Functional diversity of chalcone synthase

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The chalcone synthase (CHS) superfamily of type III poyketide synthases (PKSs) are pivotal enzymes in the biosynthesis of flavonoids as well as a wide range of structurally diverse, biologically important natural products. Benzalacetone synthase (BAS) (EC2.3.1) catalyzes a one-step decarboxylative condensation of 4-coumaroyl-CoA (1) with malonyl-CoA to produce a diketide benzalacetone. While CHS (EC 2.3.1. 74) performs sequential condensations of 4-coumaroyl-CoA with three acetate units from malonyl-CoA followed by Claisen-type cyclization reaction, leading to formation of a tetraketide naringenin chalcone. BAS, which is thought to play a crucial role for construction of the C6-C4 moiety of a variety of biologically active penylbutanoids including anti-inflammatory glucoside lindleyin in rhubarb [6]-gingerol and curcumin in ginger plants accepted methyl-malonylCoA as achain elongation substrate to produce novel C6-C5 compounds. The cDNA encoded a 42kDa protein sharning 60-75% amino aci … More d sequence identity with other members of the CHS-superfamily enzymes. Recent crystallographic and protein engineering studies on alfalfa (Medicago sativa) CHS, a homodimeric 42 kDa protein, revealed the active site machinery of the chalcone forming reaction which proceeds through starter molecule loading at Cysl164, malony-CoA deearboxylation, polyketide chain elongation, followed by cyclization and aromatization of the enzyme bound tetraketide intermediate. The catalytic center of CHS is thus composed of four amino acid residues; the catalytic triad of Cys164, His303, and Asn336, and the "gatekeeper" Phe215, absolutely conserved in all the known type III expect as of BAS. In CHS, Phe215, located at the junction between the active site cavity and the CoA binding tunnel, has been proposed to facilitate decarboxylation of malonyl-CoA and help orient substrates and intermediates during the sequential condensation reactions. We successfully showed that BAS retained CHS activity by constructing the mutant R. palmatum BAS in which the residues ^<214>IL were substituted by LF. Less