Crystal structures of an oxygen-binding cytochrome c from Rhodobacter sphaeroides

The photosynthetic bacterium Rhodobacter sphaeroides produces a heme protei n (SHP), which is an unusual c-type cytochrome capable of transiently bindi ng oxygen during autooxidation, Similar proteins have not only been observe d in other photosynthetic bacteria but also in the obligate methylotroph Me thylophilus methylotrophus and the metal reducing bacterium Shewanella putr efaciens, A three-dimensional structure of SRP was derived using the multip le isomorphous replacement phasing method. Besides a model for the oxidized state (to 1.82 Angstrom resolution), models for the reduced state (2.1 Ang strom resolution), the oxidized molecule liganded with cyanide (1.90 Angstr om resolution), and the reduced molecule Liganded with nitric oxide (2.20 A ngstrom resolution) could be derived. The SHP structure represents a new va riation of the class I cytochrome c fold. The oxidized state reveals a nove l sixth heme ligand, Asn(88), which moves away from the iron upon reduction or when small molecules bind, The distal side of the heme has a striking r esemblance to other heme proteins that bind gaseous compounds. In SHP the l iberated amide group of Asn(88) stabilizes solvent-shielded ligands through a hydrogen bond.