Reactivity derails PKS prediction
Polyketide synthases (PKSs) are multidomain proteins in which eachdomain performs a specific type of chemistry on the way to a polyketide product. Because the domains are highly conserved, it is often possible to predict not only the reaction performed by each domain but also the preferred substrate. However, Strieter et al. now identify unexpected chemistry in the biosynthesis of the hydrindane ring of coronafacic acid (CFA). In the proposed pathway for CFA synthesis, the condensation of a cyclopentanone with ethyl malonate leads to a linear intermediate bound to Cfa7, a PKS with NADPH-dependent ketoreductase, dehydratase and thioesterase domains. The direct action of these domains on the intermediate would lead down a seemingly unproductive pathway, whereas an intramolecular 6-endo-trig cyclization followed by the enzymatic reactions would result in CFA formation. The authors investigated the system by first obtaining the protein in active form and using a substrate surrogate to load the cyclopentanone onto Cfa7. Addition of the malonyl cosubstrate and NADPH resulted in generation of CFA, whereas the reaction in the absence of NADPH ended in an unknown intermediate. Comparison with a model reaction system demonstrated that this intermediate was the cyclized but unmodified compound, which confirms that the intramolecular reaction precedes enzymatic tailoring. This result not only demonstrates unexpected substrate flexibility in Cfa7, but also emphasizes the need for biochemical evidence in establishing biosynthetic pathways. (J. Am. Chem. Soc., published online 22 January 2009, doi:10.1021/ja8077945) CG
This is a preview of subscription content, access via your institution