Aromatic Natural Products Research Articles

Mechanistic analysis of a type II polyketide synthase. Role of conserved residues in the beta-ketoacyl synthase-chain length factor heterodimer.

Type II polyketide synthases (PKSs) are a family of multienzyme systems that catalyze the biosynthesis of polyfunctional aromatic natural products such as actinorhodin, frenolicin, tetracenomycin, and doxorubicin. A central component in each of these systems is the beta-ketoacyl synthase-chain length factor (KS-CLF) heterodimer. In the presence of an acyl carrier protein (ACP) and a malonyl-CoA:ACP malonyl transferase (MAT), this enzyme synthesizes a polyketide chain of defined length from malonyl-CoA. We have investigated the role of the actinorhodin KS-CLF in priming, elongation, and termination of its octaketide product by subjecting the wild-type enzyme and selected mutants to assays that probe key steps in the overall catalytic cycle. Under conditions reflecting steady-state turnover of the PKS, a unique acyl-ACP intermediate is detected that carries a long, possibly full-length, acyl chain. This species cannot be synthesized by the C169S, H309A, K341A, and H346A mutants of the KS, all of which are blocked in early steps in the PKS catalytic cycle. These four residues are universally conserved in all known KSs. Malonyl-ACP alone is sufficient for kinetically and stoichiometrically efficient synthesis of polyketides by the wild-type KS-CLF, but not by heterodimers that carry the mutations listed above. Among these mutants, C169S is an efficient decarboxylase of malonyl-ACP, but the H309A, K341A, and H346A mutants are unable to catalyze decarboxylation. Transfer of label from [(14)C]malonyl-ACP to the nucleophile at position 169 in the KS can be detected for the wild-type enzyme and for the C169S and K341A mutants, but not for the H309A mutant and only very weakly for the H346A mutant. A model is proposed for decarboxylative priming and extension of a polyketide chain by the KS, where C169 and H346 form a catalytic dyad for acyl chain attachment, H309 positions the malonyl-ACP in the active site and supports carbanion formation by interacting with the thioester carbonyl, and K341 enhances the rate of malonyl-ACP decarboxylation via electrostatic interaction. Our data also suggest that the ACP and the KS dissociate after each C-C bond forming event, and that the newly extended acyl chain is transferred back from the ACP pantetheine to the KS cysteine before dissociation can occur. Chain termination is most likely the rate-limiting step in polyketide biosynthesis. Within the act CLF, neither the universally conserved S145 residue nor Q171, which aligns with the active site cysteine of the ketosynthase, is essential for PKS activity. The results described here provide a basis for a better understanding of the catalytic cycle of type II PKSs and fatty acid synthases.

Analysis of Rotenoids by High Temperature High Resolution Gas Chromatography-Mass Spectrometry

A short glass capillary column coated with PS-090 (20% phenyl–80% methylpolysiloxane, of 10 m × 0.30 mm i.d.; with 0.1 μm film thickness) was used to analyze samples from Tephrosia candida; eighteen rotenoids were separated and identified without derivatization by HT-HRGC and HT-HRGC-MS. The mass spectra show the typical fragmentation pattern for rotenoids, with the base peaks either at M+, or originating from RDA (Retro Diels-Alder) rearrangements. HT-HRGC and HT-HRGC-MS were shown to be extremely valuable and neglected techniques for structural studies as well as the routine analysis of rotenoids in crude extracts. Possible applications to other classes of aromatic natural products, e.g. flavonoids, are envisaged.

Domain Analysis of the Molecular Recognition Features of Aromatic Polyketide Synthase Subunits

Bacterial aromatic polyketide synthases (PKSs) are a family of homologous multienzyme assemblies that catalyze the biosynthesis of numerous polyfunctional aromatic natural products. In the absence of direct insights into their structures, the use of gene fusions can be a powerful tool for understanding the structural basis for their properties. A series of truncated and hybrid proteins were constructed and analyzed within a family of PKS subunits, designated aromatases/cyclases (ARO/CYCs). When expressed alone, neither the N-terminal nor the C-terminal domain of the actinorhodin (act) or the griseusin (gris) ARO/CYC exhibited substantial aromatase activity. However, in the presence of each other, the half proteins were active. Furthermore, analysis of a set of hybrid proteins derived from the act and gris ARO/CYCs allowed us to localize the chain length dependence of this aromatase activity to their N-terminal domains. Unexpectedly, however, when the C-terminal domain of the gris ARO/CYC was expressed in a context where aromatase activity was absent, it could modulate the chain length specificity of the tetracenomycin (tcm) minimal PKS, leading to the formation of a novel 18-carbon product in addition to the expected 20-carbon one. It was also found that monodomain ARO/CYCs such as tcmN cannot substitute for the the N-terminal domain of didomain ARO/CYCs, even though they exhibit high sequence similarity with the N-terminal domain. Together, these results illustrate the utility of protein engineering approaches for dissecting the structure-function relationships of PKS subunits and for the generation of mutant alleles with novel biosynthetic properties.

A New Stereocontrolled Route to (−)-Shikimic Acid

Abstract A new stereocontrolled route to (−)-shikimic acid, the component of the fruit of shikimi tree, Illicium religiosum, and the key biogenetic precursor of a variety of aromatic natural products, has been developed using the chiral 2,5-cyclohexadienol synthon.

ChemInform Abstract: Rational Design of Aromatic Polyketide Natural Products by Recombinant Assembly of Enzymatic Subunits.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Rational design of aromatic polyketide natural products by recombinant assembly of enzymatic subunits.

Recent advances in understanding of bacterial aromatic polyketide biosynthesis allow the development of a set of design rules for the rational manipulation of chain synthesis, reduction of keto groups and early cyclization steps by genetic engineering. The concept of rational design is illustrated by the preparation of Streptomyces strains that produce two new polyketides by expression of combinations of appropriate enzymatic subunits from naturally occurring polyketide synthases. The potential for generating molecular diversity within this class of molecules by genetic engineering is enormous.

ChemInform Abstract: Chromium‐Directed Regio‐ and Stereocontrol: A New Chapter in Aromatic Natural Product Synthesis

The sequence of ortho-directing abilities of a series of aryl functionalities in arenetricarbonylchromium(0) complexes has been shown to be different to that in the uncomplexed arenes. The kinetic resolution of the lithiated complexes with homochiral oxazolidinones has been rationalised via a transition state model and applied to the stereocontrolled synthesis of a series of highly functionalised, homochiral phenethylamines.

Chromium directed regio- and stereocontrol: A new chapter in aromatic natural product synthesis

Abstract

ChemInform Abstract: Benzenoid and Polycyclic Aromatic Natural Products

ChemInformVolume 19, Issue 49 Reviews ChemInform Abstract: Benzenoid and Polycyclic Aromatic Natural Products T. J. SIMPSON, Dep. Chem., Univ. Edinburgh, Edinburgh EH9 3JJ, UKSearch for more papers by this author T. J. SIMPSON, Dep. Chem., Univ. Edinburgh, Edinburgh EH9 3JJ, UKSearch for more papers by this author First published: December 6, 1988 https://doi.org/10.1002/chin.198849324Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume19, Issue49December 6, 1988 RelatedInformation

Benzenoid and polycyclic aromatic natural products

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Introduction Monobenzenoids Polyprenylphenols (Meroterpenoids) Depsides and Depsidones Biphenyls and Dibenzofurans Diphenylmethanes Arylbenzofurans, Bibenzyls, and Phenanthrenes Diarylpropanoids and Diarylheptanoids Terphenyls Naphthalenes and Nap h t hoquinones Anthracenes and Anthraquinones Anthracyclines Benz[a]anthraquinones Aromatic Terpenoids Miscellaneous Metabolites References sequence (9) + (6a) + (7) + (8). The yellow pigment wasabidienone A that is produced by Phoma wusabiae has been shown to consist of a tautomeric mixture of the cyclohexadienones (10a) and (lob).* The reaction of the mixture with diazomethane gave the corresponding methyl ethers (1 1) and (12), which could be separated, as a 1: 1 mixture. The benzoquinone (13), which co-occurs with (lo), is probably an artifact.

Regiospecific synthesis of mono- and bicyclic 6-alkoxy-2-pyrones and their use in the preparation of substituted aromatics, anthraquinones, and tetracyclic intermediates for 11-deoxyanthracycline synthesis

Several mono- and bicyclic 6-methoxy-2-pyrones having substituents at C4 and C5 can be prepared regiospecifically by either of two routes: (1) regiospecific construction of a glutaconic half-ester followed by dehydrative cyclization, and (2)regiospecific Friedel-Crafts acylation of 6-methoxy-2-pyrones at C5. These pyrones undergo clean and regiospecific Diels-Alder cycloadditions with various unsymmetrical dienophiles, e.g., quinones, unsaturated esters, etc, with subsequent loss of carbon dioxide. In this manner several polycyclic aromatic natural products have been prepared such as chrysophanol, helminthosporin, pachybasin, 2-acetylemodin and the purported structure for orientalone. The utility of this approach for the synthesis of the anthracyclines is demonstrated by its use in the preparation of various tetracyclic intermediates for anthracycline synthesis.

Synthesis of polyketide-type aromatic natural products by biogenetically modeled routes

Synthesis of polyketide-type aromatic natural products by biogenetically modeled routes

Biogenetic-type syntheses of polyketide metabolites.

The term “polyketide” was coined in 1907 by Collie (37) and later defined more explicitly by Birch (13) to describe the aromatic natural products which are formed in nature from acetic acid via β-polycarbonyl intermediates. Biosynthetically, the polyketides arise by the acyl polymalonate route, which also produces the fatty acids. The latter are formed from linear arrays of acetate units but not via polycarbonyl intermediates.