Copper-containing nitrite reductase: a DFT study of nitrite and nitric oxide adducts

Copper-containing nitrite reductases (Cu-NIRs) reduce nitrite to NO. Reported here are DFT (density functional theory) results on models of the Cu-NIR active site bound to nitrite and nitric oxide. The Cu-NIR active site appears to have been designed to exclude N-nitrite binding even though N-O bond cleavage would be equally facile in the N- and O-isomers. The active site also appears to force a side-on coordination of the end-product, nitric oxide. The latter feature has to rely on the sterics of the active site to destabilize, thermodynamically speaking, the Cu-NO adduct; under these conditions, the absence of N-nitrite coordination is proposed to be merely a side-effect. For the Cu(II)-NO adduct, sterical crowding appears to also favour the Cu-NO electromer over Cu(I)-NO+, helping to avoid the potentially damaging chemistry associated with an NO+ moiety. These conclusions are in reasonable agreement with previous conclusions drawn from experiment [Science 304 (2004) 867].