A two-pronged computational approach was taken to study the promiscuity of the SAM+-dependent methyl transferase AtHTMT1 from thale cress with several nucleophiles (Cl-, Br-, I-, NCO-, NCS-). First, enzyme-free methyl transfer reactions were studied with M05/6-311+G(2d,p) DFT calculations and electrostatic continuum models (PCM/SMD) for various chemical environments. Second, QM/MM MD simulations with semiempirical Hamiltonians (PM7, PM6-D3, AM1, PM6-D3H4) and the AMBER 14SB force field were used to study the enzyme catalyzed reaction in silico. The combination of the DFT and MD results shows that reactant desolvation generally accelerates the reaction, but it cannot explain the selectivity of the enzyme. The critical position of H2O molecules at the reactive site favors the reaction of NCS- over Cl- and Br- in agreement with experiments, but not observed in the quantum calculations for the cytosol. The addition of selected H2O molecules to the N terminus of NCS- greatly increases its reactivity, while H2O molecules attached to Cl- slow the reaction. The partial solvation of the nucleophiles in the reactive pouch holds the key to understanding the reactivity of AtHTMT1.
© 2022 The Authors. Published by American Chemical Society.