2.3.1.221 7 malonyl-CoA + hexanoyl-[acyl-carrier protein] = 7 CoA + norsolorinic acid anthrone + [acyl-carrier protein] + 7 CO2 + 2 H2O - - 2.3.1.221 7 malonyl-CoA + hexanoyl-[acyl-carrier protein] = 7 CoA + norsolorinic acid anthrone + [acyl-carrier protein] + 7 CO2 + 2 H2O a product template domain unites with the ketosynthase and thioesterase in this IPKS system to assemble precisely seven malonyl-derived building blocks to a hexanoyl starter unit and mediate a specific cyclization cascade. These mechanistic features are general for IPKS-catalyzed production of aromatic polyketides 721052 2.3.1.221 7 malonyl-CoA + hexanoyl-[acyl-carrier protein] = 7 CoA + norsolorinic acid anthrone + [acyl-carrier protein] + 7 CO2 + 2 H2O mechanism of thioesterase/Claisen cyclase-catalyzed chain-termination of fungal aromatic polyketide biosynthesis. The ACP of the ACP-bound substrate is displaced upon thioesterase-catalyzed transesterification. Rotation of the substrate side chain can occur once the ACP leaves the pocket, and the thioesterase can then close. Thioesterase conformational constraints as observed in the closed-form crystal structure guide Claisen-type cyclization to release noranthrone, i.e. norsolorinic acid anthrone, the polyketide precursor of aflatoxin B1. Domain structure and reaction mechanism, detailed overview 723638 2.3.1.221 7 malonyl-CoA + hexanoyl-[acyl-carrier protein] = 7 CoA + norsolorinic acid anthrone + [acyl-carrier protein] + 7 CO2 + 2 H2O reaction mechanism, overview. The SAT domain in PksA selects a hexanoyl starter unit. The MAT domain loads the free ACP with malonyl units. After seven successive condensation events with malonyl-ACP catalysed in te ketoacyl synthase domain, the linear ACP-bound polyketide is cyclized (C4-C9 and C2-C11 cyclization events) and aromatized in the product template domain to give the bicyclic intermediate. The thioesterase domain catalyses C-C cyclization to release anthrone, which undergoes oxidation to the anthraquinone norsolorinic acid, to initiate the complex biosynthetic pathway to aflatoxin B1 723262