Cyclic Peptides from Graspetide Biosynthesis and Native Chemical Ligation

The ribosomally synthesized and post-translationally modified peptide (RiPP) superfamily of natural products includes many examples of cyclic peptides with diverse macrocyclization chemistries. The graspetides, one family of macrocyclized RiPPs, harbor side chain–side chain ester or amide linkages. We recently reported the structure and biosynthesis of the graspetide pre-fuscimiditide, a 22-amino-acid (aa) peptide with two ester cross-links forming a stem–loop structure. These cross-links are introduced by a single graspetide synthetase, the ATP-grasp enzyme ThfB. Here we show that ThfB can also catalyze the formation of amide or thioester cross-links in prefuscimiditide, with thioester formation being especially efficient. We further show that upon proteolysis to reveal an N-terminal cysteine residue, the thioester-linked peptide rapidly and quantitatively rearranges via native chemical ligation into an isopeptide-bonded head-to-tail cyclic peptide. The solution structure of this rearranged peptide was determined by using 2D NMR spectroscopy experiments. Our methodology offers a straightforward recombinant route to head-to-tail cyclic peptides.