Spectroscopic, computational, and kinetic studies of the mu(4)-sulfide-bridged tetranuclear Cu-Z cluster in N2O reductase: pH effect on the edge ligand and its contribution to reactivity

A combination of spectroscopy and density functional theory (DFT) calculations has been used to evaluate the pH effect at the Cu-Z site in Pseudomonas nautica (Pn) nitrous oxide reductase (N2OR) and Achromobacter cycloclastes (Ac) N2OR and its relevance to catalysis. Absorption, magnetic circular dichroism, and electron paramagnetic resonance with sulfur K-edge X-ray absorption spectra of the enzymes at high and low pH show minor changes. However, resonance Raman (rR) spectroscopy of PnN(2)OR at high pH shows that the 415 cm(-1) Cu-S vibration (observed at low pH) shifts to higher frequency, loses intensity, and obtains a 9 cm(-1) O-18 shift, implying significant Cu-O character, demonstrating the presence of a OH- ligand at the CuICuIV edge. From DFT calculations, protonation of either the OH- to H2O or the mu(4)-S2- to mu(4)-SH- would produce large spectral changes which are not observed. Alternatively, DFT calculations including a lysine residue at an H-bonding distance from the CuICuIV edge ligand show that the position of the OH- ligand depends on the protonation state of the lysine. This would change the coupling of the Cu-(OH) stretch with the Cu-S stretch, as observed in the rR spectrum. Thus, the observed pH effect (pK(a) similar to 9.2) likely reflects protonation equilibrium of the lysine residue, which would both raise E degrees and provide a proton for lowering the barrier for the N-O cleavage and for reduction of the [Cu4S(im)(7)OH](2+) to the fully reduced 4Cu(I) active form for turnover.