Dirigent protein

Dirigent proteins are members of a class of proteins which dictate the stereochemistry of a compound synthesized by other enzymes. The first dirigent protein was discovered in Forsythia intermedia. This protein has been found to direct the stereoselective biosynthesis of (+)-pinoresinol from coniferyl alcohol monomers:

Lignan biosynthesis is catalysed by oxidative enzymes. In the test tube the reaction results in a heteregenous mixture of dimeric compounds. When dirigent protein is present during the reaction, one stereoisomer of one compound is highly enriched. Dirigent proteins appear to possess no oxidative radical forming activity of their own; in the absence of oxidative enzyme, no reaction will occur.

Recently, a second, enantiocomplementary dirigent protein was identified in Arabidopsis thaliana, which directs enantioselective synthesis of (-)-pinoresinol.

In lignan biosynthesis, oxidative enzymes perform proton coupled electron transfer to remove a hydrogen atom from monolignols, forming a radical intermediate. These intermediates then couple in a radical termination reaction to form one of a variety of dimers, known as lignans.In vitro reactions of coniferyl alcohol (a common monolignol) in the presence of oxidative enzymes produce a wide variety of different dimers at varying concentrations. When dirigent protein from "Forsythia intermedia" is present, production of (+)-pinoresinol is greatly enriched, and other products are far less abundant. Because this enrichment is so pronounced, the enzyme is hypothesized to produce (+)-pinoresinol exclusively, and to compete with the non-protein-mediated coupling reaction, which produces a heterologous mix of products. This has been confirmed by analyzing the various mixtures produced with different concentrations of dirigent proteins present. The mechanism by which this stereoselectivity is achieved is not well-understood at this time. However, since no reaction proceeds in the absence of oxidative enzymes, dirigent protein does not itself appear to catalyze the oxidation of coniferyl alcohol to form radicals.

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