Naringenin-chalcone synthase

As described above, the first enzyme general to all flavonoid biosynthesis, chalcone synthase (naringenin-chalcone synthase), catalyzes the cyclization of a precursor formed from p-coumaryl-CoA and three units of malonyl-CoA (Fig. 11.6) (Dewick, 1989 Gerats and Martin, 1992 Heller and Forkmann, 1988). The enzyme, usually found in plant epidermal cells, has a molecular weight of about 42,000, requires no cofactors, and has been isolated from several plant cell cultures such as French bean Phaseolus vulgaris), parsley Petroselinum crispum), and the flowers of the carnation (Dianthus caryophyllus, Caryophyllaceae) (Hutchinson, 1986). p-Coumaiic acid and malonyl-CoA are the preferred precursors. Malonyl-ACP will not serve. 

Finally, in this vein, as shown in Scheme 12.23, p-coumaric acid is converted (in the presence of 4-coumarate-CoA ligase, EC 6.2.1.12, coenzyme A, and ATP) into its corresponding coenzyme A thioester, 4-coumaroyl-CoA. Then, in the presence of the enzyme naringenin chalcone synthase (EC 2.3.1.74), 3 equivalents of malonyl-CoA (Scheme 11.36) are added sequentially, a cycUzation follows, and the chalcone (naringenin chalcone) is produced. This is the beginning of the pathway for flavo-noid (as derived from this progenitor chalcone) biosynthesis. 

Flavonoids are the largest class of phenylpropanoids in plants. The basic flavonoid structure is two aromatic rings (one from phenylalanine and the other from the condensation of three malonic acids) linked by three carbons (Fig. 3.6). Chalcone is converted to naringenin by the enzyme chalcone isomerase, which is a key enzyme in flavonoid synthesis. This enzyme, like PAL and chalcone synthase (CHS), is under precise control and is inducible by both internal and external signals. Naringenin is the... 

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... 

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. 

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... 

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 8 Chalcone and HMG-CoA synthase, (a) Chalcone synthase from Alfalfa complexed with naringenin (NAR, slate) (Icgk.pdb) superimposed on the structure of the C164A mutant complexed with malonyl-CoA (slate) (Icml.pdb). Residues shown in gray are from the NAR-enzyme complex, (b) HMG-CoA synthase from S. aureus complexed with acetoacetyl-CoA (AcAc-CoA). The active site cysteine in this structure is acetylated (Ixpk.pdb ).

The biosynthetic pathway for isoflavonoids in soybean and the relationship of the isoflavonoids to several other classes of phenylpropanoids is presented in Fig. 8.2. Production of /i-coumaryl-CoA from phenylalanine requires phenylalanine ammonia lyase to convert phenylalanine to cinnamate, cinnamic acid hydroxylase to convert cinnamate to /7-coumarate, and coumaraterCoA ligase to convert jt -coumarate to -coumaroyl-CoA. Lignins may be produced from j3-coumaroyl-CoA or from />-coumarate. Chalcone synthase catalyzes the condensation of three molecules of malonyl CoA with p-coumaroyl-CoA to form 4, 2 , 4 , 6 -tetrahydroxychalcone, which is subsequently isomerized in a reaction catalyzed by chalcone isomerase to naringenin, the precursor to genistein, flavones, flavonols, condensed tannins, anthocyanins, and others. 

Flavonoids are a diverse family of plant polyphenols and of special interest due to their potential in the treatment of various human diseases. The first attempts to produce flavonoid precursors were accomplished by cloning of the flavanone pathway consisting of cinnamate-4-hydroxylase (CYP73A5) from A. thalima together with 4-coumaroyl CoA ligase (4CL), chalcone synthase (CHS), and chalcone isomerase (CHI) in S. cerevisiae [410], The generated strain was able to convert cinnamic acid to 200 pg naringenin... 

Tomato (Petunia) (alfalfa)/ oveiexpiession CaMV35S/ CHSl + CHR Chalcone synthase + chalcone reductase Chalcones -I- deoxychalcones Higher butein, isoliquiritigenin, naringenin, chalcone, and rutin [63]... 

Tomato Grape/ overexpression CaMVd35S/ STS Stilbene synthase Resveratrol High stilbenes (resveratrol and piceid), naringenin chalcone, rutin [63]... 

Using 4-courmaroyl-CoA (in most species) and three molecules of malonyl-CoA, chalcone synthase (CHS) carries out a series of sequential decarboxylation and condensation reactions, to produce a polyketide 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. In a few species, caffeoyl-CoA and feruloyl-CoA may also be used as substrates for chalcone formation. Malonyl-CoA is formed from acetyl-CoA by acetyl-CoA carboxylase (ACC). Acetyl-CoA may be produced in mitochondria, plastids, peroxisomes, and the cytosol by a variety of routes. It is the cytosolic acetyl-CoA that is used for flavonoid biosynthesis, and it is produced by the multiple subunit enzyme ATP-citrate lyase that converts citrate, ATP, and CoA to acetyl-CoA, oxaloacetate, ADP, and inorganic phosphate [15]. 

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