Benzalacetone synthase - A novel polyketide synthase that plays a crucial role in the biosynthesis of phenylbutanones in Rheum palmatum

Benzalacetone synthase (BSA) is a novel plant-specific polyketide synthase that catalyzes a one step decarboxylative condensation of 4-coumaroyl-CoA w ith malonyl-CoA to produce the C-6-C-4 skeleton of phenylbutanoids in highe r plants. A cDNA encoding BAS was for the first time cloned and sequenced f rom rhubarb (Rheum palmatum), a medicinal plant rich in phenylbutanoids inc luding pharmaceutically important phenylbutanone glucoside, lindleyin. The cDNA encoded a 42-kDa protein that shares 60-75% amino-acid sequence identi ty with other members of the CHS-superfamily enzymes. Interestingly, R. pal matum BAS lacks the active-site Phe215 residue (numbering in CHS) which has been proposed to help orient substrates and intermediates during the seque ntial condensation of 4-coumaroyl-CoA with malonyl-CoA in CHS. On the other hand, the catalytic cysteine-histidine dyad (Cys164-His303) in CHS is well conserved in BAS. A recombinant enzyme expressed in Escherichia coli effic iently afforded benzalacetone as a single product from 4-coumaroyl-CoA and malonyl-CoA. Further, in contrast with CHS that showed broad substrate spec ificity toward aliphatic CoA esters, BAS did not accept hexanoyl-CoA, isobu tyryl-CoA, isovaleryl-CoA, and acetyl-CoA as a substrate. Finally, besides the phenylbutanones in rhubarb, BAS has been proposed to play a crucial rol e for the construction of the C-6-C-4 moiety of a variety of natural produc ts such as medicinally important gingerols in ginger plant.