Solid-Phase-Based Total Synthesis and Stereochemical Assignment of the Cryptic Natural Product Aurantizolicin

Natural Cyclic Peptides: Synthetic Strategies and Biomedical Applications

Natural cyclic peptides, a diverse class of bioactive compounds, have been isolated from various natural sources and are renowned for their extensive structural variability and broad spectrum of medicinal properties. Over 40 cyclic peptides or their derivatives are currently approved as medicines, underscoring their significant therapeutic potential. These compounds are employed in diverse roles, including antibiotics, antifungals, antiparasitics, immune modulators, and anti-inflammatory agents. Their unique ability to combine high specificity with desirable pharmacokinetic properties makes them valuable tools in addressing unmet medical needs, such as combating drug-resistant pathogens and targeting challenging biological pathways. Due to the typically low concentrations of cyclic peptides in nature, effective synthetic strategies are indispensable for their acquisition, characterization, and biological evaluation. Cyclization, a critical step in their synthesis, enhances metabolic stability, bioavailability, and receptor binding affinity. Advances in synthetic methodologies-such as solid-phase peptide synthesis (SPPS), chemoenzymatic approaches, and orthogonal protection strategies-have transformed cyclic peptide production, enabling greater structural complexity and precision. This review compiles recent progress in the total synthesis and biological evaluation of natural cyclic peptides from 2017 onward, categorized by cyclization strategies: head-to-tail; head-to-side-chain; tail-to-side-chain; and side-chain-to-side-chain strategies. Each account includes retrosynthetic analyses, synthetic advancements, and biological data to illustrate their therapeutic relevance and innovative methodologies. Looking ahead, the future of cyclic peptides in drug discovery is bright. Emerging trends, including integrating computational tools for rational design, novel cyclization techniques to improve pharmacokinetic profiles, and interdisciplinary collaboration among chemists, biologists, and computational scientists, promise to expand the scope of cyclic peptide-based therapeutics. These advancements can potentially address complex diseases and advance the broader field of biological drug development.

Analysis of the cryptic biosynthetic gene cluster encoding the RiPP curacozole reveals a phenylalanine-specific peptide hydroxylase

Curacozole is representative of a cyanobactin-like sub-family of ribosomally synthesized and post-translationally modified peptides (RiPPs). The molecule is distinguished by its small macrocyclic structure, a poly-azole sequence that includes a phenyloxazole moiety, and a d-allo-Ile residue. The enzymatic steps required for its formation are not well understood. The predicted biosynthetic gene cluster (BGC) for curacozole in Streptomyces curacoi is cryptic, but is shown to be potently activated upon constitutive expression of the bldA-specified Leu-tRNA(UUA) molecule. Heterologous expression and gene deletion studies have defined the minimum BGC as consisting of seven genes, czlA, D, E, B1, C1, F, and BC. The biosynthetic pathway is highly substrate tolerant, accepting six variants of the precursor peptide CzlA to form new curacozole derivatives. This includes replacing the phenyloxazole moiety of curacozole with indole and p-hydroxyphenyloxazole groups by conversion of the corresponding CzlA Phe18Trp and Phe18Tyr variants. In vitro experiments with purified enzymes demonstrate that CzlD and CzlBC perform cyclodehydration and dehydrogenation reactions, respectively, to form a single oxazole from Ser 22 of CzlA. The curacozole BGC is flanked by czlI, a non-essential but conserved gene of unknown function. In vitro studies demonstrate CzlI to be a non-heme iron(ii) and 2-oxoglutarate-dependent dioxygenase, catalyzing the hydroxylation of Phe18 on CzlA to form the CzlA Phe18Tyr variant, which is then processed to form the p-hydroxyphenyloxazole derivative of curacozole. Overall, this work highlights the amenability of RiPP biosynthesis for engineering the production of new compounds and adds to the repertoire of known RiPP enzymes.

Enzyme-Primed Native Chemical Ligation Produces Autoinducing Cyclopeptides in Clostridia

Clostridia coordinate many important processes such as toxin production, infection, and survival by density‐dependent communication (quorum sensing) using autoinducing peptides (AIPs). Although clostridial AIPs have been proposed to be (thio)lactone‐containing peptides, their true structures remain elusive. Here, we report the genome‐guided discovery of an AIP that controls endospore formation in Ruminiclostridium cellulolyticum. Through a combination of chemical synthesis and chemical complementation assays with a mutant strain, we reveal that the genuine chemical mediator is a homodetic cyclopeptide (cAIP). Kinetic analyses indicate that the mature cAIP is produced via a cryptic thiolactone intermediate that undergoes a rapid S→N acyl shift, in a manner similar to intramolecular native chemical ligation (NCL). Finally, by implementing a chemical probe in a targeted screen, we show that this novel enzyme‐primed, intramolecular NCL is a widespread feature of clostridial AIP biosynthesis.

Marine natural products

This review covers the literature published in 2019 for marine natural products (MNPs), with 719 citations (701 for the period January to December 2019) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1490 in 440 papers for 2019), together with the relevant biological activities, source organisms and country of origin. Pertinent reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. Methods used to study marine fungi and their chemical diversity have also been discussed.