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Tyrosinase (EC 1.14.18.1), a copper-containing monooxygenase, catalyzes the conversion of phenol to the corresponding ortho-quinone. The  Streptomyces  tyrosinase is generated as a complex with a “caddie” protein that facilitates the transport of two copper ions into the active center. In our previous study, the Tyr 98  residue in the caddie protein, which is accommodated in the pocket of active center of tyrosinase, has been found to be converted to a reactive quinone through the formations of the μ-η 2 :η 2 -peroxo-dicopper(II) and Cu(II)-dopasemiquinone intermediates. Until now—despite extensive studies for the tyrosinase reaction based on the crystallographic analysis, low-molecular-weight models, and computer simulations—the catalytic mechanism has been unable to be made clear at an atomic level. To make the catalytic mechanism of tyrosinase clear, in the present study, the cryo-trapped crystal structures were determined at very high resolutions (1.16–1.70 Å). The structures suggest the existence of an important step for the tyrosinase reaction that has not yet been found: that is, the hydroxylation reaction is triggered by the movement of Cu A , which induces the syn-to-anti rearrangement of the copper ligands after the formation of μ-η 2 :η 2 -peroxo-dicopper(II) core. By the rearrangement, the hydroxyl group of the substrate is placed in an equatorial position, allowing the electrophilic attack to the aromatic ring by the Cu 2 O 2  oxidant.

Catalytic mechanism of the tyrosinase reaction toward the Tyr98 residue in the caddie protein