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

Many O-methyl and N-methyl groups in natural products are depleted in C-13 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 C-13 NMR spectrometry to measure the position-specific distribution of C-13 within L-methionine of natural origin, it is shown that the S-methyl group is depleted in C-13 by similar to 20%o 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 C-13 kinetic isotope effect (1.087) associated with this enzyme. Hence, depletion in C-13 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.