CYP73 enzymes are highly conserved cytochromes P450 in plant species that catalyse the regiospecific 4-hydroxylation ofcinnamicacid to formprecursorsofligninandmanyother phenolic compounds. A CYP73A1 homology model based on P450 experimentally solved structures was used to iden-tify active site residues likely togovern substrate binding and regio-specific catalysis. The functional significance of these residues was assessed using site-directedmutagenesis. Active site modelling predicted that N302 and I371 form a hydro-gen bond and hydrophobic contacts with the anionic site or aromatic ringofthe substrate
