Plant polyketides synthase , 2-pyrone

Fig. 10A-D. Pathways for plant polyketide synthase catalyzed biosynthesis of A chalcone and stilbene B 6 -deoxychalcone C methylchalcone D 2-pyrone...

In this chapter, we describe the atomic resolution structural elucidation of several plant type III polyketide synthases, including chalcone synthase, 2-pyrone synthase, and stilbene synthase. Manipulation of the catalytic activity and specificity of these biosynthetic enzymes by using a structurally guided approach offers a novel... 

Abe I, Watanabe T, Noguchi H (2004) Enzymatic formation of long-chain polyketide pyrones by plant type III polyketide synthases. Phytochemistry 65 2447-2453... 

Fig. 2 Reaction types catalyzed by plant type III polyketide synthases. PS, pyrone synthase CHS, chalcone synthase STS, stilbene synthase VPS, valerophenone synthase ACS, acridone synthase BPS, benzophe-none synthase...

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. 

Aloe (Aloe arborescens) is a medicinal plant rich in aromatic polyketides such as pharmaceutically important aloenin (a hexaketide pyrone), aloesin (a heptaketide chromone), and barbaloin (an octaketide anthrone) (Fig. 4a). Pentaketide chromone synthase (PCS) and octaketide synthase (OKS) are novel plant-specific type III PKSs, which were obtained from the aloe plant by RT-PCR cloning using degenerate oligonucleotide primers based on the conserved sequences of known CHS enzymes [30-33]. The deduced amino acid sequences of PCS and OKS are 91% identical (368/403), and show 50-60% identity to those of other CHS superfamily type III PKSs of plant origin OKS shares 60% identity (240/403) with CHS from... 

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