NMR-STUDIES AND REDOX TITRATION OF THE TETRAHEME CYTOCHROME C(3) FROM DESULFOMICROBIUM-BACULATUM - IDENTIFICATION OF THE LOW-POTENTIAL HEME

The tetraheme cytochromes c(3) isolated from two strains of Desulfomicrobium baculatum were studied by monitoring the spectral changes undergone during redox titrations followed by H-1 NMR. The evolution of the three-proton intensity signals at low field allowed the partial identification of the heme methyl resonances in the spectrum of the fully oxidized state. The chemical shift variation shown by the protons of the aromatic sidechains as well as of the substituents of the higher-potential heme HIII [Coutinho, I. B., Turner, D. L., LeGall, J. and Xavier, A. V. (1993) Biochem. J. 294, 899-908] yielded the assignment of the lower midpoint redox potential to heme HII in the three-dimensional structure. This cross-assignment is achieved by comparing the chemical shifts of the resonances in the spectra obtained at intermediate oxidation levels with the pseudocontact shifts predicted to arise from the three lower-potential hemes. The cross-assignment for the cytochromes from these two strains is different from that of the cytochromes from Desulfovibrio vulgaris and Desulfovibrio gigas.