Project Summary PLP-dependent enzymes are one of the most versatile biocatalysts and catalyze a diverse range of chemical transformations. They are widespread in nature and play critical roles in metabolism and numerous cellular processes. Studying PLP enzymes is hence important for us to understand biology and develop therapeutics. Because of their exquisite and versatile catalytic activity, PLP-dependent enzymes are also remarkable biocatalysts to build diverse structurally complex and bioactive natural products; and are indispensable biocatalytic tools for asymmetric synthesis of noncanonical amino acids and chiral amine pharmaceuticals. However, despite the vast number of PLP-dependent enzymes characterized to date, our abilities to predict, manipulate, and harness their activities are still largely limited. This research program desires to fill the knowledge gap by integrating discovery, mechanistic investigation, and biocatalytic application to systematically and comprehensively study four types of carbon-carbon (C-C) bond forming and cleaving PLP enzymes, including our recently discovered PLP-dependent Mannich cyclase. These enzymes represent the frontier of PLP enzymology because of their unusual activity, complementary synthetic utility to existing biocatalysts, and unexpected evolutionary relationship with well-characterized PLP enzyme family. All proposed aims are supported by strong preliminary data gathered in our laboratory. Our overarching goal is to understand the chemical and substrate specificity and leverage this understanding to uncover previously unknown functions of PLP-dependent enzymes and explore their non-native catalytic utility. Ultimately, the proposed research will expand our mechanistic understanding on PLP enzymology, shed new light on metabolism, and provide novel biocatalytic tools for amino acid biosynthesis.