cinnamat4hydroxylase

Cinnamate 4-hydroxylase, commonly abbreviated C4H, is a cytochrome P450 monooxygenase that catalyzes the hydroxylation of cinnamic acid to p-coumaric acid, the first hydroxylation step in the phenylpropanoid pathway of plants. It is a membrane-bound enzyme located in the endoplasmic reticulum and relies on electron transfer from NADPH via a cytochrome P450 reductase.

The enzyme uses molecular oxygen to introduce a hydroxyl group at the para position of the aromatic ring of cinnamic acid. As a member of the cytochrome P450 family, C4H typically operates in conjunction with plant NADPH-cytochrome P450 reductases and other components of the electron transport chain. Genes encoding C4H belong mainly to the CYP73 family, with CYP73A being the most widely studied in angiosperms and CYP73B identified in some species.

Biologically, C4H supplies p-coumaric acid, a central precursor for a broad range of phenylpropanoid derivatives, including lignin monomers, hydroxycinnamic esters, flavonoids, and other secondary metabolites involved in plant structure and defense. Expression and activity of C4H are developmentally regulated and responsive to environmental cues such as light, pathogen attack, wounding, and various phytohormones.

Research and practical relevance include the use of inhibitors like piperonylic acid to block C4H activity and study phenylpropanoid flux. C4H is a key control point in carbon flow toward lignin and other protective compounds, and variations in its activity influence plant growth, defense, and stress responses.