Genome sequencing of environmental bacteria allows identification of biosynthetic gene clusters encoding unusual combinations of enzymes that produce unknown natural products. We identified a pathway in which a ribosomally synthesized small peptide serves as a scaffold for nonribosomal peptide extension and chemical modification. Amino acids are transferred to the carboxyl terminus of the peptide through adenosine triphosphate and amino acyl-tRNA–dependent chemistry that is independent of the ribosome. Oxidative rearrangement, carboxymethylation, and proteolysis of a terminal cysteine yields an amino acid–derived small molecule. Microcrystal electron diffraction demonstrates that the resulting product is isosteric to glutamate. We show that a similar peptide extension is used during the biosynthesis of the ammosamides, which are cytotoxic pyrroloquinoline alkaloids. These results suggest an alternative paradigm for biosynthesis of amino acid–derived natural products.

The pool of abundant chiral terpene building blocks (i.e. “chiral pool terpenes”) has long served as a starting point for the chemical synthesis of complex natural products, including many terpenes themselves. As inexpensive and versatile starting materials, such compounds continue to influence modern synthetic chemistry. This review highlights 21st century terpene total syntheses which themselves use small, terpene-derived materials as building blocks. An outlook to the future of research in this area is highlighted as well.

Aromatic amines in nature are typically installed with Glu or Gln as the nitrogen donor. Here we report a pathway that features glycyl-tRNA as the nitrogen donor. During the biosynthesis of pyrroloiminoquinone-type natural products such as ammosamide, pe ptide- a minoacyl t R NA l igases (PEARLs) append amino acids to the C-terminus of a ribosomally synthesized peptide. First, adds Trp in a Trp-tRNA dependent reaction, and the flavoprotein AmmC 1 then carries out three hydroxylations of the indole ring of Trp. After oxidation to the corresponding ortho -hydroxy para- quinone, attaches Gly to the indole ring in a Gly-tRNA dependent fashion. Subsequent decarboxylation and hydrolysis results in an amino-substituted indole. Similar transformations are catalyzed by orthologous enzymes from Bacillus halodurans . This pathway features three previously unknown biochemical processes using a ribosomally synthesized peptide as scaffold for non-ribosomal peptide extension and chemical modification to generate an amino acid derived natural product.

A short total synthesis of podophyllotoxin, the prototypical aryltetralin lignan natural product, is reported. Key to the success of this synthetic pathway is a Pd-catalyzed C(sp(3))-H arylation reaction enabled by a conformational biasing element to promote C-C reductive elimination over an alternative C-N bond-forming pathway. This strategy allows for access to the natural product in five steps from commercially available bromopiperonal, and sheds light on subtle conformational effects governing reductive elimination pathways from high-valent palladium centers.

The domain of unknown function 692 (DUF692) is an emerging family of post-translational modification enzymes involved in the biosynthesis of ribosomally synthesized and post-translationally modified peptide (RiPP) natural products. Members of this family are multinuclear iron-containing enzymes, and only two members have been functionally characterized to date: MbnB and TglH. Here, we used bioinformatics to select another member of the DUF692 family, ChrH, that is encoded in the genomes of the Chryseobacterium genus along with a partner protein ChrI. We structurally characterized the ChrH reaction product and show that the enzyme complex catalyzes an unprecedented chemical transformation that results in the formation of a macrocycle, an imidazolidinedione heterocycle, two thioaminals, and a thiomethyl group. Based on isotopic labeling studies, we propose a mechanism for the four-electron oxidation and methylation of the substrate peptide. This work identifies the first SAM-dependent reaction catalyzed by a DUF692 enzyme complex, further expanding the repertoire of remarkable reactions catalyzed by these enzymes. Based on the three currently characterized DUF692 family members, we suggest the family be called multinuclear non-heme iron dependent oxidative enzymes (MNIOs).