Biotransformation-coupled mutasynthesis for the generation of novel pristinamycin derivatives by engineering the phenylglycine residue

Streptogramins are the last line of defense antimicrobials with pristinamycin as a representative substance used as therapeutics against highly resistant pathogenic bacteria. However, the emergence of (multi)drug-resistant pathogens renders these valuable antibiotics useless; making it necessary to derivatize compounds for new compound characteristics, which is often difficult by chemical de novo synthesis due to the complex nature of the molecules. An alternative to substance derivatization is mutasynthesis. Herein, we report about a mutasynthesis approach, targeting the phenylglycine (Phg) residue for substance derivatization, a pivotal component of streptogramin antibiotics. Mutasynthesis with halogenated Phg(-like) derivatives altogether led to the production of two new derivatized natural compounds, as there are 6-chloropristinamycin I and 6-fluoropristinamycin I based on LC-MS/MS analysis. 6-Chloropristinamycin I and 6-fluoropristinamycin I were isolated by preparative HPLC, structurally confirmed using NMR spectroscopy and tested for antimicrobial bioactivity. In a whole-cell biotransformation approach using an engineered E. coli BL21(DE3) pET28-hmo/pACYC-bcd-gdh strain, Phg derivatives were generated fermentatively. Supplementation with the E. coli biotransformation fermentation broth containing 4-fluorophenylglycine to the pristinamycin mutasynthesis strain resulted in the production of 6-fluoropristinamycin I, demonstrating an advanced level of mutasynthesis.

New geldanamycin derivatives with anti Hsp properties by mutasynthesis

Mutasynthetic supplementation of the AHBA blocked mutant strain of S. hygroscopicus, the geldanamycin producer, with 21 aromatic and heteroaromatic amino acids provided new nonquinoid geldanamycin derivatives. Large scale (5 L) fermentation provided four new derivatives in sufficient quantity for full structural characterisation. Among these, the first thiophene derivative of reblastatin showed strong antiproliferative activity towards several human cancer cell lines. Additionally, inhibitory effects on human heat shock protein Hsp90α and bacterial heat shock protein from H. pylori HpHtpG were observed, revealing strong displacement properties for labelled ATP and demonstrating that the ATP-binding site of Hsps is the target site for the new geldanamycin derivatives.

Nonribosomal Peptide Synthesis-Principles and Prospects

Nonribosomal peptide synthetases (NRPSs) are large multienzyme machineries that assemble numerous peptides with large structural and functional diversity. These peptides include more than 20 marketed drugs, such as antibacterials (penicillin, vancomycin), antitumor compounds (bleomycin), and immunosuppressants (cyclosporine). Over the past few decades biochemical and structural biology studies have gained mechanistic insights into the highly complex assembly line of nonribosomal peptides. This Review provides state-of-the-art knowledge on the underlying mechanisms of NRPSs and the variety of their products along with detailed analysis of the challenges for future reprogrammed biosynthesis. Such a reprogramming of NRPSs would immediately spur chances to generate analogues of existing drugs or new compound libraries of otherwise nearly inaccessible compound structures.

Bioreduction of aryl azides during mutasynthesis of new ansamitocins

Supplementing a culture of a mutant strain of Actinosynnema pretiosum that is unable to biosynthesize aminohydroxy benzoic acid (AHBA), with 3-azido-5-hydroxy-benzoic acid and 3-azido-5-amino-benzoic acid, unexpectedly yielded anilino ansamitocins instead of the expected azido derivatives. This is the first example of the bioreduction of organic azides. The unique nature of these results was demonstrated when 3-azido-5-amino-benzoic acid was fed to the corresponding AHBA blocked mutant of Streptomyces hygroscopicus, the geldanamycin producer. This mutasynthetic experiment yielded the fully processed azido derivative of geldanamycin.

Mutasynthesis, chemobiosynthesis, and back to semi-synthesis: combining synthetic chemistry and biosynthetic engineering for diversifying natural products

The combination of biological and chemical approaches for the generation of new and diverse natural products holds much promise. While mutasynthesis based approaches are still very relevant, more recent approaches have utilised genetic and metabolic engineering to generate key intermediates for chemical syntheses. This new semi-synthetic approach exploits the ability of biological systems to efficiently generate complex chiral molecules and of synthetic chemistry to elaborate these into new, or difficult to source, molecules.

Total synthesis approaches to natural product derivatives based on the combination of chemical synthesis and metabolic engineering

Secondary metabolites are an extremely diverse and important group of natural products with industrial and biomedical implications. Advances in metabolic engineering of both native and heterologous secondary metabolite producing organisms have allowed the directed synthesis of desired novel products by exploiting their biosynthetic potentials. Metabolic engineering utilises knowledge of cellular metabolism to alter biosynthetic pathways. An important technique that combines chemical synthesis with metabolic engineering is mutasynthesis (mutational biosynthesis; MBS), which advanced from precursor-directed biosynthesis (PDB). Both techniques are based on the cellular uptake of modified biosynthetic intermediates and their incorporation into complex secondary metabolites. Mutasynthesis utilises genetically engineered organisms in conjunction with feeding of chemically modified intermediates. From a synthetic chemist's point of view the concept of mutasynthesis is highly attractive, as the method combines chemical expertise with Nature's synthetic machinery and thus can be exploited to rapidly create small libraries of secondary metabolites. However, in each case, the method has to be critically compared with semi- and total synthesis in terms of practicability and efficiency. Recent developments in metabolic engineering promise to further broaden the scope of outsourcing chemically demanding steps to biological systems.

The Impact of Bacterial Genomics on Natural Product Research

"There's life in the old dog yet!" This adage also holds true for natural product research. After the era of natural products was declared to be over, because of the introduction of combinatorial synthesis techniques, natural product research has taken a surprising turn back towards a major field of pharmaceutical research. Current challenges, such as emerging multidrug-resistant bacteria, might be overcome by developments which combine genomic knowledge with applied biology and chemistry to identify, produce, and alter the structure of new lead compounds. Significant biological activity is reported much less frequently for synthetic compounds, a fact reflected in the large proportion of natural products and their derivatives in clinical use. This Review describes the impact of microbial genomics on natural products research, in particularly the search for new lead structures and their optimization. The limitations of this research are also discussed, thus allowing a look into future developments.

Mutational biosynthesis—a tool for the generation of structural diversity in the biosynthesis of antibiotics

Natural products represent an important source of drugs in a number of therapeutic fields, e.g. antiinfectives and cancer therapy. Natural products are considered as biologically validated lead structures, and evolution of compounds with novel or enhanced biological properties is expected from the generation of structural diversity in natural product libraries. However, natural products are often structurally complex, thus precluding reasonable synthetic access for further structure-activity relationship studies. As a consequence, natural product research involves semisynthetic or biotechnological approaches. Among the latter are mutasynthesis (also known as mutational biosynthesis) and precursor-directed biosynthesis, which are based on the cellular uptake and incorporation into complex antibiotics of relatively simple biosynthetic building blocks. This appealing idea, which has been applied almost exclusively to bacteria and fungi as producing organisms, elegantly circumvents labourious total chemical synthesis approaches and exploits the biosynthetic machinery of the microorganism. The recent revitalization of mutasynthesis is based on advancements in both chemical syntheses and molecular biology, which have provided a broader available substrate range combined with the generation of directed biosynthesis mutants. As an important tool in supporting combinatorial biosynthesis, mutasynthesis will further impact the future development of novel secondary metabolite structures.

Bioactive agents from natural sources: trends in discovery and application

About 30% of the worldwide sales of drugs are based on natural products. Though recombinant proteins and peptides account for increasing sales rates, the superiority of low-molecular mass compounds in human diseases therapy remains undisputed mainly due to more favorable compliance and bioavailability properties. In the past, new therapeutic approaches often derived from natural products. Numerous examples from medicine impressively demonstrate the innovative potential of natural compounds and their impact on progress in drug discovery and development. However, natural products are currently undergoing a phase of reduced attention in drug discovery because of the enormous effort which is necessary to isolate the active principles and to elucidate their structures. To meet the demand of several hundred thousands of test samples that have to be submitted to high-throughput screening (HTS) new strategies in natural product chemistry are necessary in order to compete successfully with combinatorial chemistry. Today, pharmaceutical companies have to spend approximately US $350 million to develop a new drug. Currently, approaches to improve and accelerate the joint drug discovery and development process are expected to arise mainly from innovation in drug target elucidation and lead finding. Breakthroughs in molecular biology, cell biology, and genetic engineering in the 1980 s gave access to understanding diseases on the molecular or on the gene level. Subsequently, constructing novel target directed screening assay systems of promising therapeutic significance, automation, and miniaturization resulted in HTS approaches changing the industrial drug discovery process drastically. Furthermore, elucidation of the human genome will provide access to a dramatically increased number of new potential drug targets that have to be evaluated for drug discovery. HTS enables the testing of an increasing number of samples. Therefore, new concepts to generate large compound collections with improved structural diversity are desirable.

An erythromycin derivative produced by targeted gene disruption in Saccharopolyspora erythraea

Derivatives of erythromycin with modifications at their C-6 position are generally sought for their increased stability at acid pH, which in turn may confer improved pharmacological properties. A recombinant mutant of the erythromycin-producing bacterium, Saccharopolyspora erythraea, produced an erythromycin derivative, 6-deoxyerythromycin A, that could not be obtained readily by chemical synthesis. This product resulted from targeted disruption of the gene, designated eryF (systematic nomenclature, CYP107), that apparently codes for the cytochrome P450, 6-deoxyerythronolide B (DEB) hydroxylase, which converts DEB to erythronolide B (EB). Enzymes normally acting on EB can process the alternative substrate DEB to form the biologically active erythromycin derivative lacking the C-6 hydroxyl group.