Cystathionine β-lyase is involved in D-amino acid metabolism

Non-canonical D-amino acids play important roles in bacteria including control of peptidoglycan metabolism and biofilm disassembly. Bacteria appear to produce non-canonical D-amino acids to adapt to various environmental changes, and understanding the biosynthetic pathways is important. We identified novel amino acid racemases possessing the ability to produce non-canonical D-amino acids in Escherichia coli and Bacillus subtilis in our previous study, whereas the biosynthetic pathways of these D-amino acids still remain unclear. In the present study, we demonstrated that two cystathionine beta-lyases (MetC and MalY) from E. coli produce non-canonical D-amino acids including non-proteinogenic amino acids. Furthermore, MetC displayed D-and L-serine (Ser) dehydratase activity. We characterised amino acid racemase, Ser dehydratase and cysteine lyase activities, and all were higher for MetC. Interestingly, all three activities were at a comparable level for MetC, although optimal conditions for each reaction were distinct. These results indicate that MetC and MalY are multifunctional enzymes involved in L-methionine metabolism and the production of D-amino acids, as well as D-and L-Ser metabolism. To our knowledge, this is the first evidence that cystathionine beta-lyase is a multifunctional enzyme with three different activities.