Crystal structure of a novel red copper protein from Nitrosomonas europaea

Nitrosocyanin (NC) is a mononuclear red copper protein isolated from the am monia oxidizing bacterium Nitrosomonas europaea. Although NC exhibits some sequence homology to classic blue copper proteins, its spectroscopic and el ectrochemical properties are drastically different. The 1.65 Angstrom resol ution crystal structure of oxidized NC reveals an unprecedented trimer of s ingle domain cupredoxins. Each copper center is partially covered by an unu sual extended beta -hairpin structure from an adjacent monomer. The copper ion is coordinated by His 98, His 103, Cys 95, a single side chain oxygen o f Glu 60, and a solvent molecule. In the 2.3 Angstrom resolution structure of reduced NC, His 98 shifts away from the copper ion, and the solvent mole cule is not observed. The arrangement of these ligands renders the coordina tion geometry of the NC red copper center distinct from that of blue copper centers. In particular, the red copper center has a higher coordination nu mber and lacks the long Cu-S(Met) and short Cu-S(Cys) bond distances charac teristic of blue copper. Moreover, the red copper center is square pyramida l whereas blue copper is typically distorted tetrahedral. Analysis of the N C structure provides insight into possible functions of this new type of bi ological copper center.