The role of cysteines in polyketide synthases. Site-directed mutagenesis of resveratrol and chalcone synthases, two key enzymes in different plant-specific pathways.

Resveratrol and chalcone synthases are related plant-specific polyketide synthases that are key enzymes in the biosynthesis of stilbenes and flavonoids, respectively. The stepwise condensing reactions correspond to those in other polyketide and fatty-acid synthases. This predicts that the two proteins also contain cysteines that are essential for enzyme activity because they bind the substrates. We exchanged, in both enzymes, all of the 6 conserved cysteines into alanine by site-directed mutagenesis and tested the mutants after expression of the proteins in the Escherichia coli heterologous system. Only cysteine 169 was essential in both enzymes, and inhibitor studies suggest that it is the main target of cerulenin, an antibiotic reacting with the cysteine in the active center of condensing enzymes. Most of the other exchanges led to reduced activities. In two cases, the enzymes responded differently, suggesting that the cysteines at positions 135 and 195 may be involved in the different product specificity of the two enzymes. The sequences surrounding the essential cysteine 169 revealed no similarity to the active sites of condensing enzymes in other polyketide synthases and in fatty acid biosynthesis. The available data indicate that resveratrol and chalcone synthases represent a group of enzymes that evolved independently of other condensing enzymes.