Mechanistic implications of variable stoichiometries of oxygen consumption during tyrosinase catalyzed oxidation of monophenols and o-diphenols

The stoichiometry of oxygen consumption during tyrosinase-catalyzed oxidation of an o-diphenol (4-tert-butylcatechol, TBC) and a monophenol (4-tert-butylphenol, TBP) has been determined. At high [substrate]/[enzyme] ratios, in the case of o-diphenols, the stoichiometry of the enzyme-catalyzed reaction was always 1 O₂/2 o-diphenols, although if the o-quinone product was unstable, the apparent stoichiometry could tend to 1 O₂/1 o-diphenol due to regeneration of an o-diphenol in a side reaction. In the case of monophenols, the stoichiometry could be 1 O₂/1 monophenol or 1.5 O₂/1 monophenol depending if the o-quinone product was stable or unstable, respectively. However, at low [substrate]/[enzyme] ratios, the oxygen/substrate stoichiometry could, even in the case where stable products are formed, be lower than 1 O₂/2 substrates for o-diphenols or higher than 1 O₂/1 substrate for monophenols. These data supported the mechanism proposed by Rodrı́guez-López et al. [J. Biol. Chem. 267 (1992) 3801–3810], in which, during hydroxylation of monophenols, tyrosinase first transformed monophenol to o-diphenol and then either catalyzed a further oxidation to form o-quinone or released it into the reaction medium. In this second case, subsequent oxidation of the o-diphenol resulted in additional oxygen consumption.