Add time:08/30/2019         Source:sciencedirect.com

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.

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