Natural products have been a rich source of compounds for drug discovery. However, their use has diminished in the past two decades, in part because of technical barriers to screening natural products in high-throughput assays against molecular targets. Here, we review strategies for natural product screening that harness the recent technical advances that have reduced these barriers. We also assess the use of genomic and metabolomic approaches to augment traditional methods of studying natural products, and highlight recent examples of natural products in antimicrobial drug discovery and as inhibitors of protein-protein interactions. The growing appreciation of functional assays and phenotypic screens may further contribute to a revival of interest in natural products for drug discovery.

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The re-emergence of natural products for drug discovery in the genomics era

From a Novel

In recent years, there has been an ever-increasing need for rapid reactions that meet the three main criteria of an ideal synthesis: efficiency, versatility, and selectivity. Such reactions would allow medicinal chemistry to keep pace with the multitude of information derived from modern biological screening techniques. The present review describes one of these reactions, the 1,3-dipolar cycloaddition ("click-reaction") between azides and alkynes catalyzed by copper (I) salts. The simplicity of this reaction and the ease of purification of the resulting products have opened new opportunities in generating vast arrays of compounds with biological potential. The present review will outline the accomplishments of this strategy achieved so far and outline some of medicinal chemistry applications in which click-chemistry might be relevant in the future.

Click chemistry reactions in medicinal chemistry: applications of the 1,3-dipolar cycloaddition between azides and alkynes.

Bacteria and fungi use large multifunctional enzymes, the so-called nonribosomal peptide synthetases (NRPSs), to produce peptides of broad structural and biological activity. Biochemical studies have contributed substantially to the understanding of the key principles of these modular enzymes that can draw on a much larger number of catalytic tools for the incorporation of unusual features compared with the ribosomal system. Several crystal structures of NRPS-domains have yielded deep insight into the catalytic mechanisms involved and have led to a better prediction of the products assembled and to the construction of hybrid enzymes. In addition to the structure-function relationship of the core- and tailoring-domains of NRPSs, which is the main focus of this review, different biosynthetic strategies and essential enzymes for posttranslational modification and editing are discussed.

Biosynthesis of nonribosomal peptides1.

METLIN originated as a database to characterize known metabolites and has since expanded into a technology platform for the identification of known and unknown metabolites and other chemical entities Through this effort it has become a comprehensive resource containing over 1 million molecules including lipids, amino acids, carbohydrates, toxins, small peptides, and natural products, among other classes METLIN’s high-resolution tandem mass spectrometry (MS/MS) database, which plays a key role in the identification process, has data generated from both reference standards and their labeled stable isotope analogues, facilitated by METLIN-guided analysis of isotope-labeled microorganisms The MS/MS data, coupled with the fragment similarity search function, expand the tool’s capabilities into the identification of unknowns Fragment similarity search is performed independent of the precursor mass, relying solely on the fragment ions to identify similar structures within the database Stable isotope data al

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METLIN: A Technology Platform for Identifying Knowns and Unknowns

In nature, the attachment of sugars to small molecules is often used to mediate targeting, mechanism of action and/or pharmacology. As an alternative to pathway engineering or total synthesis, we report a useful method, in vitro glycorandomization (IVG), to diversify the glycosylation patterns of complex natural products. We have used flexible glycosyltransferases on nucleotide diphosphosugar (NDP-sugar) libraries to generate glycorandomized natural products and then applied chemoselective ligation to produce monoglycosylated vancomycins that rival vancomycin.

Antibiotic optimization via in vitro glycorandomization.

The trisaccharide 2′-fucosyllactose (2′-FL) is one of the most abundant oligosaccharides found in human milk. Due to its prebiotic and anti-infective properties, 2′-FL is discussed as nutritional additive for infant formula. Besides chemical synthesis and extraction from human milk, 2′-FL can be produced enzymatically in vitro and in vivo. The most promising approach for a large-scale formation of 2′-FL is the whole cell biosynthesis in Escherichia coli by intracellular synthesis of GDP-L-fucose and subsequent fucosylation of lactose with an appropriate α1,2-fucosyltransferase. Even though whole cell approaches have been demonstrated for the synthesis of 2′-FL, further improvements of the engineered E. coli host are required to increase product yields. Furthermore, an antibiotic-free method of whole cell synthesis of 2′-FL is desirable to simplify product purification and to avoid traces of antibiotics in a product with nutritional purpose. Here we report the construction of the first selection marker-free E. coli strain that produces 2′-FL from lactose and glycerol. To construct this strain, recombinant genes of the de novo synthesis pathway for GDP-L-fucose as well as the gene for the H. pylori fucosyltransferase futC were integrated into the chromosome of E. coli JM109 by using the λ-Red recombineering technique. Strains carrying additional copies of the futC gene and/or the gene fkp (from Bacteroides fragilis) for an additional salvage pathway for GDP-L-fucose production were used and shown to further improve production of 2′-FL in shake flask experiments. An increase of the intracellular GDP-L-fucose concentration by expression of fkp gene as well as an additional copy of the futC gene lead to an enhanced formation of 2′-FL. Using an improved production strain, feasibility of large scale 2′-FL production was demonstrated in an antibiotic-free fed-batch fermentation (13 l) with a final 2′-FL concentration of 20.28 ± 0.83 g l-1 and a space-time-yield of 0.57 g l-1 h-1. By chromosomal integration of recombinant genes, altering the copy number of these genes and analysis of 2′-FL and intracellular GDP-L-fucose levels, we were able to construct and improve the first selection marker-free E. coli strain which is capable to produce 2′-FL without the use of expression plasmids. Analysis of intracellular GDP-L-fucose levels identified the de novo synthesis pathway of GDP-L-fucose as one bottleneck in 2′-FL production. In antibiotic-free fed-batch fermentation with an improved strain, scale-up of 2′-FL could be demonstrated.

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Construction of Escherichia coli strains with chromosomally integrated expression cassettes for the synthesis of 2′-fucosyllactose

The xanthophyll astaxanthin is a high-value compound with applications in the nutraceutical, cosmetic, food, and animal feed industries. Besides chemical synthesis and extraction from naturally producing organisms like Haematococcus pluvialis, heterologous biosynthesis in non-carotenogenic microorganisms like Escherichia coli, is a promising alternative for sustainable production of natural astaxanthin. Recent achievements in the metabolic engineering of E. coli strains have led to a significant increase in the productivity of carotenoids like lycopene or β-carotene by increasing the metabolic flux towards the isoprenoid precursors. For the heterologous biosynthesis of astaxanthin in E. coli, however, the conversion of β-carotene to astaxanthin is obviously the most critical step towards an efficient biosynthesis of astaxanthin. Here we report the construction of the first plasmid-free E. coli strain that produces astaxanthin as the sole carotenoid compound with a yield of 1.4 mg/g cdw (E. coli BW-ASTA). This engineered E. coli strain harbors xanthophyll biosynthetic genes from Pantoea ananatis and Nostoc punctiforme as individual expression cassettes on the chromosome and is based on a β-carotene-producing strain (E. coli BW-CARO) recently developed in our lab. E. coli BW-CARO has an enhanced biosynthesis of the isoprenoid precursor isopentenyl diphosphate (IPP) and produces β-carotene in a concentration of 6.2 mg/g cdw. The expression of crtEBIY along with the β-carotene-ketolase gene crtW148 (NpF4798) and the β-carotene-hydroxylase gene (crtZ) under controlled expression conditions in E. coli BW-ASTA directed the pathway exclusively towards the desired product astaxanthin (1.4 mg/g cdw). By using the λ-Red recombineering technique, genes encoding for the astaxanthin biosynthesis pathway were stably integrated into the chromosome of E. coli. The expression levels of chromosomal integrated recombinant biosynthetic genes were varied and adjusted to improve the ratios of carotenoids produced by this E. coli strain. The strategy presented, which combines chromosomal integration of biosynthetic genes with the possibility of adjusting expression by using different promoters, might be useful as a general approach for the construction of stable heterologous production strains synthesizing natural products. This is the case especially for heterologous pathways where excessive protein overexpression is a hindrance.

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Christoph Albermann

Papers

Antibiotic optimization via in vitro glycorandomization.

Construction of Escherichia coli strains with chromosomally integrated expression cassettes for the synthesis of 2′-fucosyllactose

Engineering of a plasmid-free Escherichia coli strain for improved in vivo biosynthesis of astaxanthin

Synthesis of the milk oligosaccharide 2'-fucosyllactose using recombinant bacterial enzymes.

Production of human milk oligosaccharides by enzymatic and whole-cell microbial biotransformations.