Polyketides stilbenes

Watts, K. T., R C. Lee et al. (2006). Biosynthesis of plant-specific stilbene polyketides in metabolically engineered Escherichia coli. BMC Biotechnol. 6 22. 

Chalcone Synthase / Stilbene Synthase Family of Plant Polyketide Synthases. 199... 

Recently, a new polyketide biosynthetic pathway in bacteria that parallels the well studied plant PKSs has been discovered that can assemble small aromatic metabolites.8,9 These type III PKSs10 are members of the chalcone synthase (CHS) and stilbene synthase (STS) family of PKSs previously thought to be restricted to plants.11 The best studied type III PKS is CHS. Physiologically, CHS catalyzes the biosynthesis of 4,2, 4, 6 -tetrahydroxychalcone (chalcone). Moreover, in some organisms CHS works in concert with chalcone reductase (CHR) to produce 4,2 ,4 -trihydroxychalcone (deoxychalcone) (Fig. 12.1). Both natural products constitute plant secondary metabolites that are used as precursors for the biosynthesis of anthocyanin pigments, anti-microbial phytoalexins, and chemical inducers of Rhizobium nodulation genes.12... 

In this chapter, we describe the atomic resolution structural elucidation of several plant type III polyketide synthases, including chalcone synthase, 2-pyrone synthase, and stilbene synthase. Manipulation of the catalytic activity and specificity of these biosynthetic enzymes by using a structurally guided approach offers a novel... 

CHALCONE SYNTHASE / STILBENE SYNTHASE FAMILY OF PLANT POLYKETIDE SYNTHASES... 

Members of the CHS/STS family of condensing enzymes are relatively modest-sized proteins of 40-47 kDa that function as homodimers. Each enzyme typically reacts with a cinnamoyl-CoA starter unit and catalyzes three successive chain extensions with reactive acetyl groups derived from enzyme catalyzed decarboxylation of malonyl-CoA.11 Release of the resultant tetraketide together with or prior to polyketide chain cyclization and/or decarboxylation yields chalcone or resveratrol (a stilbene). Notably, CHS and STS catalyze identical reactions up to the formation of the intermediate tetraketide. Divergence occurs during the termination step of the biosynthetic cascade as each tetraketide intermediate undergoes a distinct cyclization reaction (Fig. 12.2). 

SCHRODER, J., The chalcone/stilbene synthase-type family of condensing enzymes. In Comprehensive Natural Products Chemistry, vol. 1, Polyketides and Other Secondary Metabolites Including Fatty Acids and Their Derivatives (U. Sankawa ed.), Elsevier, Amersterdam, 1999, pp. 749-771. 

PKSs are characterized by their ability to catalyze the formation of polyketide chains from the sequential condensation of acetate units from malonate thioesters. In plants they produce a range of natural products with varied in vivo and pharmacological properties. PKSs of particular note include acridone synthase, bibenzyl synthase, 2-pyrone synthase, and stilbene synthase (STS). STS forms resveratrol, a plant defense compound of much interest with regard to human health. STS shares high sequence identity with CHS, and is considered to have evolved from CHS more than once. ° Knowledge of the molecular structure of the CHS-like enzymes has allowed direct engineering of CHS and STS to alter their catalytic activities, including the number of condensations carried out (reviewed in Refs. 46, 51, 52). These reviews also present extensive, and superbly illustrated, discussions of CHS enzyme structure and reaction mechanism. 

Tropf, S. et al., Reaction mechanisms of homodimeric plant polyketide synthases (stilbenes and chalcone synthase) a single active site for the condensing reaction is sufficient for synthesis of stilbenes, chalcones, and 6 -deoxychalcones. J. Biol. Chem., 270, 7922, 1995. 

The polyketide synthases responsible for chain extension of cinnamoyl-CoA starter units leading to flavonoids and stilbenes, and of anthraniloyl-CoA leading to quinoline and acridine alkaloids (see page 377) do not fall into either of the above categories and have now been termed Type TTT PKSs. These enzymes differ from the other examples in that they are homodimeric proteins, they utilize coenzyme A esters rather than acyl carrier proteins, and they employ a single active site to perform a series of decarboxylation, condensation, cyclization, and aromatization reactions. 

Austin MB, Bowman ME, Ferrer J-L, Schroder J, Noel JP (2004) An aldol switch discovered in stilbene synthases mediates cyclization specificity of Type III polyketide synthases. Chem Biol 11 1179-1194... 

Tropf S, Karcher B, Schroder G, Schroder J (1995) Reaction mechanisms of homodimeric plant polyketide syntahses (stilbene synthase and chalcone synthase). J Biol Chem 270 7922-7928... 

Shi S-P, Wanibuchi K, Morita H, Endo K, Noguchi H, Abe I (2009) Enzymatic formation of unnatural novel chalcone, stilbene, and benzophenone scaffolds by plant type III polyketide synthase. Org Lett 11 551-554... 

Morita H, Noguchi H, Schroder J, Abe I (2001) Novel polyketides synthesized with a higher plant stilbene synthase. Eur J Biochem 268 3759-3766... 

The extension of a cinnamoyl-CoA chain (126) with malonate leads to the polyke-tide 127, which may be folded differently, depending on the enzymes present in the plant (Fig. 32). Stilbene synthase, for example, converts the polyketide into a stilbene (171) through an aldol-type condensation (86). 

Stilbenoids are derived from cinnamic acid and three acetate units from mal-onyl coenzym A. The first part of the biosynthesis is in common to flavonoids. The two biosynthetic routes are diverging at the point of cyclization of a styryl-3,5,7-triketoheptanoic acid. A C-acylation produces a chalcone and subsequent modifications lead to the flavonoids. An aldol condensation of the same intermediate polyketide produces a stilbene-2-carboxylic acid that is unstable and constitutes a range of structures known as stilbenoids. Figure 9C.5 shows an overview of the biosynthetic pathway (Gorham 1995). 

Figure 8 Polyketides of the chalcone super family of chalcone and stilbene (The illustrated [ 2]acetate labeling is extrapolated from related incorporation studies)...

Some groups applied C NMR to study the stilbene biosynthesis pathway [18,372], for example mulberrosides A [373] and D (32) [18]. The type III PKS stilbene synthase (STS) was also employed to investigate the potential of a nonhydrolyzable malonyl-coenzyme A analog to trap intermediates of polyketide biosynthesis [374],... 

Type III polyketide synthases are responsible for the biosynthesis of a vast number of plant-derived natural products, including flavonoids derived from the important branch metabolite 4 ,2 ,4 ,6 -tetrahydroxychalcone, the product of the enzyme chalcone synthase (39). Because chalcone synthase was the first type III enzyme discovered, and a second flavonoid pathway type III enzyme, stilbene synthase, was discovered shortly thereafter, type III PKSs are also collectively referred to as the chalcone synthase/stilbene synthase superfamily of enzymes (24,25). 

Figure 1. Type III polyketide synthases and their reaction products. CHS chalcone synthase, STS stilbene synthase BPS benzophenone synthase ACS acridone synthase VPS valerophenone synthase CTAS p-coumaryltriacetic acid synthase 2PS 2-pyrone synthase SPS styrylpyrone synthase BAS benzalacetone synthase H/EDS homoeryodyctiol/eriodyctiol chalcone synthase ALS Aloesone synthase PCS phertylchromone synthase BIS biphenyl synthase...

J. Schroder, The Chalcone/Stilbene Synthase-Type Family ot Condensing Enzymes. In Oomprehensive Natural Products Chemistry, Vol. 1 Polyketides and Other Secondary Metabolites Including Fatty Acids and Their Derivatives U. Sankawa, Ed. Sir D. H. R. Barton, K. Nakanishi, O. Meth-Cohn, Series Eds. Elsevier Oxford, 1999 pp 749-771. 

The biosynthesis of the stilbenoids, including 1, has been previously reviewed. Briefly, the synthesis of 1 is dependent upon a single key enzyme known as stilbene synthase or resveratrol synthase as part of a mixed phenylpropanoid-polyketide pathway [2,3,4,5,6] (Fig. (1)). Stilbene synthase catalyzes the formation of 1 through the condensation of one p-coumaroyl CoA and three malonyl CoA molecules, both of which are ubiquitous intermediary plant metabolites. 

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