Conserved Mechanism of 2'-Phosphorylation-Aided Amide Ligation in Peptidyl Nucleoside Biosynthesis

Biosyntheses of azetidine-containing natural products

Azetidine is a four-membered polar heterocycle including a basic secondary amine, and is characterized by its high ring-strain energy, strong molecular rigidity and satisfactory stability. As a result, azetidine exhibits great challenges in its chemical synthesis and biosynthesis, which may explain the limited number of azetidine-containing natural products uncovered to date. In particular, the biosynthetic mechanisms of naturally occurring azetidines are poorly understood. Only some of them have been intensively investigated and few reviews have been published for the summarization of azetidine biosynthesis. In this review, we provide a comprehensive description of the biosyntheses of all the azetidine-containing natural products, especially the biosyntheses of azetidine moieties. We hope that this review will draw much attention to the biosynthetic research of the largely unexplored azetidine moieties as well as the discovery of novel azetidine-containing natural products in the near future.

Biosynthesis and Genome Mining Potentials of Nucleoside Natural Products

Nucleoside natural products show diverse biological activities and serve as leads for various application purposes, including human and veterinary medicine and agriculture. Studies in the past decade revealed that these nucleosides are biosynthesized through divergent mechanisms, in which early steps of the pathways can be classified into two types (C5' oxidation and C5' radical extension), while the structural diversity is created by downstream tailoring enzymes. Based on this biosynthetic logic, we investigated the genome mining discovery potentials of these nucleosides using the two enzymes representing the two types of C5' modifications: LipL-type α-ketoglutarate (α-KG) and Fe-dependent oxygenases and NikJ-type radical S-adenosyl-L-methionine (SAM) enzymes. The results suggest that this approach allows discovery of putative nucleoside biosynthetic gene clusters (BGCs) and the prediction of the core nucleoside structures. The results also revealed the distribution of these pathways in nature and implied the possibility of future genome mining discovery of novel nucleoside natural products.

Cryptic Phosphorylation-Mediated Divergent Biosynthesis of High-Carbon Sugar Nucleoside Antifungals

Establishing a general biosynthetic scheme for natural products is critical for a broader understanding of natural product biosynthesis and the structural prediction of metabolites based on genome sequence information. High-carbon sugar nucleoside antimicrobials are an underexplored class of natural products with unique structures and important biological activities. Recent studies on C6 sugar nucleoside antifungal natural products, such as nikkomycins and polyoxins, revealed a novel biosynthetic mechanism involving cryptic phosphorylation. However, the generality of this biosynthetic mechanism remained unexplored. We here report in vitro characterization of the biosynthesis of a C7 sugar nucleoside antifungal, malayamycin A. Our results demonstrate that the malayamycin biosynthetic enzymes specifically accept 2'-phosphorylated biosynthetic intermediates, suggesting that cryptic phosphorylation-mediated biosynthesis is conserved beyond C6 sugar nucleosides. Furthermore, the results suggest a generalizable divergent biosynthetic mechanism for high-carbon sugar nucleoside antifungals. In this model, C6 and C7 sugar nucleoside biosyntheses proceed via a common C8 sugar nucleoside precursor, and the sugar size is determined using the functions of α-ketoglutarate (α-KG)-dependent dioxygenases (NikI/PolD for C6 sugar nucleosides and MalI for C7 sugar nucleosides). These results provide an important guidance for the future genome-mining discovery of high-carbon sugar nucleoside antimicrobials.