Stereoselective Biocatalytic α-Deuteration of L-Amino Acids by a Pyridoxal 5'-Phosphate-Dependent Mannich Cyclase.

α-Deuterated amino acids are valuable building blocks for developing deuterated drugs, and are important tools for studying biological systems. Biocatalytic deuteration represents an attractive strategy to directly access enantiopure α-deuterated amino acids. Here, we show that a PLP-dependent Mannich cyclase, LolT, involved in the biosynthesis of loline alkaloids, is capable of deuterating a diverse range of L-amino acids, including basic and acidic, nonpolar and polar, aliphatic and aromatic amino acids. Furthermore, complete deuteration of many amino acids can be achieved within minutes with exquisite control on the site- and stereoselectivity. During the course of this investigation, we also unexpectedly discovered that LolT exhibits β-elimination activity with L-cystine and O-acetyl-L-serine, confirming our previous hypothesis based on structural and phylogenetic analysis that LolT, a Cα-C bond forming enzyme, is evolved from a primordial Cβ-S lyase family. Overall, our study demonstrates that LolT is an extremely versatile biocatalyst, and can be used for not only heterocyclic quaternary amino acid biosynthesis, but also biocatalytic amino acid deuteration.