The Electronic Structure of the two Copper Sites in Nitrite Reductase by 9 and 95 GHz EPR on Cavity Mutants

Electron paramagnetic resonance at 9 and 95 GHz on frozen solutions of the wild-type nitrite reductase (wt NiR) from Alcaligenes faecalis and on cavity mutants of its type I site has been performed to determine copper-hyperfine and g-tensor principal values of the type I and the type 2 copper sites. The mutants H145G, H145A, and M150G have a gap in the first coordination shell of the copper in the type I site. The reconstitution of the Cu site of the mutants by means of an external ligand such as imidazole or chloride was investigated. Information on the electronic structure of the type I site was obtained. Indications were found that the position of the histidine 145 in the native protein is not constrained by the protein environment but reflects the equilibrium position of this ligand with respect to Cu(II). Furthermore, changes in the electronic structure at the type 2 site induced by the modification of the type I site were detected, providing evidence for interaction between the two copper sites of the enzyme.