Spectroscopic features and enzymatic activity of H135R nitrite reductase mutant, in which a His ligand at the type 2 copper site is replaced with Arg, have been investigated to get information on unique nitrite reductases having one Arg at the type 2 copper site. Although the substitution of Arg for His slightly perturbed the structure of copper sites, the enzymatic activity was considerably lower than that of wild-type enzyme.