Exogenous acetate ion reaches the type II copper centre in CueO through the water-excretion channel and potentially affects the enzymatic activity.

The acetate-bound form of the type II copper was found in the X-ray structure of the multicopper oxidase CueO crystallized in acetate buffer in addition to the conventional OH−-bound form as the major resting form. The acetate ion was retained bound to the type II copper even after prolonged exposure of a CueO crystal to X-ray radiation, which led to the stepwise reduction of the Cu centres. However, in this study, when CueO was crystallized in citrate buffer the OH−-bound form was present exclusively. This fact shows that an exogenous acetate ion reaches the type II Cu centre through the water channel constructed between domains 1 and 3 in the CueO molecule. It was also found that the enzymatic activity of CueO is enhanced in the presence of acetate ions in the solvent water.