The mutation Met121-->His creates a type-1.5 copper site in Alcaligenes denitrificans azurin.

The Cu ligand Met121 in azurin of Alcaligenes denitrificans was mutated to His. The spectroscopic and mechanistic properties of [M121H]azurin appear to be pH dependent with a pKa of 3.8 due to the ionization of His121. The [M121H]azurin mutant exhibits two major distinct metal-site-coordination geometries which coexist in solution according to pH-dependent equilibrium. Both species have been spectroscopically characterized by ultraviolet-visible, EPR and resonance Raman spectroscopies. At neutral pH, His121 is deprotonated and acts as the fourth ligand of the Cu; the spectroscopic characteristics of the Cu site at this pH are halfway between those of a type-1 and a type-2 Cu site, and the site is referred to as a type-1.5 or intermediate Cu site. The spectral data are compatible with a tetrahedral geometry of this site. At low pH, the spectroscopic data indicate that [M121H]azurin has a trigonal type-1 rhombic Cu site.