Solution structure of a monoheme ferrocytochrome c from Shewanella putrefaciens and structural analysis of sequence-similar proteins: functional implications.

Within the frame of the characterization of the structure and function of cytochromes c, an 81-amino acid cytochrome c was identified in the genome of Shewanella putrefaciens. Because of the scarce information about bacterial cytochromes of this type and the large variability in sequences and possibly function, we decided to proceed to its structural characterization. This protein was expressed in Escherichia coli and purified. The oxidized species is largely high spin, with a detached methionine, whereas the reduced species has the classical His/Met axial ligation to iron. The NMR solution structure of the reduced form was determined on a (15)N-labeled sample, for which 99% of all non-proline backbone (1)H and (15)N resonances have been assigned. One thousand three hundred two meaningful NOEs, out of 1775 NOEs, together with 66 dihedral angles provide a structure with rmsd values from the mean of 0.50 and 0.96 A for backbone and all heavy atoms, respectively. A search of gene banks allowed us to locate 10 different cytochromes c, the sequences of which are more than 30% identical to that of the S. putrefacienscytochrome. For two of them, the structures are known. The structures of the others have been modeled by using the available templates and internally validated. Structural similarities in terms of surface properties account for their biophysical features and provide hints about the function.