Biosynthesis of octosyl acid A: Incorporation of C-13 labeled glucose

Recent advances in the biosynthesis of nucleoside antibiotics

Nucleoside antibiotics are a diverse class of natural products with promising biomedical activities. These compounds contain a saccharide core and a nucleobase. Despite the large number of nucleoside antibiotics that have been reported, biosynthetic studies on these compounds have been limited compared with those on other types of natural products such as polyketides, peptides, and terpenoids. Due to recent advances in genome sequencing technology, the biosynthesis of nucleoside antibiotics has rapidly been clarified. This review covering 2009-2019 focuses on recent advances in the biosynthesis of nucleoside antibiotics.

Construction of an octosyl acid backbone catalyzed by a radical S-adenosylmethionine enzyme and a phosphatase in the biosynthesis of high-carbon sugar nucleoside antibiotics

Unique bicyclic octosyl uronic acid nucleosides include ezomycin, malayamycin, and octosyl acid (OA). They are structurally characterized by OA, an unusual 8-carbon furanosyl nucleoside core proposed to be the precursor to polyoxin and nikkomycin. Despite the well-known bioactivity of these nucleoside antibiotics, the biosynthesis of OA has not been elucidated yet. Here we report the two pivotal enzymatic steps in the polyoxin biosynthetic pathway leading to the identification of OA as a key intermediate. Our data suggest that this intermediate is formed via a free radical reaction catalyzed by the radical S-adenosylmethionine (SAM) enzyme, PolH, and using 3'-enolpyruvyl uridine 5'-monophosphate (3'-EUMP) as a substrate. Subsequent dephosphorylation catalyzed by phosphatase PolJ converts the resulting octosyl acid 5'-phosphate (OAP) to OA. These results provide, for the first time, significant in vitro evidence for the biosynthetic origins of the C8 backbone of OA.

The biosynthesis of nitrogen-, sulfur-, and high-carbon chain-containing sugars

Carbohydrates serve many structural and functional roles in biology. While the majority of monosaccharides are characterized by the chemical composition (CH2O)n, modifications including deoxygenation, C-alkylation, amination, O- and N-methylation, which are characteristic of many sugar appendages of secondary metabolites, are not uncommon. Interestingly, some sugar molecules are formed via modifications including amine oxidation, sulfur incorporation, and "high-carbon" chain attachment. Most of these unusual sugars have been identified over the past several decades as components of microbially produced natural products, although a few high-carbon sugars are also found in the lipooligosaccharides of the outer cell walls of Gram-negative bacteria. Despite their broad distribution in nature, these sugars are considered "rare" due to their relative scarcity. The biosynthetic steps that underlie their formation continue to perplex researchers to this day and many questions regarding key transformations remain unanswered. This review will focus on our current understanding of the biosynthesis of unusual sugars bearing oxidized amine substituents, thio-functional groups, and high-carbon chains.

Total synthesis of polyoximic acid

The structure and stereochemistry of polyoximic acid, a degradation product of polyoxins, was originally designated as trans-3-ethylidene-L-azetidine-2-carboxylic acid. However, total synthesis revealed that the correct structure was in fact cis-3-ethylidene-L-azetidine-2-carboxylic acid, which was confirmed by X-ray crystallography. The synthesis of the trans-isomer was also done and its identity was confirmed by X-ray analysis as well. The key step for constructing the four-membered ring was a rhodium catalyzed carbenoid insertion into the N—H bond of a beta-amino acid derivative. The stereoselectivity of the exo-double bond was controlled by conducting a Horner-Emmons-Wadsworth or a Wittig reaction to generate the trans- and cis-isomers, respectively. Weinreb's amide was used as a latent methyl group for the separation of trans and cis mixtures. The double bond stereochemistry of polyoximic acid in the parent polyoxin was also confirmed to be cis by extensive 2D NMR studies.Key words: diazoinsertion, azetidine, olefination.

Current progress on nucleoside antibiotics

Structure and biological activity of thirty-six new nucleoside antibiotics which appeared after the 1988 review are described. New synthetic analogs of neplanocin and oxetanocin are also described with special emphasis on their antiviral activities. New biosynthetic findings on nikkomycins, blasticidin S, and griseolic acid are also reviewed.