The structure of the Met144Leu mutant of copper nitrite reductase from Alcaligenes xylosoxidans provides the first glimpse of a protein–protein complex with azurin II

Cu-containing nitrite reductases (NiRs) perform the reduction of nitrite to NO via an ordered mechanism in which the delivery of a proton and an electron to the catalytic type 2 Cu site is highly orchestrated. Electron transfer from a redox partner protein, azurin or pseudoazurin, to the type 1 Cu site is assumed to occur through the formation of a protein–protein complex. We report here a new crystal form in space group P212121 of the Met144Leu mutant of NiR from Alcaligenes xylosoxidans (AxNiR), revealing a head-to-head packing motif involving residues around the hydrophobic patch of domain 1. Superposition of the structure of azurin II with that of domain 1 of one of the Met144Leu molecules provides the first glimpse of an azurin II–NiR protein–protein complex. Mutations of two of the residues of AxNiR, Trp138His (Barrett et al. in Biochemistry 43:16311–16319, 2004) and Met87Leu, highlighted in the AxNiR–azurin complex, results in substantially decreased activity when azurin is used as the electron donor instead of methyl viologen, providing direct evidence for the importance of this region for complex formation.