Stilbene synthase reaction products

PKSs are characterized by their ability to catalyze the formation of polyketide chains from the sequential condensation of acetate units from malonate thioesters. In plants they produce a range of natural products with varied in vivo and pharmacological properties. PKSs of particular note include acridone synthase, bibenzyl synthase, 2-pyrone synthase, and stilbene synthase (STS). STS forms resveratrol, a plant defense compound of much interest with regard to human health. STS shares high sequence identity with CHS, and is considered to have evolved from CHS more than once. ° Knowledge of the molecular structure of the CHS-like enzymes has allowed direct engineering of CHS and STS to alter their catalytic activities, including the number of condensations carried out (reviewed in Refs. 46, 51, 52). These reviews also present extensive, and superbly illustrated, discussions of CHS enzyme structure and reaction mechanism. 

Rg.6. Reactions catalyzed by chalcone and stilbene synthases. Each enzyme condenses three malonyl CoA extender units onto p-coumaroyl-CoA. Stilbene synthases catalyze an additional decarboxylation, resulting in a different pattern of cyclization for chalcone versus stilbene products... 

Figure 1. Type III polyketide synthases and their reaction products. CHS chalcone synthase, STS stilbene synthase BPS benzophenone synthase ACS acridone synthase VPS valerophenone synthase CTAS p-coumaryltriacetic acid synthase 2PS 2-pyrone synthase SPS styrylpyrone synthase BAS benzalacetone synthase H/EDS homoeryodyctiol/eriodyctiol chalcone synthase ALS Aloesone synthase PCS phertylchromone synthase BIS biphenyl synthase...

Type I and Type II PKSs catalyze multiple rounds of reactions by catalytic modules encoded either by a single polypeptide (PKS I) or on separate polypeptides (PKS II) by analogy to FAS-I and FAS-II. In contrast, PKS Ills are dimers of KASs that catalyze multiple condensation reactions in one active site and include chalcone synthase, stilbene synthase, and 2-pyrone synthase (see Chapters 1.05, 1.07, and 1.04). In the case of chalcone synthase, three consecutive condensation reactions each utilizing malonyl-CoA, followed by a cyclization reaction, lead to the formation of 4, 2, 4, 6 -tetrahydroxychalcone from 4-hydroxycinnamoyl-CoA (Figure 3). Recruitment of a reductase leads to the formation of a product lacking the 6 -hydroxy group, a reaction that requires an intermediate in the synthesis of chalcone to dissociate from the synthase active site. 

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