Mycinamicins, new macrolide antibiotics. VIII. Chemical degradation and absolute configuration of mycinamicins

A comprehensive review of glycosylated bacterial natural products

A systematic analysis of all naturally-occurring glycosylated bacterial secondary metabolites reported in the scientific literature up through early 2013 is presented. This comprehensive analysis of 15 940 bacterial natural products revealed 3426 glycosides containing 344 distinct appended carbohydrates and highlights a range of unique opportunities for future biosynthetic study and glycodiversification efforts.

Salinipyrone and Pacificanone Are Biosynthetic By-products of the Rosamicin Polyketide Synthase

Salinipyrones and pacificanones are structurally related polyketides from Salinispora pacifica CNS-237 that are proposed to arise from the same modular polyketide synthase (PKS) assembly line. Genome sequencing revealed a large macrolide PKS gene cluster that codes for the biosynthesis of rosamicin A and a series of new macrolide antibiotics. Mutagenesis experiments unexpectedly correlated salinipyrone and pacificanone biosynthesis to the rosamicin octamodule Spr PKS. Remarkably, this bifurcated polyketide pathway illuminates a series of enzymatic domain- and module-skipping reactions that give rise to natural polyketide product diversity. Our findings enlarge the growing knowledge of polyketide biochemistry and illuminate potential challenges in PKS bioengineering.

Substrate recognition by the multifunctional cytochrome P450 MycG in mycinamicin hydroxylation and epoxidation reactions

The majority of characterized cytochrome P450 enzymes in actinomycete secondary metabolic pathways are strictly substrate-, regio-, and stereo-specific. Examples of multifunctional biosynthetic cytochromes P450 with broader substrate and regio-specificity are growing in number and are of particular interest for biosynthetic and chemoenzymatic applications. MycG is among the first P450 monooxygenases characterized that catalyzes both hydroxylation and epoxidation reactions in the final biosynthetic steps, leading to oxidative tailoring of the 16-membered ring macrolide antibiotic mycinamicin II in the actinomycete Micromonospora griseorubida. The ordering of steps to complete the biosynthetic process involves a complex substrate recognition pattern by the enzyme and interplay between three tailoring modifications as follows: glycosylation, methylation, and oxidation. To understand the catalytic properties of MycG, we structurally characterized the ligand-free enzyme and its complexes with three native metabolites. These include substrates mycinamicin IV and V and their biosynthetic precursor mycinamicin III, which carries the monomethoxy sugar javose instead of the dimethoxylated sugar mycinose. The two methoxy groups of mycinose serve as sensors that mediate initial recognition to discriminate between closely related substrates in the post-polyketide oxidative tailoring of mycinamicin metabolites. Because x-ray structures alone did not explain the mechanisms of macrolide hydroxylation and epoxidation, paramagnetic NMR relaxation measurements were conducted. Molecular modeling based on these data indicates that in solution substrate may penetrate the active site sufficiently to place the abstracted hydrogen atom of mycinamicin IV within 6 Å of the heme iron and ~4 Å of the oxygen of iron-ligated water.

Astonishing diversity of natural surfactants: 2. Polyether glycosidic ionophores and macrocyclic glycosides

Polyether glycosidic ionophores and macrocyclic glycosides are of great interest, especially for the medicinal and pharmaceutical industries. These biologically active natural surfactants are good prospects for the future chemical preparation of compounds useful as antibiotics, anticancer agents, or in industry. More than 300 interesting and unusual natural surfactants are described in this review article, including their chemical structures and biological activities.

Chemical transformation of tylosin derivatives into neutral macrolides having a 3'-methoxyl group

This paper describes the chemical transformation of the basic 16-membered macrolides, tylosin derivatives, into neutral macrolides having a 3'-methoxyl group. 2',4'-Di-O-acetyl-3,23-bis(O-tert-butyldimethylsilyl)mycaminosyltylon olide 9,20-bis(ethylene acetal) N-oxide (1b) was treated with Ac2O-pyridine in CH2Cl2 to afford the 3'-ketone 1c and the 3'-N-acetyl-3'-N-demethyl derivative 1d in 67 and 5% yield; respectively. Reduction of 1c with Zn(BH4)2 gave the 3'-alcohol 1e in 84% yield stereoselectively. O-Methylation of 1e with MeOTf and 2,6-di-tert-butylpyridine gave the 3'-methyl ether 1f in 49% yield in spite of the presence of the adjacent acetoxyl groups. Deprotection of 1f provided the desired neutral macrolide 1g. Similar synthetic routes were also used for transformation of the suitably protected 4'-deoxymycaminosyltylonolide 2b and desmycosin 3c into neutral macrolides having a 3'-methoxyl group. It was found that the mycinose moiety of a neutral macrolide plays an important role in its antimicrobial activity.