Dirigentins

Dirigentins, or dirigent proteins (DIR), are a family of plant secreted proteins first described in Forsythia intermedia in 1992. The name derives from the Latin dirigir, meaning to guide. Dirigentins are implicated in the stereoselective coupling of coniferyl alcohol-derived radicals during lignan biosynthesis, directing the formation of specific lignans such as the enantiomer (+)-pinoresinol without performing the radical coupling themselves. In vivo, radical formation is carried out by oxidative enzymes such as laccases or peroxidases, while the dirigent protein provides the proper orientation to yield a defined product.

Most characterized dirigentins are secreted into the apoplast and are thought to be glycoproteins. The dirigent-domain family is present in many vascular plants, with multiple DIR genes per genome; some proteins related to the family, termed dirigent-like proteins, share sequence similarity but have divergent functions or unknown roles.

Functionally, dirigentins contribute to the diversification of plant phenylpropanoid metabolites, including lignans such as pinoresinol and lariciresinol, and may play a role in plant defense by shaping lignan-based defensive compounds. They are not enzymes themselves and do not alter the redox chemistry directly; instead they bias the outcome of radical coupling reactions.

From an applied perspective, dirigentins have attracted interest for biocatalysis and metabolic engineering to produce enantioselective lignans, which are of pharmaceutical and industrial interest. Research continues to elucidate their exact mechanisms, substrate scope, and regulation.