Insights into the role of methionine synthase in the universal 13C depletion in O- and N-methyl groups of natural products

Abstract

Many O-methyl and N-methyl groups in natural products are depleted in ¹³C relative to the rest of the molecule. These methyl groups are derived from the C-1 tetrahydrofolate pool via l-methionine, the principle donor of methyl units. Depletion could occur at a number of steps in the pathway. We have tested the hypothesis that methionine biosynthesis is implicated in this depletion by using a combined experimental and theoretical approach. By using isotope ratio monitoring ¹³C NMR spectrometry to measure the position-specific distribution of ¹³C within l-methionine of natural origin, it is shown that the S-methyl group is depleted in ¹³C by ∼20‰ relative to the other positions in the molecule. In parallel, we have conducted a basic theoretical analysis of the reaction pathway of methionine synthase to assess whether the enzyme cobalamin-independent l-methionine synthase (EC 2.1.1.14)-that catalyzes the synthesis of l-methionine from 5-methyl-tetrahydrofolate and homocysteine-plays a role in causing this depletion. Calculation predicts a strong normal ¹³C kinetic isotope effect (1.087) associated with this enzyme. Hence, depletion in ¹³C in the S-methyl of l-methionine during biosynthesis can be identified as an important factor contributing to the general depletion seen in many O-methyl and N-methyl groups of natural products.

Keywords: (13)C NMR; (13)C/(12)C isotope fractionation; O-methyl and N-methyl groups; QM/MM theoretical calculation; l-methionine; l-methionine synthase (EC 2.1.1.14).