Dirigent protein is a protein which dictates the stereochemistry of a compound synthesized by other enzymes.[1] Originally they were discovered in coniferous trees, where they participate in lignan biosynthesis.[2] Lignan synthesis is catalysed by oxidative enzymes. In the test tube the reaction results in a heteregenous mixture of compounds. When dirigent protein is present during the reaction, only one stereoisomer of one compound is produced. Dirigent proteins possess no oxidative radical forming activity of their own; but catalyze a radical radical coupling.