Abstract: In Cu-containing nitrite reductase from Alcaligenes faecalis S-6 the axial methionine ligand of the type 1 site was replaced (M150G) to make the copper atom accessible to external ligands that might affect the enzyme's catalytic activity. The type-1 site optical spectrum of M150G (A460/A600=0.71) differs significantly from that of the native nitrite reductase (A460/A600=1.3). The reduction potential of the type-1 site of nitrite reductase M150G (EM=312?5 mV versus hydrogen) is higher than that of the native enzyme (EM=213?5 mV). M150G has a lower catalytic activity (kcat=133?6 s?1) than the wild-type nitrite reductase (kcat=416?10?s 1). The binding of external ligands to M150G restores spectral properties, reduction potential (EM<225 mV), and catalytic activity (kcat=374?28 s?1). Also the M150H (A460/A600=7.7, EM=104?5 mV, kcat=0.099?0.006 s?1) and M150T (A460/A600=0.085, EM=340?5 mV, kcat=126?2 s?1) variants were characterized to compare their properties with those of M150G.