Davin L B, Wang H B, Crowell A L, Bedgar D L, Martin D M, Sarkanen S, Lewis N G
Institute of Biological Chemistry, Washington State University, Pullman, WA 99164-6340, USA.
Science. 1997 Jan 17;275(5298):362-6. doi: 10.1126/science.275.5298.362.
The regio- and stereospecificity of bimolecular phenoxy radical coupling reactions, of especial importance in lignin and lignan biosynthesis, are clearly controlled in some manner in vivo; yet in vitro coupling by oxidases, such as laccases, only produce racemic products. In other words, laccases, peroxidases, and comparable oxidases are unable to control regio- or stereospecificity by themselves and thus some other agent must exist. A 78-kilodalton protein has been isolated that, in the presence of an oxidase or one electron oxidant, effects stereoselective bimolecular phenoxy radical coupling in vitro. Itself lacking a catalytically active (oxidative) center, its mechanism of action is presumed to involve capture of E-coniferyl alcohol-derived free-radical intermediates, with consequent stereoselective coupling to give (+)-pinoresinol.
双分子苯氧基自由基偶联反应的区域和立体特异性在木质素和木脂素生物合成中尤为重要，在体内显然以某种方式受到控制；然而，诸如漆酶等氧化酶在体外进行的偶联反应只产生外消旋产物。换句话说，漆酶、过氧化物酶及类似的氧化酶自身无法控制区域或立体特异性，因此必定存在某种其他因子。现已分离出一种78千道尔顿的蛋白质，该蛋白质在氧化酶或单电子氧化剂存在的情况下，可在体外实现立体选择性双分子苯氧基自由基偶联反应。其自身缺乏催化活性（氧化）中心，据推测其作用机制涉及捕获源自E-松柏醇的自由基中间体，随后进行立体选择性偶联生成(+)-松脂醇。
