O-Methyltransferase from Soybean Uses Both Caffeoyl-CoA and Flavonoids as Substrates

A gene encoding O-methyltransferase (SOMT)-10 from soybean, SOMT-10, was cloned by reverse transcription polymerase chain reaction. Phylogenetic analysis revealed that SOMT-10 belonged to caffeoyl-CoA O-methyltransferase (CCoAOMT) and contained conserved catalytic residues found in CCoAOMT. SOMT-10 was expressed in Escherichia coli as a glutathione S-transferase fusion protein and purified to determine its substrate. Several compounds including caffeoyl-CoA, naringenin, quercetin, caffeic acid, kaempferol and luteonin were tested as substrates for the purified recombinant SOMT-10. Analysis of reaction products using high performance liquid chromatography revealed that SOMT-10 used caffeoyl-CoA, quercetin and luteolin as substrates. This result indicated that SOMT-10 used flavones having vicinal hydroxyl groups. The methylation position was determined to be the 3' hydroxyl group. It is likely that SOMT-10 is a new class of OMT that uses not only caffeoyl-CoA, but also flavonoids. Molecular docking of tricetin with the modeled structure SOMT-10 disclosed that SOMT-10 showed all combinations of the O-methylated products.