title: A model of dirigent proteins derived from structural and functional similarities with allene oxide cyclase and lipocalins.
abstract: Dirigent proteins impart stereoselectivity on the phenoxy radical-coupling reaction, yielding optically active lignans from two molecules of coniferyl alcohol. By an unknown mechanism, they direct the coupling of two phenoxy radicals toward the formation of optically active (+)- or (-)-pinoresinol. We show here that the dirigent protein AtDIR6 from Arabidopsis thaliana is a homodimeric all-beta protein in the superfamily of calycins. Based on its homology with calycins, the structure of AtDIR6 was modeled using allene oxide cyclase as template. The structural model of AtDIR6 was supported experimentally by confirmation of a predicted disulfide bridge and by the characterization of two N-linked glycans at the solvent-exposed protein surface. The model shows AtDIR6 as an eight-stranded antiparallel β-barrel with a central hydrophobic cavity for substrate binding, suggesting that dirigent proteins evolved from hydrophobic ligand-binding proteins. The data are fully consistent with the current view of the dirigent protein mode of action, according to which each subunit of the homodimer captures one of the substrate radicals and orients them in a way that precludes undesired reaction channels, thus favoring the formation of the optically pure coupling product.
authors.affiliation: Institute of Plant Physiology and Biotechnology, University of Hohenheim, Stuttgart, Germany.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Pickel
authors.forename: Jens
authors.initials: J
authors.lastname: Pfannstiel
authors.forename: Alexander
authors.initials: A
authors.lastname: Steudle
authors.forename: Axel
authors.initials: A
authors.lastname: Lehmann
authors.forename: Uwe
authors.initials: U
authors.lastname: Gerken
authors.forename: Jürgen
authors.initials: J
authors.lastname: Pleiss
authors.forename: Andreas
authors.initials: A
authors.lastname: Schaller
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology*; D029681:Arabidopsis Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D005663:Furans / Q000378:metabolism; D057927:Hydrophobic and Hydrophilic Interactions; D019746:Intramolecular Oxidoreductases / Q000737:chemistry; D017705:Lignans / Q000378:metabolism; D054834:Lipocalins / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010636:Phenols / Q000378:metabolism; D011485:Protein Binding; D055503:Protein Multimerization; D017433:Protein Structure, Secondary; D017386:Sequence Homology, Amino Acid; D013237:Stereoisomerism; D040681:Structural Homology, Protein
chemical_list: D029681:Arabidopsis Proteins; C586363:DIR6 protein, Arabidopsis; D005663:Furans; D017705:Lignans; D054834:Lipocalins; D010636:Phenols; C010559:coniferyl alcohol; D019746:Intramolecular Oxidoreductases; C021377:hydroperoxide isomerase; C103298:pinoresinol