Beyond Phenolics: Alternative Substrates for Type III Copper Enzymes

The type III copper enzyme family of tyrosinases (TYRs) catalyzes the ortho-hydroxylation and oxidation of phenols as well as the two-electron oxidation of catechols to ortho-quinones. TYRs use copper ions as their tightly bound cofactors and utilize molecular oxygen as their cosubstrate. They are responsible for physiologically important reactions like the formation of melanin, the primary pigment animals apply for protection against UV light. While the reactivity of TYRs on substrates containing aromatic hydroxy groups (i. e. phenols) is well recognized, reports clearly demonstrating that TYRs are active on aromatic amines as well have gone largely unnoticed. In this perspective we aim to bring together the sparse data on non-phenolic TYR substrates to illustrate the potential of TYRs for the oxidation of aminophenols and anilines. The activity of TYRs on aromatic amines extends the substance classes amenable to biotechnological production with TYRs from catechols to N-phenyl imines and phenoxazinone derivates and calls for the inclusion of TYRs among the candidates for oxidative modification of aromatic amines in metabolic pathways.