Analysis of the Valgamicin Biosynthetic Pathway Reveals a General Mechanism for Cyclopropanol Formation across Diverse Natural Product Scaffolds

Nitrocyclopropanes as Valuable Building Blocks in Organic Synthesis and Biology: Exploring the Origin of the Nitro Group

Cyclopropane derivatives serve as important building blocks in organic synthesis, due to their three-membered stretched ring. Nitro-substituted cyclopropanes (NCPs) represent a special class of donor-acceptor cyclopropanes (DACs) that contain a strong electron-withdrawing nitro group as substituent on the cyclopropyl moiety. Due to the versatile nature of the nitro group, which can be converted into other functional azo groups or heterocyclic scaffolds, NCPs are considered important building blocks in the design of complex organic compounds. Herein, the present review is organized in two main sections focused on the synthesis and application of NCPs. The part containing the synthetic methodologies of NCPs has been further divided into distinct sections based mainly on the nature of the starting materials, such as: a) from nitroalkenes, b) from nitro-bearing diazo compounds, c) from nitroalkanes in combination with alkenes, d) from aminocyclopropanes under oxidative conditions, and e) from various nitro compounds, through miscellaneous processes. In the part concerning the applications and use of NCPs, the categorization includes three main subcategories, concerning their reduction to 1-aminocyclopropane-1-carboxylic acids (ACC), the ring opening of NCPs leading to valuable open-chain heterocycles, and the ring expansion to larger cyclic and heterocyclic compounds.

Biosynthesis and recruitment of reactive amino acids in nonribosomal peptide assembly lines

Reactive amino acid side chains play important roles in the binding of peptides to specific targets. In addition, their reactivity enables selective peptide conjugation and functionalization for pharmaceutical purposes. Diverse reactive amino acids are incorporated into nonribosomal peptides, which serve as a source for drug candidates. Notable examples include (poly)unsaturated (enamine, alkyne, and furyl) and halogenated residues, strained carbacycles (cyclopropyl and cyclopropanol), small heterocycles (oxirane and aziridine), and reactive N-N functionalities (hydrazones, diazo compounds, and diazeniumdiolates). Their biosynthesis requires diverse biocatalysts for sophisticated reaction mechanisms. Several avenues have been identified for their incorporation into peptides, the recruitment by adenylation domains or ligases, on-line modifications, and enzymatic tailoring reactions. Combined with protein engineering approaches, this knowledge provides new opportunities in synthetic biology and bioorthogonal chemistry.

Direct α-Hydroxy Acid Loading onto a Bacterial Thiotemplate Assembly Line via a Multienzyme Gateway

Various nonribosomal peptide synthetases (NRPSs) create structural and functional diversity by incorporating α-hydroxy acids into peptide backbones. Trigonic acid, an unusual cyclopropanol-substituted hydroxy acid, is the source of the molecular warhead of malleicyprol, a critical virulence factor of human and animal pathogens of the Burkholderia pseudomallei (BP) group. The process of selecting and loading this building block remained enigmatic as the NRPS module designated for this task is incomplete. Using a combination of bioinformatics, mutational analyses, targeted metabolomics, and in vitro biochemical assays, we show that two trans-acting enzymes are required to load this central building block onto the modular assembly line. An adenylation-thiolation didomain enzyme (BurJ) activates trigonic acid, followed by the translocation of the enzyme-bound α-hydroxy acid thioester by an FkbH-like protein with a mutated phosphatase domain (BurH). This specialized gateway is the first reported direct loading of an α-hydroxy acid onto a bona fide NRPS module in bacteria and expands the synthetic biology toolbox for the site-specific incorporation of non-canonical building blocks. Moreover, insight into the biochemical basis of virulence factor biosynthesis can provide a foundation for developing enzyme inhibitors as anti-virulence therapeutics against BP pathogen infections.

Hot off the Press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as eugeniinaline A from Leuconotis eugeniifolia.