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

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 show...