Genomic analysis reveals widespread occurrence of new classes of copper nitrite reductases

Recently, the structure of a Cu-containing nitrite reductase (NiR) from Hyphomicrobium denitrificans (HdNiR) has been reported, establishing the existence of a new family of Cu-NiR where an additional type 1 Cu (T1Cu) containing cupredoxin domain is located at the N-terminus (Nojiri et al. in Proc. Natl. Acad. Sci. USA 104:4315-4320, 2007). HdNiR retains the well-characterised coupled T1Cu-type 2 Cu (T2Cu) core, where the T2Cu catalytic site is also built utilising ligands from neighbouring monomers. We have undertaken a genome analysis and found the wide occurrence of these NiRs, with members clustering in two groups, one showing an amino acid sequence similarity of around 80% with HdNiR, and a second group, including the NiR from the extremophile Acidothermus cellulolyticus, clustering around 50% similarity to HdNiR. This is reminiscent of the difference observed between the blue (Alcaligenes xylosoxidans) and green (Achromobacter cycloclastes and Alcaligenes faecalis) NiRs which have been extensively studied and may indicate that these also form two distinct subclasses of the new family. Genome analysis also showed the presence of Cu-NiRs with a C-terminal extension of 160-190 residues containing a class I cytochrome c domain with a characteristic beta-sheet extension. Currently no structural information exists for any member of this family. Genome analysis suggests the widespread occurrence of these novel NiRs with representatives in the alpha, beta and gamma subclasses of the Proteobacteria and in two species of the fungus Aspergillus. We selected the enzyme from Ralstonia pickettii for comparative modelling and produced a plausible structure highlighting an electron transfer mode in which the cytochrome c haem at the C-terminus can come within 16-A reach of the T1Cu centre of the T1Cu-T2Cu core.