Leader-Independent C‑Terminal Modification by a Radical <i>S</i>‑Adenosyl‑l‑methionine Maturase Enables Macrocyclic GLP-1-Like Peptides.
This study investigated PapB, a radical S-adenosyl-l-methionine (rSAM) enzyme involved in the biosynthesis of ribosomally synthesized and post-translationally modified peptides (RiPPs). Classically, RiPP maturase enzymes require an N-terminal leader sequence on the precursor peptide to guide substrate recognition and modification. The researchers discovered that PapB can function in a "leader-independent" manner — meaning it can process peptide substrates that entirely lack canonical leader sequences. To demonstrate the practical utility of this finding, the team applied PapB to three analogues of glucagon-like peptide (GLP-1) pathway agonists — a therapeutically relevant class of peptides — and showed that the enzyme achieved complete conversion of each linear peptide into a thioether-macrocyclized (C-terminally cyclized) product. The study is primarily a biochemical and enzymological characterization conducted in vitro, with no human or animal subjects involved. Limitations include that all work was performed outside of a biological system, and the therapeutic relevance of the resulting macrocyclic GLP-1 analogues in vivo remains to be established. The findings position PapB as a potentially versatile biocatalytic tool for generating conformationally constrained peptide drug candidates.
Why this grade: All experimental work was conducted using purified enzyme and synthetic peptide substrates in vitro, with no animal models or human subjects employed.
Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a rapidly expanding family of natural products in which biosynthetic maturase enzymes tailor ribosomal precursors into bioactive products. Classical RiPP maturation relies on an N-terminal leader sequence in the precursor peptide and a complementary RiPP-recognition element in the enzyme to guide substrate binding. Herein, we interrogated PapB, a radical S -adenosyl-l-methionine RiPP maturase known to introduce thio-(seleno)-ether cross-links and discovered that its catalytic reach extends well beyond this paradigm. PapB efficiently processes substrates that lack any canonical leader sequence, demonstrating bona fide leader-independent activity. To highlight the practical value of this capability, we applied PapB to three therapeutically relevant analogues of glucagon-like peptide pathway agonists to generate C-terminal cyclic structures. In every case, the enzyme achieved complete conversion of the linear to the thioether macrocyclized peptide. These results establish PapB as a versatile, plug-and-play biocatalyst for late-stage macrocyclization of structurally diverse peptides, opening a direct route to conformationally constrained therapeutic candidates without the need for leader tags.
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