The enzymatic properties of octopus vulgaris hemocyanin: o-Diphenol oxidase activity

Hemocyanin and tyrosinase are dinuclear copper proteins capable of reversibly binding dioxygen. Despite the great similarity of structure and properties of their active site, the two proteins perform different biological functions (oxygen transport/storage versus monooxygenase and oxidase activity). In this paper, we show that Octopus Vulgaris hemocyanin exhibits a tyrosinase-like activity; namely, it is capable of utilizing dioxygen for the oxidation of o-diphenol to quinone. The reaction is specific for this isomer of diphenol, the meta and para isomers being unreactive, and is strongly controlled by steric factors. Dioxygen represents a cosubstrate of the reaction, and it is involved in the catalytic turnover by binding to the dinuclear copper site of the protein to form, under steady-state conditions, oxy-Hc, which is the active species. The generation of semiquinone radicals, detected by EPR and by their reaction with N,N,N¢,N¢-tetramethyl-1,4-phenylenediamine, strongly supports a reaction mechanism in which such radicals represent the reaction products of one-electron oxidation of the substrate, quinone being generated by dismutation of semiquinones. Met-Hc is regenerated by the substrate to the deoxy form. To close the catalytic cycle, the proposed reaction mechanism also involves the participation of two transient protein forms with the total oxidation state of the active site (V and IV) intermediate between that of oxy-Hcy, [CuIIO22-CuII]VI, and deoxy-Hc, [CuICuI]II. A mathematical model has been elaborated to describe the reaction kinetics. The differences in reaction mechanisms between hemocyanin and tyrosinase are discussed in terms of accessibility to exogenous molecules of their active sites.