Identification of O-Methyltransferases Potentially Contributing to the Structural Diversity of 2-(2-Phenylethyl)chromones in Agarwood

2-(2-Phenylethyl)chromones (PECs) are the primary constituents responsible for the promising pharmacological activities and unique fragrance of agarwood. However, the O-methyltransferases (OMTs) involved in the formation of diverse methylated PECs have not been reported. In this study, we identified one Mg2+-dependent caffeoyl-CoA-OMT subfamily enzyme (AsOMT1) and three caffeic acid-OMT subfamily enzymes (AsOMT2–4) from NaCl-treated Aquilaria sinensis calli. AsOMT1 not only converts caffeoyl-CoA to feruloyl-CoA but also performs nonregioselective methylation at either the 6-OH or 7-OH position of 6,7-dihydroxy-PEC. On the other hand, AsOMT2–4 preferentially utilizes PECs as substrates to produce structurally diverse methylated PECs. Additionally, AsOMT2–4 also accepts nonPEC-type substrates such as caffeic acid and apigenin to generate methylated products. Protein structure prediction and site-directed mutagenesis revealed that residues of L313 and I318 in AsOMT3, as well as S292 and F313 in AsOMT4 determine the distinct regioselectivity of these two OMTs toward apigenin. These findings provide important biochemical evidence of the remarkable structural diversity of PECs in agarwood.