Stereochemical Course of Methyl Transfer by Cobalamin-Dependent Radical SAM Methyltransferase in Fosfomycin Biosynthesis

The methyl groups of [<i>methyl</i>-(<i>S</i>)]- and [<i>methyl</i>-(<i>R</i>)]-[<i>methyl</i>-D,T]-l-methionine fed to <i>Streptomyces fradiae</i> were incorporated into fosfomycin, which was chemically degraded to chiral AcONa. The enzymatic test gave the (<i>S</i>)-configuration for the chiral AcONa derived from methionine with the (<i>S</i>)-[D,T]­methyl group (<i>F</i> = 31.7) and (<i>R</i>) for the one derived from methionine with the (<i>R</i>)-[D,T]­methyl group (<i>F</i> = 83.0). The radical SAM methyltransferase transfers the methyl group of MeCbl to HEP-CMP with inversion of configuration