Precursor-Directed Biosynthesis and Biological Testing of omega-Alicyclic- and neo-Branched Tunicamycin N-Acyl Variants

Tunicamycins (TUNs) are Streptomyces-derived natural products, widely used to block protein N-glycosylation in eukaryotes or cell wall biosynthesis in bacteria. Modified or synthetic TUN analogues that uncouple these activities have considerable potential as novel mode-of-action antibacterial agents. Chemically modified TUNs reported previously with attenuated activity on yeast have pinpointed eukaryotic-specific chemophores in the uridyl group and the N-acyl chain length and terminal branching pattern. A small molecule screen of fatty acid biosynthetic primers identified several novel alicyclic- and neo-branched TUN N-acyl variants, with primer incorporation at the terminal omega-acyl position. TUNs with unique 5- and 6-carbon ω-cycloalkane and ω-cycloalkene acyl chains are produced under fermentation and in yields comparable with the native TUN. The purification, structural assignments, and the comparable antimicrobial properties of 15 of these compounds are reported, greatly extending the structural diversity of this class of compounds for potential medicinal and agricultural applications.

Branched Chain Lipid Metabolism As a Determinant of the N-Acyl Variation of Streptomyces Natural Products

Branched-chain fatty acids (BCFA) are encountered in Gram-positive bacteria, but less so in other organisms. The bacterial BCFA in membranes are typically saturated, with both odd- and even-numbered carbon chain lengths, and with methyl branches at either the ω-1 (iso) or ω-2 (anteiso) positions. The acylation with BCFA also contributes to the structural diversity of microbial natural products and potentially modulates biological activity. For the tunicamycin (TUN) family of natural products, the toxicity toward eukaryotes is highly dependent upon N-acylation with trans-2,3-unsaturated BCFA. The loss of the 2,3-unsaturation gives modified TUN with reduced eukaryotic toxicity but crucially with retention of the synergistic enhancement of the β-lactam group of antibiotics. Here, we infer from genomics, mass spectrometry, and deuterium labeling that the trans-2,3-unsaturated TUN variants and the saturated cellular lipids found in TUN-producing Streptomyces are derived from the same pool of BCFA metabolites. Moreover, non-natural primers of BCFA metabolism are selectively incorporated into the cellular lipids of TUN-producing Streptomyces and concomitantly produce structurally novel neo-branched TUN N-acyl variants.

Exploring the Active Site of the Antibacterial Target MraY by Modified Tunicamycins

The alarming growth of antibiotic resistance that is currently ongoing is a serious threat to human health. One of the most promising novel antibiotic targets is MraY (phospho-MurNAc-pentapeptide-transferase), an essential enzyme in bacterial cell wall synthesis. Through recent advances in biochemical research, there is now structural information available for MraY, and for its human homologue GPT (GlcNAc-1-P-transferase), that opens up exciting possibilities for structure-based drug design. The antibiotic compound tunicamycin is a natural product inhibitor of MraY that is also toxic to eukaryotes through its binding to GPT. In this work, we have used tunicamycin and modified versions of tunicamycin as tool compounds to explore the active site of MraY and to gain further insight into what determines inhibitor potency. We have investigated tunicamycin variants where the following motifs have been modified: the length and branching of the tunicamycin fatty acyl chain, the saturation of the fatty acyl chain, the 6″-hydroxyl group of the GlcNAc ring, and the ring structure of the uracil motif. The compounds are analyzed in terms of how potently they bind to MraY, inhibit the activity of the enzyme, and affect the protein thermal stability. Finally, we rationalize these results in the context of the protein structures of MraY and GPT.

Rhodium-catalyzed reductive modification of pyrimidine nucleosides, nucleotide phosphates, and sugar nucleotides

Nucleosides and nucleotides are a group of small molecule effectors and substrates which include sugar nucleotides, purine and pyrimidine-based nucleotide phosphates, and diverse nucleotide antibiotics. We previously reported that hydrogenation of the nucleotide antibiotic tunicamycin leads to products with reduced toxicity on eukaryotic cells. We now report the hydrogenation of diverse sugar nucleosides, nucleotide phosphates, and pyrimidine nucleotides. UDP-sugars and other uridyl and thymidinyl nucleosides are quantitatively reduced to the corresponding 5,6-dihydro-nucleosides. Cytidyl pyrimidines are reduced, but the major products are the corresponding 5,6-dihydrouridyl nucleosides resulting from a deamination of the cytosine ring.

Thiazolidine Peracetates: Carbohydrate Derivatives that Readily Assign cis-, trans-2,3-Monosaccharides by Gas Chromatography-Mass Spectrometry Analysis

A novel group of carbohydrate derivatives is described that uniquely assign cis/ trans-2,3-aldose stereoisomers at low nanomolar concentrations. Aldopentoses, aldohexoses, or component aldoses from hydrolysis of polysaccharides or oligosaccharides react with cysteamine in pyridine to give quantitative formation of thiazolidines, which are subsequently peracetylated in a one-pot reaction. The nonpolar thiazolidines peracetate (TPA) derivatives are analyzed by gas chromatography and electron impact mass spectrometry (GC/EI-MS), each aldose giving rise to two TPA geometric isomers. The quantitative ratio of these diastereomers is dependent upon whether the parent monosaccharide is cis-2,3-(Rib, Lyx, Man, All, Gul, and Tal), or trans-2,3-aldose (Xyl, Ara, Glc, Gal, Ido, and Alt). TPAs generate observed EI-MS fragment ions characteristic of C1-C2 and C3-C4 bond cleavage of the parent sugars. This has been used to estimate the extent of metabolic labeling of microbial cell-wall carbohydrates, especially into the defining anomeric carbons and during aldolase / ketolase -catalyzed rearrangements.

Evaluation of carbohydrate-cysteamine thiazolidines as pro-drugs for the treatment of cystinosis

Cystinosis is a genetic disorder caused by malfunction of cystinosin and is characterized by accumulation of cystine. Cysteamine, the medication used in cystinosis, causes halitosis resulting in poor patient compliance. Halitosis is mainly caused by the formation of dimethylsulfide as the final product in the cysteamine metabolism pathway. We have synthesized carbohydrate-cysteamine thiazolidines, and hypothesized that the hydrolytic breakdown of cysteamine-thiazolidines can result in free cysteamine being released in target organs. To examine our hypothesis, we tested these analogs in vitro in patient-derived fibroblasts. Cystinotic fibroblasts were treated with different concentrations of arabinose-cysteamine, glucose-cysteamine and maltose-cysteamine. We demonstrated that the analogs break down into cysteamine extracellularly and might therefore not be fully taken up by the cells under the form of the pro-drug. Potential modifications of the analogs that enable their intracellular rather than extracellular breakdown, is necessary to pursue the potential of these analogs as pro-drugs.

Galactoglucomannan Oligosaccharides (GGMO) from a molasses byproduct of pine ( Pinus taeda ) fiberboard production

"Temulose" is the trade name for a water-soluble molasses produced on a large scale (300-400 tonnes per year) as a byproduct of the fiberboard industry. The feedstock for Temulose is predominantly a single species of pine ( Pinus taeda ) grown and harvested in stands in southeastern Texas. Because of the method of production, the molasses was predicted to consist of water-soluble hemicelluloses, mainly arabinoxylan-type and galactoglucomannan-type oligosaccharides, plus minor components of lignin, but no detailed structural study had been reported. The structure and composition of the molasses has now been deduced by a combination of MALDI-TOF mass spectrometry, size exclusion chromatography, proton and (13)C NMR techniques, and classic carbohydrate analysis. Limited acid hydrolysis released a series of galactoglucomannan oligosaccharides (GGMO) that were selectively recovered from the acid-labile arabinogalactan by precipitation with ethanol. The precipitate was named "Temulose brown sugar" because of its appearance, and is shown to consist of GGMO with a degree of polymerization (DP) from 4 to 13, with the major component being DP 5-8. The structure of these oligosaccharides is a β-1,4-linked backbone of Man and Glc residues, with occasional α-1,6 branching by single galactosyl units.

Do indium 111-labeled polymorphonuclear leukocytes detect neutrophil-prominent, fibrosing lung disease in rats?

The role of polymorphonuclear leukocytes (PMNs) in fibrotic lung disease remains poorly defined, but PMN prominence may predict a poor response to therapy in some patients. The bleomycin injury to rats represents a model of PMN-prominent alveolitis leading to fibrosis. We examined a method of identifying the alveolitis during its PMN-prominent phase with indium 111-labeled homologous PMNs (In-PMNs). When pairs of injured and uninjured rats were compared, greater activity was consistently found persisting in the lung images of the injured rats, both several hours (early) and a day (late) after injection of In-PMNs. Differences could likewise be identified when lung activity was normalized to activity of the injectate, the spleen, and variably the liver, as determined from organ images. These differences in gamma camera images were supported by direct measurement of activity in organs obtained by killing the animals after scanning. When groups of animals were compared, lung/spleen ratios appeared to most consistently differentiate the group of normal animals from those with alveolitis. These methods may provide a noninvasive means of identifying PMN prominence in the lung, which occurs in some fibrotic processes.