AbstractTyrosinases mediate the ortho‐hydroxylation and two‐electron oxidation of monophenols to ortho‐quinones. Catechol oxidases only catalyze the oxidation of diphenols. Although it is of significant interest, the origin of the functional discrimination between tyrosinases and catechol oxidases has been unclear. Recently, it has been postulated that a glutamate and an asparagine bind and activate a conserved water molecule towards deprotonation of monophenols. Here we demonstrate for the first time that a polyphenoloxidase, which exhibits only diphenolase activity, can be transformed to a tyrosinase by mutation to introduce an asparagine. The asparagine and a conserved glutamate are necessary to properly orient the conserved water in order to abstract a proton from the monophenol. These results provide direct evidence for the crucial importance of a proton shuttle for tyrosinase activity of type 3 copper proteins, allowing a consistent understanding of their different chemical reactivities.