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Nargenicin
Nargenicin is a 28 carbon macrolide with a tricyclic lactone containing a unique ether bridge. The polyketide antibiotic was isolated from ''Nocardia argentinensis''. Nargenicin is effective towards gram-positive bacteria and been shown to have strong antibacterial activity against ''Staphylococcus aureus'', a methicillin resistance bacterium. It has also been shown to induce cell differentiation and be used as a possible treatment for neoplastic diseases.
==Biosynthesis==
The biosynthesis of nargenicin is believed to be closely related to fatty acid biosynthesis. David E. Cane and colleagues have shown, through feeding experiments, that nargenicin is derived from common precursors acetate and propionate, specifically 4 propionate and 5 acetate building blocks. Experiments using ()-, ()-, and ()acetate and ()- and () propionate established the carbon skeleton of nargenicin. To determine origin of oxygen atoms, (113C )acetate and ()propionate where incorporated in the experimental feeding. It was then determined that oxygens located at C-1 and C-11 originate from carboxylate oxygens of the acetate precursors while oxygens at C-9 and C-17 positions originate from propoinate precursors.〔 The rest of the oxygens are believed to originate from molecular oxygen. In a later study, David E. Cane and colleagues incorporated the synthesized deutirated N-acetylcysteamine thioester (2S,3R)-(3-13C )-2-methyl-3-hydroxypentanoic acid into nargenicin. The feeding of the labeled intermediate, and other deuterinated ketide fragments, incorporated in nargenicin provide support of the proposed biosynthesis of the polyketide chain. The following illustrates a proposed biosynthesis of nargenicin.
The polyketide chain produced is electronically capable to go through a lactonization and a diels alder cyclization, as shown below. Experiments have shown the incorporation a complex intermediate of a synthesized pentaketide substrate containing four stereocenters and an E,E-diene into the biosynthesis of nargenicin to result in the labeled macrolide nargenicin. The study provides support for the biosynthetic route of nargenicin and absolute configuration. The last biosynthesis stages include the oxidations at C-2, C-8, C-13 and C-18 positions as well as the attachment of the pyrrole carboxylate.〔
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