PNNL

Abstract

Control over phenoxy radical-radical coupling reactions in vivo in vascular plants was enigmatic until our discovery of dirigent proteins (DPs, latin: dirigere: to guide or align). The first 3D structure of a DP [(+)-pinoresinol forming DP, 1.95 Å resolution, rhombohedral space group H32)] is reported herein. It has a tightly packed trimeric structure with an eight stranded ß-barrel topology for each DP monomer. Each putative substrate binding and orientation coupling site is located on the trimer surface, but too far apart for intermolecular coupling between sites. It is proposed that each site enables stereoselective coupling (using either two coniferyl alcohol radicals or a radical and a monolignol). Interestingly, there are 6 differentially conserved residues in DPs affording either the (+)- or (-)-antipodes in the vicinity of the putative binding site and region known to control stereoselectivity. DPs are involved in lignan biosynthesis, whereas dirigent domains/sites have been implicated in lignin deposition.