Reaction Mechanism of Plant Polyketide Synthase

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Chalcone synthase (CHS) and stilbene synthase (STS) are plant-specific polyketide synthases (PKSs). They catalyze common three consecutive decarboxylative condensations and specific cyclization reactions. They are highly homologous to each other, and are likely to fall into a family of PKSs along with acridone synthase and bibenzyl synthase. Two cDNA clones (named as HmC and HmS) both of which show high homology to the known CHSs, were obtained from leaves of Hydrangea macrophylla var. thunbergii. They were expressed in Escherichia coli in order to determine their enzyme functions. Detection of chalcone formation clearly indicated that HmC encoded CHS, while HmS protein catalyzed the formation of neither chalcone nor stilbene. However, a novel pyrone, a lactonization product of a linear tetraketide was detected in reaction products of HmS protein. This proves that HmS encodes a novel PKS which catalyzes only chain elongation without cyclization.