Threonine aldolase-catalyzed stereoselective radical α-alkylation of amino acids.

Pyridoxal 5'-phosphate (PLP)-dependent enzymes comprise a structurally and functionally diverse enzyme superfamily underlying amino acid metabolism and other biologically significant processes. While PLP enzymes that use a two-electron mechanism for substrate activation have found widespread applications in biocatalysis and biotechnology, synthetically useful PLP radical enzymes have remained rare until recently. In 2023, our group demonstrated that naturally occurring two-electron PLP enzymes can be reprogrammed to catalyze radical-mediated transformations via a photoredox mechanism. In this chapter, we describe the repurposing and engineering of PLP-dependent threonine aldolases to allow for the enantioselective α-alkylation of abundant amino acid donors as a useful means to prepare valuable non-canonical amino acids. Our engineered radical PLP enzymes allow intermolecular C-C bond forming reactions to occur with excellent enantioselectivity, giving rise to α-tri and tetrasubstituted non-canonical amino acids with excellent enantioselectivity. By integrating photochemistry with PLP enzymology, we expect a broad spectrum of traditionally closed-shell PLP enzymes to be reprogrammed to catalyze novel radical-mediated stereoselective C-C bond formation.