High-resolution structures of the oxidized and reduced states of cytochrome c554 from Nitrosomonas europaea

Cytochrome c554 (cyt c554) is a tetra-heme cytochrome involved in the oxida tion of NH, by Nitrosomonas europaea. The X-ray crystal structures of both the oxidized and dithionite-reduced states of cyt c554 in a new, rhombohedr al crystal form have been solved by molecular replacement, at 1.6 Angstrom and 1.8 Angstrom resolution, respectively. Upon reduction, the conformation of the polypeptide chain changes between residues 175 and 179, which are a djacent to hemes III and TV. Cyt c554 displays conserved heme-packing motif s that are present in other heme-containing proteins. Comparisons to hydrox ylamine oxidoreductase, the electron donor to cyt c554, and cytochrome c ni trite reductase, an enzyme involved in nitrite ammonification, reveal subst antial structural similarity in the polypeptide chain surrounding the heme core environment. The structural determinants of these heme-packing motifs extend to the buried water molecules that hydrogen bond to the histidine Li gands to the heme iron. In the original structure determination of a tetrag onal crystal form, a cis peptide bond between His129 and Phe130 was identif ied that appeared to be stabilized by crystal contacts. In the rhombohedral crystal form used in the present high-resolution structure determination, this peptide bond adopts the trans conformation, but with disallowed angles of phi and psi.