phenylpropanoids

Phenylpropanoids are a large class of plant secondary metabolites derived from the amino acid phenylalanine (and, in some cases, tyrosine) through the phenylpropanoid pathway. They share a basic C6–C3 skeleton consisting of a phenyl ring attached to a three-carbon side chain, and they form a diverse range of compounds including hydroxycinnamic acids, monolignols, lignans, neolignans, and many more with various decorations.

Biosynthesis begins with phenylalanine ammonia-lyase (PAL), which deaminates phenylalanine to cinnamic acid. Cinnamate 4-hydroxylase (C4H) introduces a second hydroxyl, yielding p-coumaric acid, which is then activated by 4-coumarate CoA ligase (4CL). Further enzymatic steps—hydroxylation, methylation, reduction, and polymerization—produce hydroxycinnamic acids (such as caffeic and ferulic acids) and the lignin precursors, known as monolignols (coniferyl, sinapyl, and p-coumaryl alcohols). These monolignols polymerize to lignin, a major structural component of vascular plants. Lignans and neolignans arise via different coupling reactions of phenylpropanoid units.

Roles include providing structural support in cell walls, contributing to wood formation, defense against pathogens and herbivores, protection from UV radiation, and participation in signaling and pigmentation. Industrially, phenylpropanoids are important sources of lignin for paper and bio-based materials and serve as precursors to a wide array of natural products.

Note: In some classifications, flavonoids and related compounds are considered part of the phenylpropanoid family because they are biosynthetically derived from p-coumaroyl-CoA.