EPR−ENDOR of the Cu(I)NO Complex of Nitrite Reductase

• Published in: Journal of the American Chemical Society 2006-10, Vol.128 (40), p.13102-13111
• Main Authors: Usov, Oleg M, Sun, Yan, Grigoryants, Vladimir M, Shapleigh, James P, Scholes, Charles P
• Format: Article
• Language: English
• Subjects: Chemistry; Copper - chemistry; Copper - metabolism; Electron Spin Resonance Spectroscopy - methods; Exact sciences and technology; General and physical chemistry; Histidine - chemistry; Histidine - metabolism; Models, Molecular; Nitric Oxide - chemistry; Nitric Oxide - metabolism; Nitrite Reductases - chemistry; Nitrite Reductases - metabolism
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja056166n

With limited reductant and nitrite under anaerobic conditions, copper-containing nitrite reductase (NiR) of Rhodobacter sphaeroides yielded endogenous NO and the Cu(I)NO derivative of NiR. 14N- and 15N-nitrite substrates gave rise to characteristic 14NO and 15NO EPR hyperfine features indicating NO involvement, and enrichment of NiR with 63Cu isotope caused an EPR line shape change showing copper involvement. A markedly similar Cu(I)NONiR complex was made by anaerobically adding a little endogenous NO gas to reduced protein and immediately freezing. The Cu(I)NONiR signal accounted for 60−90% of the integrated EPR intensity formerly associated with the Type 2 catalytic copper. Analysis of NO and Cu hyperfine couplings and comparison to couplings of inorganic Cu(I)NO model systems indicated ∼50% spin on the N of NO and ∼17% spin on Cu. ENDOR revealed weak nitrogen hyperfine coupling to one or more likely histidine ligands of copper. Although previous crystallography of the conservative I289V mutant had shown no structural change beyond the 289 position, this mutation, which eliminates the Cδ1 methyl of I289, caused the Cu(I)NONiR EPR spectrum to change and proton ENDOR features to be significantly altered. The proton hyperfine coupling that was significantly altered was consistent with a dipolar interaction between the Cδ1 protons of I289 and electron spin on the NO, where the NO would be located 3.0−3.7 Å from these protons. Such a distance positions the NO of Cu(I)NO as an axial ligand to Type 2 Cu(I).