Lignaanipolku

Lignaanipolku is the set of enzymatic reactions by which plants synthesize lignans, a class of phenylpropanoid-derived secondary metabolites. This pathway originates in the general phenylpropanoid metabolism and produces diverse lignan structures that vary by species and tissue. The pathway is often studied in the context of plant defense, development, and responses to environmental stress, and it also has relevance in human nutrition through the transformation of dietary lignans in the gut.

A typical lignan biosynthesis route begins with the phenylalanine-derived phenylpropanoid flux, including enzymes such as phenylalanine ammonia-lyase, cinnamate 4-hydroxylase, and 4-coumarate–CoA ligase, which funnel precursors toward coniferyl alcohol. A key step is the stereoselective coupling of coniferyl alcohol molecules to form the lignan skeleton, a reaction guided by dirigent proteins to produce pinoresinol. Through a series of reductases, including pinoresinol/lariciresinol reductases, pinoresinol is converted into lariciresinol and secoisolariciresinol. Further oxidation and reduction steps lead to the formation of enterodiol and its oxidized form enterolactone, along with other lignans that may be produced in different plant species.

Plants regulate the pathway in response to developmental cues and environmental factors such as light, temperature, and pathogen or herbivore attack. Lignans contribute to plant defense and can affect lignin composition and cell wall properties. In humans, dietary lignans are precursors that gut microbiota convert into enterolignans (e.g., enterodiol and enterolactone), which are studied for potential health effects, including phytoestrogenic activity and antioxidant properties. Flaxseed is a notable dietary source of a major lignan precursor, secoisolariciresinol diglucoside (SDG).