Mechanism of S -Adenosyl-l-methionine C -Methylation by Cobalamin-dependent Radical S -Adenosyl-l-methionine Methylase in 1-Amino-2-methylcyclopropanecarboxylic Acid Biosynthesis.

The radical S -adenosyl-l-methionine (SAM) methylase Orf29 catalyzes the C -methylation of SAM in the biosynthesis of 1-amino-2-methylcyclopropanecarboxylic acid. Here, we determined that the methylation product is (4″ R )-4″-methyl-SAM. Furthermore, we found that the 5'-deoxyadenosyl radical generated by Orf29 abstracts the pro-R hydrogen atom from the C-4″ position of SAM to generate the radical intermediate, which reacts with methylcobalamin to give (4″ R )-4″-methyl-SAM. Consequently, the Orf29-catalyzed C -methylation was confirmed to proceed with retention of configuration.