1.Cyclopiazonic acid biosynthesis in Aspergillus sp.: characterization of a reductase-like R* domain in cyclopiazonate synthetase that forms and releases cyclo-acetoacetyl-L-tryptophan.
Liu X1, Walsh CT. Biochemistry. 2009 Sep 15;48(36):8746-57. doi: 10.1021/bi901123r.
The fungal neurotoxin alpha-cyclopiazonic acid (CPA), a nanomolar inhibitor of Ca2+-ATPase, has a pentacyclic indole tetramic acid scaffold that arises from one molecule of tryptophan, acetyl-CoA, malonyl-CoA, and dimethylallyl pyrophosphate by consecutive action of three enzymes, CpaS, CpaD, and CpaO. CpaS is a hybrid, two module polyketide synthase-nonribosomal peptide synthetase (PKS-NRPS) that makes and releases cyclo-acetoacetyl-L-tryptophan (cAATrp), the tetramic acid that serves as substrate for subsequent prenylation and oxidative cyclization to the five ring CPA scaffold. The NRPS module in CpaS has a predicted four-domain organization of condensation, adenylation, thiolation, and reductase* (C-A-T-R*), where R* lacks the critical Ser-Tyr-Lys catalytic triad of the short chain dehydrogenase/reductase (SDR) superfamily. By heterologous overproduction in Escherichia coli of the 56 kDa Aspergillus flavus CpaS TR* didomain and the single T and R* domains, we demonstrate that CpaS catalyzes a Dieckmann-type cyclization on the N-acetoacetyl-Trp intermediate bound in thioester linkage to the phosphopantetheinyl arm of the T domain to form and release cAATrp.
