Merging Flow Synthesis and Enzymatic Maturation to Expand the Chemical Space of Lasso Peptides

Many peptidic natural products—such as lasso peptides, cyclic peptides, and cyclotides—are conformationally constrained and show biological stability, making them attractive scaffolds for drug development. Although many peptides can be synthesized and modified through chemical methods, knot-like lasso peptides such as microcin J25 (MccJ25) and their analogs remain elusive. As the chemical space of MccJ25 analogs accessible through purely biological methods is also limited, we proposed a hybrid approach: flow-based chemical synthesis of non-natural precursor peptides followed by in vitro transformation with recombinant maturation enzymes to yield a more diverse array of lasso peptides. Herein, we established the rapid, flow-based synthesis of chemically modified MccJ25 precursor peptides (57 amino acids). Heterologous expression of enzymes McjB and McjC was extensively optimized to improve yields and facilitate the synthesis of multiple analogs of MccJ25, including the incorporation of non-canonical tyrosine and histidine derivatives into the lasso scaffold. Finally, using our chemoenzymatic strategy, we produced a biologically active analog containing three D-amino acids in the loop region and incorporated backbone N-methylations. Our method provides rapid access to chemically modified lasso peptides that could be used to investigate structure-activity relationships, epitope grafting, and improvement of therapeutic properties.