Altered regioselectivity of a poplar O-methyltransferase, POMT-7

O-Methylated flavonoids are biosynthesized by regioselective flavonoid O-methyltransferases (OMTs), which may account for the limited number of naturally occurring flavonoids in nature. It was previously shown that poplar POMT-7 regioselectively methylates the 7-hydroxyl group of flavones, whereas rice ROMT-9 regioselectively methylates the 3′-hydroxyl group of the substrate. We co-expressed both OMT genes (POMT-7 and ROMT-9) in E. coli and carried out biotransformation experiments of some flavonoids with the transformed E. coli strain. Contrast to the predicted regioselectivity of both POMT-7 and ROMT-9, unexpected methylation reaction products, i.e. 3′,4′-O-methylated flavonoids, in addition to the predicted ones, were obtained with luteolin (5,7,3′,4′-tetrahydroxyflavone) and quercetin (3,5,7,3′,4′-pentahydroxyflavone) as substrates. Reactions using the 3′-O-methyl derivative of luteolin and quercetin by POMT-7 revealed that the enzyme has altered its regioselectivity from the 7- to the 4′-hydroxyl groups. These results are discussed in terms of molecular modeling of POMT-7 in relation to its methyl donor.