Are glutamate and asparagine necessary for tyrosinase activity of type-3 copper proteins?

Type-3 copper proteins (T3CPs) are complex proteins which share similar active sites. Two copper atoms (CuA and CuB) bind dioxygen as a peroxide in a side on coordination. This protein family comprises the enzymes tyrosinase and catechol oxidase as well as the oxygen transporter hemocyanin. T3CPs occur in almost all organisms and exhibit a number of essential functions. In particular, they are involved in all kinds of enzymatic browning reactions and immune defense. The chemical basis of the two catalytic processes, i.e., the o-hydroxylation of monophenols and the two-electron oxidation to o-quinones, is still discussed. Investigations on natural enzymes with known crystal structures, along with site directed mutagenesis experiments, provide deeper insight into mechanistic details of these reactions. In particular, they shed light on the deprotonation of monophenolic substrates which appears to be required for the enzymatic conversion to o-quinones. Moreover, they allow drawing definite conclusions regarding the role of particular amino acids (glutamate and asparagine/aspartate) in these processes, which in turn provides a structure-based argument for the functional difference between tyrosinases and catechol oxidases. (C) 2017 Elsevier B.V. All rights reserved.