The amino acid methionine is formed by a methylthio reaction of homo?cysteine with N-methyltetrahydrofolate. The stereochemistry of the reaction has been probed by carrying out the transformation using a donor with a ?chiral methyl group? that contains protium (H), deuterium (D), and tritium (T) isotopes of hydrogen. Does the methylation reaction occur with inversion or retention of configuration?

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CO2 HS. CO2 Methionine synthase H3N H H3N H Homocysteine Methionine NHAI H2N N-Methyltetrahydrofolate Tetrahydrofolate