Enzymatic Synthesis of L-Methionine Analogues and Application in a Methyltransferase Catalysed Alkylation Cascade

Chemical modification of small molecules is a key step for the development of pharmaceuticals. S-adenosyl-L-methionine (SAM) analogues are used by methyltransferases (MTs) to transfer alkyl, allyl and benzyl moieties chemo-, stereo- and regioselectively onto substrates, enabling an enzymatic way for specific derivatization of a wide range of molecules. L-Methionine analogues are required for the synthesis of SAM analogues. Most of these are not commercially available. In nature, O-acetyl-L-homoserine sulfhydrolases (OAHS) catalyse the synthesis of L methionine from O-acetyl-L-homoserine or L homocysteine, and methyl mercaptan. Here, we investigated the substrate scope of ScOAHS from Saccharomyces cerevisiae for the production of L methionine analogues from L homocysteine and organic thiols. The promiscuous enzyme was used to synthesise nine different L methionine analogues with modifications on the thioether residue up to a conversion of 75%. ScOAHS was combined with an established MT dependent three-enzyme alkylation cascade, allowing transfer of in total seven moieties onto two MT substrates. Ethylation was nearly doubled with the new four-enzyme cascade, indicating a beneficial effect of the in situ production of L methionine analogues with ScOAHS.