Deoxychalcone

Bomati EK, Austin MB, Bowman ME, Dixon RA and Noel IP. 2005. Structural elucidation of chalcone reductase and implications for deoxychalcone biosynthesis. J Biol Chem 280 30496-30503. 

The peptide sequences obtained for codeinone reductase aligned well with the amino acid sequences for 6 -deoxychalcone synthase (chalcone reductase) from alfalfa, Glycerrhiza, and soybean. Knowledge of the relative positions of the peptides allowed for a quick RT-PCR based isolation of cDNAs encoding codeinone reductase from P. somniferum. The codeinone reductase isoforms are 53 % identical to chalcone reductase from soybean.25 By sequence comparison, both codeinone reductase and chalcone reductase belong to the aldo/keto reductase family, a group of structurally and functionally related NADPH-dependent oxidoreductases, and thereby possibly arise from primary metabolism. Six alleles encoding codeinone... 

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... 

The stereospecific cyclization of chalcones to (2S)-flavanones is a prerequisite for the synthesis of the majority of fiavonoid subclasses derived from this branch point metabolite. This reaction is catalyzed by chalcone isomerase (CHI, CFI EC 5.5.1.6). CHI exists in two forms, one that accepts only 6 -hydroxychalcones and another that accepts both 6 -hydroxy-(naringenin chalcone) and 6 -deoxychalcones (isoliquirgentin), the latter generally found in legumes. Although 6 -hydroxychalcones will spontaneously convert to a racemic flavanone mixture, the CHI-catalyzed reaction proceeds at a rate 36 million-fold faster and is highly stereoselective for the formation of (25)-flavanones [60]. Spontaneous isomerization of 6 -deoxychalcones does not substantially occur without enzyme catalysis. 

This enzyme [EC 2.3.1.74] (also known as narmgenm-chalcone synthase, flavonone synthase, and 6 -deoxy-chalcone synthase) catalyzes the reaction of three malonyl-CoA with 4-coumaroyl-CoA to produce four coenzyme A, three carbon dioxide, and naringeninchal-cone. If both NADH and a particular reductase is also present, the final product is 6 -deoxychalcone. 

The structure of the recombinant M. sativa CHI-II protein has been determined to 1.85 A resolution. The progress of the reaction in the reactive site cleft has been elucidated, and a full reaction sequence proposed. However, the basis for the specificities toward 6 -hydroxy-and 6 -deoxychalcones was not resolved, although potentially key amino acid residues were identified. 

With 6 -hydroxychalcones, such as naringenin chalcone, the isomerization reaction can readily occur nonenzymically to form racemic (2R,2S) flavanone. This occurs easily in vitro and has been reported to occur in vivo to the extent that moderate levels of anthocyanin can be formed. However, 6 -deoxychalcones are stable under physiological conditions, due to an... 

A characteristic of legumes is the biosynthesis of 6 -deoxychalcones (chalcones lacking a hydroxyl at the C-6 position), which are the substrates for the production of 5-deoxyflavo-noids. The formation of 6 -deoxychalcones requires the activity of polyketide reductase (PKR) (also known as chalcone reductase or chalcone ketide reductase) in conjunction with CHS. It is thought that CoA-linked polyketide intermediates diffuse in and out of the CHS active site, and while unbound are reduced to alcohols by PKR. The resultant hydroxyl groups are then removed from the PKR products in the final cyclization and aromatization steps catalyzed by CHS. 

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. 

Welle, R. and Grisebach, H., Phytoalexin synthesis in soybean cells elicitor induction of reductase involved in biosynthesis of 6 -deoxychalcone. Arch. Biochem. Biophys., 272, 97, 1989. 

Halbwirth, H. et al., Enzymatic glucosylation of 4-deoxyaurones and 6 -deoxychalcones with... 

FIGURE 16.3 Overview of the biosynthesis of (I) chalcones and (II) 6 -deoxychalcones. The sequential condensation of three molecules of malonyl-CoA (acetate pathway) and p-coumaroyl-CoA (shikimate pathway) is catalyzed by the enzyme chalcone synthase.The production of 6 -deoxychalcones is thought to involve an additional reduction step at the tri- or tetraketide level, catalyzed by polyketide reductase.The origin of the A-ring carbons derived from the acetate pathway is indicated in bold. CoA, coenzyme A. 

Wimmer, G. et al., Enz5matic hydroxylation of 6 -deoxychalcones with protein preparations from petals of Dahlia variabilis. Phytochemistry, 47, 1013, 1998. 

The main flavonoid skeleton derives from the stepwise condensation of three molecules of malonyl CoA with one molecule of 4-coumaroyl CoA, a reaction catalyzed by chalcone synthase (CHS) to form naringenin (2, 4,4 ,6,-tetrahydroxy) chalcone, the common intermediate in the formation of all flavonoids with 5,7-dihydroxy (flavone numbering) A-ring substitution. In some plants, however, an NADP-dependent chalcone-ketide reductase coacts with CHS to form 6 -deoxychalcone, the precursor of 5-deoxyflavonoids. The resulting chalcones undergoe a stereospecific cyclization to the corresponding (2S) flavanones, the... 

Microsomal preparations of Dahlia variabilis flowers have been reported to catalyze the 3-hydroxylation of the 6 -deoxychalcone, isoliquiritigenin, to butein.46 This enzyme activity was shown to be a typical cytochrome P450 monooxygenase that appears to be different from the F3 H, although its occurrence in other species remains to be investigated. 

Alternatively, chalcone reductase (CHR also known as deoxychalcone synthase) together with chalcone synthase and NADPH as a cofactor act in the formation of isoliquiritigenin, which is then isomerized, again by the enzyme chalcone isomerase, to form liquiritigenin, the precursor to daidzein, and the pterocarpan phytoalexins. A type II chalcone isomerase that seems to be found exclusively in the legumes catalyzes this isomerization reaction. Glycitein synthesis is not yet clearly defined, but is likely derived from liquiritigenin via flavonoid 6-hydroxylase, and an unidentified methyltransferase. 

Ayabe, S., A. Udagawa, and T. Furuya Studies on Plant Tissue Cultures. 54. NAD(P)H-Dependent 6 -Deoxychalcone Synthase Activity in Glycyrrhiza echinata Cells Induced by Yeast Extract. Arch. Biochem. Biophys., 261, 458 (1988). 

Harano, K., N. Okada, T. Furuno, T. Takahashi, S. Ayabe, and R. Welle Enzymatic Synthesis of 6 -Deoxychalcone in Cultured Glycyrrhiza echinata Cells. Plant Cell Rep., 12, 66 (1993). 

Discrete plant KRs have been identified that interact with CHS for the biosynthesis of 6 -deoxychalcone (20) (Fig. 3D) [133, 136-139]. Interestingly, the plant KR has no similarity with those that catalyze the reduction of the poly- -ketone intermediates in polyketide or fatty acid biosynthesis [49, 50, 81-84]. Instead it is similar to various aldo/keto-reductases, mostly from carbohydrate metabolism [139-141], and contains a leucine zipper motif known to be involved in protein-protein interaction [142]. Plant 0-methyltransferases are well known [143], but enzymes for C-methylation have not been described. The... 

The biochemistry and enzymology of CHS and its closely related plant PKSs have been extensively studied [72, 75]. The native enzymes have been isolated from various plants [70,71,73,74,124,125,127,128], and the recombinant proteins have been produced in E. coli and purified [71,76,124,125,132-135,139]. The active site for the condensation reaction has been mapped to a single Cys residue [132]. The dimeric nature of CHS and STS has been established with each homodimer to constitute two active sites [76,127,133]. The subtle differences among the CHS and STS proteins have been compared, which seems to have a profoimd effect on substrate specificity and regiospedfidty for the condensation and cyclization reactions, respectively [74, 127, 128, 133, 134]. The interactions between CHS and other proteins, such as KR [133,136-139] and methylmalonyl CoA-specific condensing enzyme [76], have been examined to account for the biosynthesis of deoxychalcones and methylchalcones. 

---