Chalcone/stilbene synthases

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. 

Schroder J (1999) The chalcone/stilbene-synthase family of condensing enzymes. In Sankawa U (ed) Comprehensive natural products chemistry, vol 1. Pergamon, Oxford, pp 749-771... 

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. 

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... 

TROPF, S LANZ, T., RENSING, S.A., SCHRODER, J., SCHRODER, G., Evidence that stilbene synthases have developed from chalcone synthases several times in the course of evolution, J. Mol. Evol., 1994,38, 610-618. 

ZUURBIER, K.W.M., LESER, J., BERGER, T., HOFTE, A.J.P., SCHRODER, G., VERPOORTE, R., SCHRODER, J., Hydroxy-2-pyrone formation by chalcone and stilbene synthase with nonphysiological substrates, Phytochemistry, 1998,49, 1945-1951. 

SUH, D.-Y., FUKUMA, K., KAGAMI, J., YAMAZAKI, Y., SHIBUYA, M., EBIZUKA, Y., SANKAWA, U., Identification of amino acid residues important in the cyclization reactions of chalcone and stilbene synthases, Biochem. J., 2000,350, 229-235. 

Stilbene synthase shows similarity to chalcone synthase, which is not surprising given that stilbenes (3.97) also originate from the condensation of... 

Figure 1.36 Schematic diagram of the stilbene and flavonoid biosynthetic pathway. Enzyme abbreviations SS, stilbene synthase CHS, chalcone synthase CHR, chalcone reductase CHI, chalcone isomerase IFS, isoflavone synthase FNS, flavone synthase F3H, flavanone 3-hydroxylase FLS, flavonol synthase F3 H, flavonoid 3 -hydroxylase DFR, dihydroflavonol 4-reductase LAR, leucoanthocyanidin 4-reductase LDOX, leucocyanidin deoxygenase ANR, anthocyanidin reductase EU, extension units TU, terminal unit.

Fig. 1 Biosynthesis of plant polyphenols by CHS-superfamily type III PKSs. CHS chalcone synthase STS stilbene synthase ACS acridone synthase VPS valerophenone synthase SPS styrylpyrone synthase BPS benzophenone synthase OAS olivetolic acid synthase 2PS 2-pyrone synthase BAS benzalacetone synthase CUS curcumin synthase...

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... 

Type III PKSs are small, homodimeric enzymes which can be found in both plants and a wide array of microbes. They generally construct fairly low molecular weight 1-2 ring aromatic molecules.36 Examples include plant chalcone and stilbene synthases and the S. coelicolor gene tetrahydroxy naphthalene synthase (THNS), each of which has been structurally characterised by X-ray diffraction. These enzymes are often easy to express het-erologously in E. coli and much work has been done to understand their complicated enzymology as a model for the larger, more complex Type I and II systems.2... 

Chalcone and stilbene synthases are related plant PKSs [ 132]. Chalcones, such as naringenin chalcone, are produced as the biosynthetic precursors of flavinoids, while stilbenes are produced for their antifungal properties. Plant PKSs are likely to have evolved independendy from any of the aforementioned PKS and FAS systems [133, 134] and are atypical in many respects. These homodimeric enzymes consist of a single 40 kDa gene product (Fig. 2) [135]. The two active sites of the dimer function independently of one another [136]. Plant PKSs lack an AGP component, are not phosphopantetheinlyated, and act direcdy on CoA thioesters [134,137]. 

The peanut chalcone synthase and parsley stilbene synthases have been cloned, expressed in E. colt, and purified to homogeneity [135,137]. The enzymes appear to be mechanistically similar each catalyzes the formation of a tetraketide from three molecules of malonyl CoA that are decarboxylated and condensed with a starter unit derived from p-coumaroyl CoA or a similar CoA thioester (Fig. 6). No reductions or dehydrations occur during either chalcone or stilbene synthesis, and some products spontaneously cyclize following their release from the enzyme. A major feature that distinguishes chalcone and stilbene synthases is that the latter perform an additional decarboxylation to remove a carbon atom that is present in chalcone products [132,138]. The presence of this additional carboxyl group results in a different cyclization pattern for chalcone products. The precise mechanisms by which chalcone and stilbene synthases determine the fate of this carbon atom are not known. 

Substrates are processed directly through cysteine thiols, and cerulenin and iodoacetamide are able to inhibit the activity of plant PKSs by modifying sulf-hydryl groups of these residues [ 134]. Although chalcone and stilbene synthases do not share a high level of overall sequence similarity with FASs and PKSs, the cysteine that is modified by cerulenin is conserved, suggesting that the active sites may be structurally related [51]. 

Rg.6. Reactions catalyzed by chalcone and stilbene synthases. Each enzyme condenses three malonyl CoA extender units onto p-coumaroyl-CoA. Stilbene synthases catalyze an additional decarboxylation, resulting in a different pattern of cyclization for chalcone versus stilbene products... 

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). 

Type I and Type II PKSs catalyze multiple rounds of reactions by catalytic modules encoded either by a single polypeptide (PKS I) or on separate polypeptides (PKS II) by analogy to FAS-I and FAS-II. In contrast, PKS Ills are dimers of KASs that catalyze multiple condensation reactions in one active site and include chalcone synthase, stilbene synthase, and 2-pyrone synthase (see Chapters 1.05, 1.07, and 1.04). In the case of chalcone synthase, three consecutive condensation reactions each utilizing malonyl-CoA, followed by a cyclization reaction, lead to the formation of 4, 2, 4, 6 -tetrahydroxychalcone from 4-hydroxycinnamoyl-CoA (Figure 3). Recruitment of a reductase leads to the formation of a product lacking the 6 -hydroxy group, a reaction that requires an intermediate in the synthesis of chalcone to dissociate from the synthase active site. 

It is also important to consider any possible deleterious effects that resveratrol and its metabolites may have on plants. Production of defensive stilbenes in plants results in resource competition between parallel biosynthetic pathways. For example, one group of metabolites that may be severely impacted by this competition is the chalcones and their metabolites that play a variety of important roles in plants. These compounds share a pool of basic precursors with the stilbenoids [134,135]. Alterations in stilbenoid production could have far reaching effects on the health of the plant. The severity of this problem has been highlighted by Fischer and colleagues [136] who reported that overexpression of a stilbene synthase gene in transgenic tobacco and petunia plants resulted male in sterility. 

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