Functional Characterization of a Novel Flavonoid O-methyltransferase From Polar Pedobacter sp. PAMC26386 and...

Flavonoid O-methyltransferases (OMTs) catalyze the methylation of flavonoid hydroxyl groups, enhancing structural diversity and biological activity. In this study, we identified and characterized a novel Class I flavonoid OMT from the Antarctic bacterium Pedobacter sp. PAMC26386. Despite originating from a cold-adapted organism, the enzyme exhibited high catalytic activity at 55 °C and a strong preference for Co²⁺ as a cofactor. Sequence and phylogenetic analyses confirmed its classification as a flavonoid-specific OMT and revealed conserved motifs for S-adenosyl-L-methionine (SAM) binding and metal coordination. The enzyme accepted a wide range of flavonoid substrates, with quercetin and fisetin showing the highest activities. Kinetic analysis indicated greater substrate affinity for quercetin (K_m = 36.55 µM) than for fisetin (K_m = 49.41 µM). Whole-cell biotransformation using recombinant Escherichia coli C41 co-expressing the OMT and metK enabled efficient intracellular methylation, yielding 62.5 mg L⁻¹ of methylated quercetin and 55.5 mg L⁻¹ of 3'-O-methyl fisetin. The predicted methylation site at the 3'-hydroxyl of fisetin, determined by molecular docking, was confirmed by NMR spectroscopy. Notably, the previously reported 3'-O-methyl fisetin showed enhanced in vitro anticancer activity against mouse breast cancer cells and selectively improved antimycobacterial activity against Mycobacterium tuberculosis compared to the parent compound. To our knowledge, this study is among the first to report the enzymatic production of 3'-O-methyl fisetin using a polar microbial OMT that is optimally active at elevated temperatures in the presence of Co²⁺. The increased bioactivity compared to the parent compounds highlights the potential of this OMT as a biocatalyst for the sustainable production of pharmaceutically relevant methylated flavonoids.