Deuterium Solvent Kinetic Isotope Effect on Enzymatic Methyl Transfer Catalyzed by Catechol O-methyltransferase.

Introduction: Catechol o-methyltransferase plays a key role in the metabolism of catecholamine neurotransmitters. At present, its catalytic mechanism, overall structure, and kinetic characteristics have been basically clarified, but few people have paid attention to the function of solvents on enzymatic methyl transfer reactions. The influence of solvents on enzymatic reactions has always been a fuzzy hot topic. In addition, as a well-studied typical methyltransferase, COMT is a good test bed for exploring the source of the solvent isotope effect, which is a powerful tool in enzymatic mechanism research.

Methods: We have measured the kinetic parameters of methyl transfer catalyzed by COMT in both normal water (H₂O) and heavy water (D₂O) by high-performance liquid chromatography (HPLC) in the range of pL 6 ~ 11.

Results: The kinetic characteristics of COMT in H₂O and D₂O were significantly different under different pH/pD conditions. Significant solvent kinetic isotope effects (SKIE) were obtained, especially inverse solvent kinetic isotope effects (SKIE < 1) were observed in this methyl transfer reaction for the first time.

Conclusion: Traditional factors which could interpret the solvent isotope effect were ruled out. It's suggested that the solvent might affect the overall conformation as well as the flexibility of protein through non-covalent forces, thus altering the catalytic activity of COMT and leading to the solvent isotope effect.