Elucidation of the Enzymatic Conversions in Lignan Biosynthesis: ()-Pinoresinol Synthase, ()- Pinoresinol and ()- Lariciresinol Reductase

摘要

()-Pinoresinol synthase, an enzyme catalyzing the highly unusual stereoselective coupling of two achiral E-coniferyl alcohol molecules, has been discovered for the first time; this discovery is the first example of stereoselective coupling in phenylpropanoid metabolism. The enzyme is present in the 'insoluble residue' from Forsythia species, obtained following removal of readily soluble enzymes. Interestingly, this preparation is capable of engendering()-pinoresinol formation, even in the absence of exogenously supplied cofactors; however, enzymatic activity is stimulated when malate and NAD are added. By contrast, the soluble enzyme preparation from Forsythia intermedia contains a NAD(P)H-dependent reductase (or reductases) which converts ()-pinoresinol into ()-lariciresinol, and subsequently transforms the latter into (-)-secoisolariciresinol; significantly, the corresponding (-)-antipodes of pinoresinol or lariciresinol do not serve as substrates for this reduction. These enzymes are highly unusual since they apparently catalyze direct benzylic ether reduction (or a quinone methide intermediate derived thereof) at their active sites.