Discovery and Biocatalytic Application of a PLP-Dependent Amino Acid γ-Substitution Enzyme that Catalyzes C-C Bond Formation

Pyridoxal phosphate (PLP)-dependent enzymes can catalyze various transformations of amino acids at alpha, beta, and gamma positions. These versatile enzymes are prominently involved in the biosynthesis of nonproteinogenic amino acids as building blocks of natural products, and are attractive biocatalysts. Here, we report the discovery of a two-step enzymatic synthesis of (2S, 6S)-6-methyl pipecolate 1, from the biosynthetic pathway of indole alkaloid citrinadin. The key enzyme CndF is PLP-dependent and catalyzes synthesis of (S)-2-amino-6-oxoheptanoate 3 that is in equilibrium with the cyclic Schiff base. The second enzyme CndE is a stereoselective imine reductase that gives 1. Biochemical characterization of CndF showed this enzyme performs gamma-elimination of O-acetyl L-homoserine to generate the vinylglycine ketimine, which is subjected to nucleophilic attack by acetoacetate to form the new C_gamma-C_delta bond in 3 and complete the gamma-substitution reaction. CndF displays substrate promiscuity towards different beta-keto carboxylate and esters. Using a recombinant Aspergillus strain expressing CndF and CndE, feeding various alkyl-beta-keto esters led to the biosynthesis of 6-substituted L-pipecolates. The discovery of CndF expands the repertoire of reactions that can be catalyzed by PLP-dependent enzymes.