Edited by Jerrold Meinwald, Cornell University, Ithaca, NY, and approved July 12, 2011 (received for review May 15, 2011)
HsPKS1 from Huperzia serrata is a type III polyketide synthase (PKS) with remarkable substrate tolerance and catalytic potential. Here we present the synthesis of unnatural unique polyketide–alkaloid hybrid molecules by exploiting the enzyme reaction using precursor-directed and structure-based approaches. HsPKS1 produced novel pyridoisoindole (or benzopyridoisoindole) with the 6.5.6-fused (or 6.6.5.6-fused) ring system by the condensation of 2-carbamoylbenzoyl-CoA (or 3-carbamoyl-2-naphthoyl-CoA), a synthetic nitrogen-containing nonphysiological starter substrate, with two molecules of malonyl-CoA. The structure-based S348G mutant not only extended the product chain length but also altered the cyclization mechanism to produce a biologically active, ring-expanded 6.7.6-fused dibenzoazepine, by the condensation of 2-carbamoylbenzoyl-CoA with three malonyl-CoAs. Thus, the basic nitrogen atom and the structure-based mutagenesis enabled additional C─C and C─N bond formation to generate the novel polyketide-alkaloid scaffold.
↵1Present address: Modern Research Center of Traditional Chinese Medicine, Beijing University of Chinese Medicine, 11 Sanhuan East Road, Zhaoyang District, Beijing 100029, China.
Author contributions: I.A. designed research; H.M., M.Y., and S.-P.S. performed research; H.M., T.W., S.K., R.K., S.S., T.K., and I.A. analyzed data; and H.M. and I.A. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: Coordinates and structure factors have been deposited in the Protein Data Bank, www.pdb.org (PDB ID codes 3AWK, for the apo structure, and 3AWJ, for the CoA-SH complexed structure).
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1107782108/-/DCSupplemental.
