Chemoenzymatic Total Synthesis of the Antiproliferative Polyketide (+)-(R)-Aureothin

Enzymatic Kinetic Resolution by Addition of Oxygen

Kinetic resolution using biocatalysis has proven to be an excellent complementary technique to traditional asymmetric catalysis for the production of enantioenriched compounds. Resolution using oxidative enzymes produces valuable oxygenated structures for use in synthetic route development. This Minireview focuses on enzymes which catalyse the insertion of an oxygen atom into the substrate and, in so doing, can achieve oxidative kinetic resolution. The Baeyer-Villiger rearrangement, epoxidation, and hydroxylation are included, and biological advancements in enzyme development, and applications of these key enantioenriched intermediates in natural product synthesis are discussed.

Potent inhibition of HIV replication in primary human cells by novel synthetic polyketides inspired by Aureothin

Overcoming the global health threat of HIV infection requires continuous pipelines of novel drug candidates. We identified the γ-pyrone polyketides Aureothin/Neoaureothin as potent hits by anti-HIV screening of an extensive natural compound collection. Total synthesis of a structurally diverse group of Aureothin-derivatives successfully identified a lead compound (#7) superior to Aureothin that combines strong anti-HIV activity (IC₉₀<45 nM), photostability and improved cell safety. Compound #7 inhibited de novo virus production from integrated proviruses by blocking the accumulation of HIV RNAs that encode the structural components of virions and include viral genomic RNAs. Thus, the mode-of-action displayed by compound #7 is different from those of all current clinical drugs. Proteomic analysis indicated that compound #7 does not affect global protein expression in primary blood cells and may modulate cellular pathways linked to HIV infection. Compound #7 inhibited multiple HIV genotypes, including HIV-type 1 and 2 and synergistically inhibited HIV in combination with clinical reverse transcriptase and integrase inhibitors. We conclude that compound #7 represents a promising new class of HIV inhibitors that will facilitate the identification of new virus-host interactions exploitable for antiviral attack and holds promise for further drug development.

P450 Monooxygenases Enable Rapid Late-Stage Diversification of Natural Products via C-H Bond Activation

The biological potency of natural products has been exploited for decades. Their inherent structural complexity and natural diversity might hold the key to efficiently address the urgent need for the development of novel pharmaceuticals. At the same time, it is that very complexity, which impedes necessary chemical modifications such as structural diversification, to improve the effectiveness of the drug. For this purpose, Cytochrome P450 enzymes, which possess unique abilities to activate inert sp3-hybridised C-H bonds in a late-stage fashion, offer an attractive synthetic tool. In this review the potential of cytochrome P450 enzymes in chemoenzymatic lead diversification is illustrated discussing studies reporting late-stage functionalisations of natural products and other high-value compounds. These enzymes were proven to extend the synthetic toolbox significantly by adding to the flexibility and efficacy of synthetic strategies of natural product chemists, and scientists of other related disciplines.

First synthesis of (S)-(+)-hymenoic acid, a DNA polymerase λ inhibitor isolated from Hymenochaetaceae sp

Hymenoic acid, isolated from cultures of the fungus, Hymenochaetaceae sp., is a specific inhibitor of DNA polymerase λ. The first synthesis of (S)-(+)-hymenoic acid was achieved by starting from trans-1,4-cyclohexanedimethanol and methyl (R)-(−)-3-hydroxyisobutyrate, and Julia–Kocienski olefination was employed as the key step.

Biosynthesis of oxygen and nitrogen-containing heterocycles in polyketides

This review highlights the biosynthesis of heterocycles in polyketide natural products with a focus on oxygen and nitrogen-containing heterocycles with ring sizes between 3 and 6 atoms. Heterocycles are abundant structural elements of natural products from all classes and they often contribute significantly to their biological activity. Progress in recent years has led to a much better understanding of their biosynthesis. In this context, plenty of novel enzymology has been discovered, suggesting that these pathways are an attractive target for future studies.

A tandem chemoenzymatic methylation by S-adenosyl-L-methionine

Keep 'em methylated: The in situ preparation of the cofactor AdoMet was achieved by allowing the biosynthetic enzyme SalL to operate in the reverse direction by presentation of 5'-chloro-5'-deoxyadenosine at low salt concentrations. This reaction was readily coupled with DNA and small molecule methyltransferases to afford a regioselective method for chemo-enzymatic methylation and isotope incorporation.

Pyran formation by an atypical CYP-mediated four-electron oxygenation-cyclization cascade in an engineered aureothin pathway

Small changes, big effect: A new aureothin derivative, aureopyran, which features an unusual pyran backbone, was generated by simply altering the enzymatic methylation topology. The α-pyrone ring hampers the correct placement of the polyketide backbone in the multifunctional cytochrome P450 monooxygenase AurH. Instead of a tetrahydrofuran ring, an oxo intermediate is formed that readily undergoes a rare electrocyclization reaction.

Structural fine-tuning of a multifunctional cytochrome P450 monooxygenase

AurH is a unique cytochrome P450 monooxygenase catalyzing the stepwise formation of a homochiral oxygen heterocycle, a key structural and pharmacophoric component of the antibiotic aureothin. The exceptional enzymatic reaction involves a tandem oxygenation process including a regio- and stereospecific hydroxylation, followed by heterocyclization. For the structural and biochemical basis of this unparalleled sequence, four crystal structures of AurH variants in different conformational states and in complex with the P450 inhibitor ancymidol were solved, which represent the first structures of the CYP151A group. Structural data in conjunction with computational docking, site-directed mutagenesis, and chemical analyses unveiled a switch function when recognizing the two substrates, deoxyaureothin and the hydroxylated intermediate, thus allowing the second oxygenation-heterocyclization step. Furthermore, we were able to modify the chemo- and regioselectivity of AurH, yielding mutants that catalyze the regioselective six-electron transfer of a nonactivated methyl group to a carboxylic acid via hydroxyl and aldehyde intermediates.

Exploiting enzymatic promiscuity to engineer a focused library of highly selective antifungal and antiproliferative aureothin analogues

Aureothin is a shikimate-polyketide hybrid metabolite from Streptomyces thioluteus with a rare nitroaryl moiety, a chiral tetrahydrofuran ring, and an O-methylated pyrone ring. The antimicrobial and antitumor activities of aureothin have caught our interest in modulating its structure as well as its bioactivity profile. In an integrated approach using mutasynthesis, biotransformation, and combinatorial biosynthesis, a defined library of aureothin analogues was generated. The promiscuity of the polyketide synthase assembly line toward different starter units and the plasticity of the pyrone and tetrahydrofuran ring formation were exploited. A selection of 15 new aureothin analogues with modifications at the aryl residue, the pyrone ring, and the oxygenated backbone was produced on a preparative scale and fully characterized. Remarkably, various new aureothin derivatives are less cytotoxic than aureothin but have improved antiproliferative activities. Furthermore, we found that the THF ring is crucial for the remarkably selective activity of aureothin analogues against certain pathogenic fungi.

Total (bio)synthesis: strategies of nature and of chemists

The biosynthetic pathways to a number of natural products have been reconstituted in vitro using purified enzymes. Many of these molecules have also been synthesized by organic chemists. Here we compare the strategies used by nature and by chemists to reveal the underlying logic and success of each total synthetic approach for some exemplary molecules with diverse biosynthetic origins.

Evolution of metabolic diversity in polyketide-derived pyrones: using the non-colinear aureothin assembly line as a model system

Polyketide-derived pyrones are structurally diverse secondary metabolites that are represented in all three kingdoms of life and are endowed with various biological functions. The aureothin family of Streptomyces metabolites was chosen as a model to study the factors governing structural diversity and the evolutionary processes involved. This review highlights recent insights into the non-colinear aureothin and neoaureothin modular type I polyketide synthase (PKS), aromatic starter unit biosynthesis, polyketide tailoring reactions, and a non-enzymatic polyene splicing cascade. Pyrone biosynthesis in bacteria, fungi, and plants is compared. Finally, various strategies to increase metabolic diversity of aureothin derivatives through mutasynthesis, pathway engineering, and biotransformation are presented. The unusual aureothin and neoaureothin assembly lines thus not only represent a model for PKS evolution, but provided important insights into non-canonical enzymatic processes that could be employed for the production of antitumor and antifungal agents.

Chasing the treasures of the sea - bacterial marine natural products

Bacterial marine natural products are an important source of novel lead structures for drug discovery. The cytotoxic properties of many of these secondary metabolites are of particular interest for the development of new anticancer agents. Tremendous advances in marine molecular biology, genome sequencing, and bioinformatics have paved the way to fully exploit the biomedical potential of marine bacterial products. In addition, unique biosynthetic enzymes discovered from bacteria from the sea have begun to emerge as powerful biocatalysts in medicinal chemistry and total synthesis. The increasingly interdisciplinary field of marine natural product chemistry thus strongly impacts future developments in medicine, chemistry, and biology.