Catalytic oxygenation of phenols by arthropod hemocyanin, an oxygen carrier protein, from Portunus trituberculatus

Abstract

The hexamer (Pt-6Hc) of swimming crab Portunus trituberculatus hemocyanin (Pt-Hc) and one of its monomeric subunits (Pt-1Hc) have been purified and converted to an efficient phenol monooxygenase (phenolase) by treatment with urea. To explore the intrinsic chemical reactivity of the dicopper center of Pt-Hc, the spectroscopic features and phenol monooxygenase (phenolase) activity of the isolated proteins have been examined in detail. The oxy-forms involving a (μ-η²:η²-peroxo)dicopper(II) species (oxy-Hc) of Pt-6Hc and Pt-1Hc are relatively stable in 0.5 M borate buffer (pH 9.0) even in the presence of a high concentration of urea (3 M) at 25 °C. The catalytic activity of monomeric Pt-1Hc in the oxygenation reaction (multi-turnover reaction) of 4-methylphenol to 4-methyl-1,2-dihydroxybenzene (4-methylcatechol) was higher than that of hexameric Pt-6Hc, and its catalytic activity was further accelerated by the addition of urea. Kinetic deuterium isotope effect analysis and Hammett analysis using a series of p-substituted phenol derivatives under anaerobic conditions (single-turnover reaction) have indicated that the monooxygenation reaction of phenols to catechols by the peroxo species of oxy-Hc proceeds via electrophilic aromatic substitution mechanism as in the case of tyrosinase (dinuclear copper monooxygenase). The effect of urea on the redox functions of oxy-Hc is discussed on the basis of spectroscopic analysis and reactivity studies.