title: Stable copper-nitrosyl formation by nitrite reductase in either oxidation state.
abstract: Nitrite reductase (NiR) is an enzyme that uses type 1 and type 2 copper sites to reduce nitrite to nitric oxide during bacterial denitrification. A copper-nitrosyl intermediate is a proposed, yet poorly characterized feature of the NiR catalytic cycle. This intermediate is formally described as Cu(I)-NO+ and is proposed to be formed at the type 2 copper site after nitrite binding and electron transfer from the type 1 copper site. In this study, copper-nitrosyl complexes were formed by prolonged exposure of exogenous NO to crystals of wild-type and two variant forms of NiR from Alcaligenes faecalis (AfNiR), and the structures were determined to 1.8 A or better resolution. Exposing oxidized wild-type crystals to NO results in the reverse reaction and formation of nitrite that remains bound at the active site. In a type 1 copper site mutant (H145A) that is incapable of electron transfer to the type 2 site, the reverse reaction is not observed. Instead, in both oxidized and reduced H145A crystals, NO is observed bound in a side-on manner to the type 2 copper. In AfNiR, Asp98 forms hydrogen bonds to both substrate and product bound to the type 2 Cu. In the D98N variant, NO is bound side-on but is more disordered when observed for the wild-type enzyme. The solution EPR spectra of the crystallographically characterized NiR-NO complexes indicate the presence of an oxidized type 2 copper site and thus are interpreted as resulting from stable copper-nitrosyls and formally assigned as Cu(II)-NO-. A reaction scheme in which a second NO molecule is oxidized to nitrite can account for the formation of a Cu(II)-NO- species after exposure of the oxidized H145A variant to NO gas.
authors.affiliation: Department of Microbiology and Immunology, Life Sciences Institute, University of British Columbia, 2350 Health Sciences Mall, Vancouver BC, V6T 1Z3, Canada.
authors.forename: Elitza I
authors.initials: EI
authors.lastname: Tocheva
authors.forename: Federico I
authors.initials: FI
authors.lastname: Rosell
authors.forename: A Grant
authors.initials: AG
authors.lastname: Mauk
authors.forename: Michael E P
authors.initials: ME
authors.lastname: Murphy
mesh_terms: D042481:Alcaligenes faecalis / Q000201:enzymology; D003300:Copper / Q000378:metabolism*; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D008958:Models, Molecular; D009572:Nitrite Reductases / Q000737:chemistry / Q000378:metabolism*; D010084:Oxidation-Reduction; D011487:Protein Conformation
chemical_list: D003300:Copper; D009572:Nitrite Reductases