Mutational and kinetic studies on Asp439, a ligand for the substrate-binding site in the copper(I) oxidase-CueO-and the deletion mutant-Delta alpha 5-7 CueO-indicated that the electron transfer from the copper(I) ion as the substrate to the type I copper is performed though the hydrogen bond between Asp439 and His443, a ligand for the type I copper, and also by means of the through-space mechanism directly.