Enzymatic formation of long-chain polyketid pyrones by plant type III polyketide synthases

Recombinant chalcone synthase (CHS) from Scutellaria baicalensis and stilbene synthase (STS) from Arachis hypogaea accepted CoA esters of long-chain fatty acid (CHS up to the C-12 ester, while STS up to the C-14 ester) as a starter substrate, and carried out sequential condensations with malonyl-CoA, leading to formation of triketide and tetraketide alpha-pyrones. Interestingly, the C-6, C-8, and C-10 esters were kinetically favored by the enzymes over the physiological starter substrate; the k(cat)/K-M values were 1.2- to 1.9-fold higher than that of p-coumaroyl-CoA. The catalytic diversities of the enzymes provided further mechanistic insights into the type III PKS reactions, and suggested involvement of the CHS-superfamily enzymes in the biosynthesis of long-chain alkyl polyphenols such as urushiol and ginkgolic acid in plants. (C) 2004 Elsevier Ltd. All rights reserved.