Research
Natural Product Discovery
Peptide natural products comprise a diverse set of pharmaceutically relevant antibiotic, antiviral, immunosuppressive, nematicidal, and anti-cancer agents. Many of these pharmaceuticals harbor backbone α-N-methylations and macrocyclizations, since these tailorings significantly improve peptide pharmacokinetics due to increased structural rigidity, proteolytic resistance, and membrane permeability as seen in the blockbuster immunosuppressant cyclosporin A. We identified a new biosynthetic route to backbone α-N-methylated peptides through the co-discovery of the borosin peptide natural product family. While natural sources of N-methylated peptides have been known for over 100 years, they were never found to be made by the ribosome. Borosins originate from an enzyme synthesized by the ribosome that self-modifies its own backbone with multiple N-methylations to eventually produce the final metabolite. This unprecedented biocatalysis affords new routes to engineered N-methylated metabolites and we are actively pursuing genome mining and metabolite discovery platforms for new borosins from basidiomycete fungi.
Unraveling the Secrets of RiPP Biosynthetic Machinery
Recently, we have uncovered a large and diverse subfamily of 'split' borosin pathways in bacteria that, unlike their fungal counterparts, follow canonical RiPP biosynthetic logic. RiPPs are Ribosomally synthesized and Post-translationally modified Peptide natural products. Precursor peptides are first transcribed and translated by the ribosome. These precursors are substrates for iteratively acting enzymes that install chemically unique post-translational modifications, like α-N-methylations, to produce the final peptide metabolites. We in the Freeman Lab are interested in uncovering the mechanisms and dynamics for how these molecular machines iteratively act on their peptide substrates. Along with our collaborators, we are using an array of kinetics, protein structure, and biophysical techniques to disentangle the complex catalytic cycles of split borosin systems. We are especially interested in the contributions of RiPP peptide backbone modifications that may affect precursor secondary structure as a means to direct future sites of modification.
