Chalcone synthase (CHS), the pivotal enzyme in the biosynthesis of flavonoids, is a plant-specific type III polyketide synthase (PKS) that catalyzes a sequential condensation of 4-coumaroyl-CoA with three molecules of malonyl-CoA to produce naringenin chalcone. Two novel CHSs were for the first time cloned and sequenced from rhubarb (Rheum palmatum), a medicinal plant rich in aromatic polyketides. Recombinant CHS1 and CHS2, sharing 90% amino acid sequence identity, showed K_M = 61.1 M, k_cat = 1.12 min^-1, and K_M = 36.1 M, k_cat = 0.79 min^-1 for 4-coumaroyl-CoA, respectively. Interestingly, CHS's conserved Thr197, the residue lining the active-site cavity, is uniquely replaced with Cys in CHS1 and CHS2. It was remarkable that both enzymes accepted long-chain fatty acyl CoAs up to the C₂₀ chain length as a starter substrate to efficiently produce triketide and tetraketide α-pyrones.


(Communicated by Shoji SHIBATA, M.J.A.)