Catecholase catalytic cycle

Scheme 2. The catecholase catalytic cycle of tyrosinase (adapted from (44)).

The mechanism of catecholase activity (outer circle) starts from the oxy and met states. A diphenol substrate binds to the met state (for example), followed by the oxidation of the substrate to the first quinone and the formation of the reduced state of the enzyme. Binding of dioxygen leads to the oxy state, which is subsequently attacked by the second diphenol molecule. Oxidation to the second quinone forms the met state again and closes the catalytic cycle. 

Fig. 5 Catalytic cycles of the two different oxygen-dependent reactions that occur consequently the oxidations of both monophenols (cresolase or monophenolase activity) and o-diphenols (catecholase or diphenolase activity). E,...

Catalytic mechanisms for tyrosinase activity have been proposed (7) based on both biochemical and model system studies (see later). The former include key recent kinetic studies from Itoh (50) and Casella (51). Mechanistic details have also been suggested based on computational studies (52,53). (Scheme 3) shows a well-accepted proposed mechanism, which includes phenolase and catecholase cycles. Here, tyrosinase is present in three redox forms during the cycle. The reduced deoxy Cu(I)-Cu(I) entity binds O2 to generate the oxyform Cu(II)-02 -Cu(II) in which the peroxide group occurs in a side-on -binding mode the... 

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