漆树漆酶的催化氧化作用──Ⅸ．含叔丁基的儿茶酚类底物的漆酶催化氧化

CATALYZED OXIDATION BY RHUS VERNICIFERA LACCASE ──Ⅸ. On the laccase-catalyzed oxidation of catechols containing tert-butyl group

用分光光度法对６个含叔丁基的儿茶酚类底物在不同ＰＨ条件下的离解和漆酶催化氧化行为进行了考察，测定了它们在ＰＨ８．０时的反应速度常数．结果表明，叔丁基的疏水性较好地适应了漆酶对底物疏水程度的要求，使这类底物更容易为漆酶所包结；漆酶活性中心周围的酸性氨基酸残基中的ω－羧基阴离子所提供的ｐＨ环境和质子转移作用，促进了这类底物的离解，使这类底物能在远低于其一级离解所需ｐＨ的环境中进行反应．速度常数数据显示，取代基的疏水作用和取代基参与双齿配位的能力是影响这类底物反应速度的主要因素；在叔丁基的存在下，取代基的电子效应对这类底物的反应速度的影响变得不明显．

The dissociation and laccase-catalyzed oxidation of six catechols containing tert-butyl group were investigated spectrophotometrically under various PH conditions,and the rate constants of enzymatic reaction of these substrates were measured in pH8. 0. The studies showed that,because of the hydrophobic tert-butyl group meet fairly laccase's requirements for hydrophobicity of substrates,these catechols are bound easyly with active site of laccase ;due to the pH environment and the abilty of proton transfer,which are caused by the free ω-carboxy group anions of acidic amino acids (Asp or Glu) around the copper(Ⅱ)active site of laccase, the dissociation of these substrates is promoted and the enzymatic oxidation reaction can occur under condition of pH6. 0. Hence,in essence,the first-order dissociation is also essential for laccase-enzymatic oxidation of these hydrophobic substrates. The data of the rate constants demonstrated that,the hydrohobic interreaction of substituents and the ability of substituents with lone pairs to function as a ligand played an important role in the reaction rate of these substrates. In the presence of tert-butyl group,the influence of electron withdrawing and donating of other substituents on the enzymatic reaction rate of these substrates are not clear.