A Model of Dirigent Proteins Derived from Structural and Functional Similarities with Allene Oxide Cyclase and Lipocalins

Publikations-Art

Zeitschriftenbeitrag (peer-reviewed)

Autoren

Pickel B., Pfannstiel J., Steudle A., Lehmann A., Gerken U.,Pleiss J., Schaller A.

Erscheinungsjahr

2012

Veröffentlicht in

FEBS Journal

Band/Volume

279(11)/

Seite (von - bis)

1980-1993

Dirigent proteins impart stereoselectivity on the phenoxy radical coupling reaction yielding optically active lignans from two molecules of coniferyl alcohol. By a mechanism unknown, they direct the coupling of two phenoxy radicals towards 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 the homology with calycins, the structure of AtDIR6 was modeled using allene oxide cyclase as template. The structural model of AtDIR6 is supported experimentally by confirmation of a redicted disulfide bridge and by the characterization of two N-linked glycans at the solvent exposed protein surface. The model shows AtDIR6 as eight-stranded anti-parallel b-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 favoring the formation of the optically pure coupling product.