Naringenin-chalcone

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. 

CHS carries out a series of sequential decarboxylation and condensation reactions, using 4-courmaroyl-CoA (in most species) and three molecules of malonyl-CoA, to produce a poly-ketide intermediate that then undergoes cyclization and aromatization reactions that form the A-ring and the resultant chalcone structure. The chalcone formed from 4-courmaroyl-CoA is naringenin chalcone. However, enzyme preparations and recombinant CHS proteins from some species have been shown to accept other HCA-CoA esters as substrates, such as cinnamoyl-CoA (see, e.g., Ref. 37). In particular, the Hordeum vulgare (barley) CHS2 cDNA encodes a CHS protein that converts feruloyl-CoA and caffeoyl-CoA at the highest rate, and cinnamoyl-CoA and 4-courmaroyl-CoA at lower rates. 

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... 

Although it has long been thought, based on genetic mutant and biochemical evidence, that aurones are derived from chalcones, the biosynthetic mechanism has only recently been clarified, and some aspects of the enzymatic process still await in vivo proof An mRNA from A. majns, specifically expressed in the petal epidermal cells, has been shown to encode a recombinant protein with aureusidin synthase (AUS) activity.AUS is a variant polyphenol oxidase (PPO) that can catalyze conversion of either 2, 4, 6, 4-etrahydroxychalcone (naringenin chalcone) or 2, 4, 6, 3,4-pentahydroxychalcone to... 

Chalcones are comparatively rare in foods. Naringenin chalcone is present in tomato skin and may be present in juice, paste and ketchup. Acid hydrolysis, commonly applied prior to HPLC, converts the chalcone to the corresponding flavanone (naringenin), which is naturally present only in trace amounts (2-15 mg/kg) in the tomato [23],... 

Krause M, Galensa G. 1992. Determination of naringenin and naringenin-chalcone in tomato skins by reversed phase HPLC after solid phase extraction. Z Lebensmittel... 

Heller, W. and Hahlbrock, K. (1980) Highly purified flavanone synthase from parsley catalyzes the formation of naringenin chalcone. Archives of Biochemistry and Biophysics 200(2), 61 7-619. 

Naringenin chalcone Dianthus caryophyllus (carnation) 17PHSOR (3)... 

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]. 

Chalcone synthase (CHS), the first plant natural product polyketide synthase (PKS) to be characterized at the molecular level (39), catalyzes the condensation of 4-coumaroyl-CoA with three molecules of malonyl-CoA to afford naringenin chalcone, a precursor of the major classes of plant flavonoids. The cloning of a novel type III pentaketide chromone synthase (PCS) from aloe (Aloe arborescens, Liliaceae) rich in aromatic polyketides, especially quinones such as aloe-emodin and emodin, resulted in... 

Figure 1. Proposed mechanism for the formation of (A) naringenin chalcone from 4-coumaroyl-CoA and three molecules of malonyUCoA by CHS, (B) triacetic lactone from acetyl-CoA and two molecules of malortyUCoA by 2PS, (C) aloesone from acetyl-CoA and six molecules of malonyUCoA by ALS, (D) 5,7-dihydroxy-2-methylchromone from five molecules of malonyl-CoA by PCS, and (E) SEK4 and SEK4b from eight molecules of malonyl-CoA by OKS. Bis-noryangonin (BNY) and 4-coumaroyltriacetic acid lactone (CTAL) are derailment by-products of the CHS reactions in vitro when the reaction mixtures are acidified before extraction. In A. arborescens PCS and OKS, acetyl-CoA, resulting from decarboxylation of malonyl-CoA, is also accepted as a starter but not so efficiently as in the case ofR. palmatum ALS.

Fig. 12 shows a characteristic chromatogram of the phenolic compounds of tomato peel. Especially interesting is the relatively large naringenin chalcone peak. Other peaks are observed in the chromatogram, corresponding to unidentified hydroxycinnamic and benzoic acid derivatives. 

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