Glycothiohexides, novel antibiotics produced by "Sebekia" sp. LL-14E605. I. Taxonomy, fermentation and biological evaluation

Importance of Noncovalent Interactions Involving Sulfur Atoms in Thiopeptide Antibiotics─Glycothiohexide α and Nocathiacin I

Noncovalent interactions involving sulfur atoms play essential roles in protein structure and function by significantly contributing to protein stability, folding, and biological activity. Sulfur is a highly polarizable atom that can participate in many types of noncovalent interactions including hydrogen bonding, sulfur-π interactions, and S-lone pair interactions, but the impact of these sulfur-based interactions on molecular recognition and drug design is still often underappreciated. Here, we examine, using quantum chemical calculations, the roles of sulfur-based noncovalent interactions in complex naturally occurring molecules representative of thiopeptide antibiotics: glycothiohexide α and its close structural analogue nocathiacin I. While donor-acceptor orbital interactions make only very small contributions, electrostatic and dispersion contributions are predicted to be significant in many cases. In pursuit of understanding the magnitudes and nature of these noncovalent interactions, we made potential structural modifications that could significantly expand the chemical space of effective thiopeptide antibiotics.

De Novo Synthesis of an l-Lemonose Thioglycoside Donor from d-Threonine

A short de novo synthesis of an l-lemonose thioglycoside is described starting from d-threonine. The synthesis leverages a Dieckmann condensation and Stork-Danheiser transposition to arrive at a key vinylogous ester intermediate on gram scale. Ensuing 1,2-addition diastereoselectively establishes the C3 tetra-substituted center and subsequent glycal hydration allows for anomeric functionalization to the thioglycoside. ¹H and NOESY NMR analyses reveal that the major α-anomer of thioglycoside deviates from the expected ¹C₄ conformation.

YcaO-Dependent Posttranslational Amide Activation: Biosynthesis, Structure, and Function

With advances in sequencing technology, uncharacterized proteins and domains of unknown function (DUFs) are rapidly accumulating in sequence databases and offer an opportunity to discover new protein chemistry and reaction mechanisms. The focus of this review, the formerly enigmatic YcaO superfamily (DUF181), has been found to catalyze a unique phosphorylation of a ribosomal peptide backbone amide upon attack by different nucleophiles. Established nucleophiles are the side chains of Cys, Ser, and Thr which gives rise to azoline/azole biosynthesis in ribosomally synthesized and posttranslationally modified peptide (RiPP) natural products. However, much remains unknown about the potential for YcaO proteins to collaborate with other nucleophiles. Recent work suggests potential in forming thioamides, macroamidines, and possibly additional post-translational modifications. This review covers all knowledge through mid-2016 regarding the biosynthetic gene clusters (BGCs), natural products, functions, mechanisms, and applications of YcaO proteins and outlines likely future research directions for this protein superfamily.

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.

Activity of the thiopeptide antibiotic nosiheptide against contemporary strains of methicillin-resistant Staphylococcus aureus

The rapid rise in antimicrobial resistance in bacteria has generated an increased demand for the development of novel therapies to treat contemporary infections, especially those caused by methicillin-resistant Staphylococcus aureus (MRSA). However, antimicrobial development has been largely abandoned by the pharmaceutical industry. We recently isolated the previously described thiopeptide antibiotic nosiheptide from a marine actinomycete strain and evaluated its activity against contemporary clinically relevant bacterial pathogens. Nosiheptide exhibited extremely potent activity against all contemporary MRSA strains tested including multiple drug-resistant clinical isolates, with MIC values ≤ 0.25 mg/L. Nosiheptide was also highly active against Enterococcus spp and the contemporary hypervirulent BI strain of Clostridium difficile but was inactive against most Gram-negative strains tested. Time-kill analysis revealed nosiheptide to be rapidly bactericidal against MRSA in a concentration- and time-dependent manner, with a nearly 2-log kill noted at 6 hours at 10X MIC. Furthermore, nosiheptide was found to be non-cytotoxic against mammalian cells at >> 100X MIC, and its anti-MRSA activity was not inhibited by 20% human serum. Notably, nosiheptide exhibited a significantly prolonged post-antibiotic effect (PAE) against both healthcare- and community-associated MRSA compared to vancomycin. Nosiheptide also demonstrated in vivo activity in a murine model of MRSA infection, and therefore represents a promising antibiotic for the treatment of serious infections caused by contemporary strains of MRSA.

Chemical conversion of nocathiacin I to nocathiacin II and a lactone analogue of glycothiohexide alpha

Nocathiacin I (1) was converted to its deoxy indole analogue, nocathiacin II (2), another natural product, by a unique and facile chemical process. This process was applied to nocathiacin IV (4), generating the lactone analogue of glycothiohexide alpha (5), which was also prepared from nocathiacin II by a mild hydrolytic process. In contrast to glycothiohexide alpha (3), this lactone analogue (5) was found to exhibit in vivo antibacterial efficacy in an animal (mouse) infection model.

Novel semi-synthetic nocathiacin antibiotics: synthesis and antibacterial activity of bis- and mono-O-alkylated derivatives

Several semi-synthetic bis- and mono-O-alkyl nocathiacin derivatives were synthesized and evaluated for antibacterial activity. Mono-O-alkyl N-hydroxyindole analogues 3a-l were prepared by regioselective alkylation. Bis-O-alkyl nocathiacins 4a-f were obtained by treatment with base and excess electrophile. A one-pot protection-alkylation-deprotection strategy was developed for the preparation of mono-O-alkyl hydroxypyridine analogues 5a,b. Most of the bis- and mono-O-alkyl nocathiacins maintained good in vitro activity but showed reduced in vivo efficacy when compared with the natural product. The excellent in vivo activity and improved water solubility of phosphate analogues 3m and 4g suggest their use as potential pro-drugs.