Mixed activation and mixed inhibition of the oxidoreductase activity of Rhus vernicifera laccase by different Pd²⁺ ion concentration

Abstract

AMracthnetic experiments were performed to study the effects of P 8 + ion on the oxidation of 5,6-&ro-~2.3-dicyanohydroqtunone catalyzed by Rhus Vern;C$ma laccase under condition of pH 4.5 and 30 * 0.1 T .' h e results showed that the mixed activation could be observed when Pd.+ ion was at low concentrations. The competitive and non-competitive activation constants were 9 x la6 and 2 x 10' moVL, respectively. With the increase of P 8 + ion concentration, the activation was g r a d d y converted into mixed inhibition, and the cornpetitive and non-competitive inhibition constants were 6 x 1@ and 32 x 10' moVL, respectively. KevwordsRhlls vemc$kra laccase, palladium(n), mixed activation, rnixed inhibition.Laccase (EC 1.10.3.2) , which contains a blue (type 1 ) copper and a trinuclear copper cluster comprised of a normal (type 2) and a binuclear (type 3 ) copper center, catalyzes the oxidation of diphenolir substrates by oxygen, producing quinones, phenol coupling products, and water as the sole oxygen reduction p d u c t . Diphenolic substrates participate laccase-enzymatic reaction in terms of the phenolate rnonoanion in the rate-determining step for electron t r a n~f e r .~ As the first-order dissociation of most diphenolic substrates occurs around pH 8.0, and at h s circumstance hydrolysis of most metal ions may take place, so the reports of effects of metal ions on the oxidation of diphenolic substrates catalyzed by laccase were rare. There only were several examples5'' that H&+ ion could be selectively substituted at the type 1 site in laccase, forming the type 1 mercury derivative of laccase, and the derivative remained 5 % catalytic activity of native laccase, but they could not study the effect of HZ + ion on the oxidation catalyzed by laccase directly as the limit mentioned above. In our previous studies we found that the first-order dissociation of 5,6-d1bmmo-2,3-dicyanohy~none ( DDBQH2) occurred at pH 4.0. This finding makes it convenient to study the effects of metal ions on catalytic activity of laccase. In addition, ran: earth metal