoichiometry and association constant were two cytochromec₃ molecules per monomer of ferredoxin I and 10⁸ M^–2 (at 53 mM ionic strength and 25°C), respectively.