STEREOSELECTIVE BIMOLECULAR PHENOXY RADICAL COUPLING BY AN AUXILIARY (DIRIGENT) PROTEIN WITHOUT AN ACTIVE-CENTER

The regio- and stereospecificity of bimolecular phenoxy radical coupli ng 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 othe r words, laccases, peroxidases, and comparable oxidases ave unable to control regio- or stereospecificity by themselves and thus some other agent must exist. A 78-kilodalton protein has been isolated that, in t he presence of an oxidase or one electron oxidant, effects stereoselec tive bimolecular phenoxy radical coupling in vitro. Itself lacking a c atalytically active (oxidative) center, its mechanism of action is pre sumed to involve capture of E-coniferyl alcohol-derived free-radical i ntermediates, with consequent stereoselective coupling to give (+)-pin oresinol.