STRUCTURE OF THE TETRAHEME CYTOCHROME FROM DESULFOVIBRIO-DESULFURICANS ATCC-27774 - X-RAY-DIFFRACTION AND ELECTRON-PARAMAGNETIC-RESONANCE STUDIES

The three-dimensional X-ray structure of-cytochrome cs from a sulfate reducing bacterium, Desulfovibrio desulfuricans ATCC 27774 (107 residu es, 4 heme groups), has been determined by the method of molecular rep lacement [Frazao et al. (1993) Acta Crystallogr. D50, 233-236] and ref ined at 1.75 Angstrom to an R-factor of 17.8%. When compared with the homologous proteins isolated from Desulfovibrio gigas, Desulfovibrio v ulgaris Hildenborough, Desulfovibrio vulgaris Miyazaki F, and Desulfom icrobium baculatus, the general outlines of the structure are essentia ly kept [heme-heme distances, heme-heme angles, His-His (axial heme li gands) dihedral angles, and the geometry of the conserved aromatic res idues]. The three-dimensional structure of D. desulfuricans ATCC 27774 cytochrome c(3)Dd was modeled on the basis of the crystal structures available and amino acid sequence comparisons within this homologous f amily of multiheme cytochromes [Palma et al. (1994) Biochemistry 33, 6 391-6407]. This model is compared with the refined crystal structure n ow reported, in order to discuss the validity of structure prediction methods and critically evaluate the steps used to predict protein stru ctures by homology modeling. The four heme midpoint redox potentials w ere determined by using deconvoluted electron paramagnetic resonance ( EPR) redox titrations. Structural criteria (electrostatic potentials, heme ligand orientation, EPR g values, heme exposure, data from protei n-protein interaction studies) are invoked to assign the redox potenti als corresponding to each specific heme in the three-dimensional struc ture.