Interrogation of global active site occupancy of a fungal iterative polyketide synthase reveals strategies for maintaining biosynthetic fidelity

Nonreducing iterative polyketide synthases (NR-PKSs) are responsible for assembling the core of fungal aromatic natural products with diverse biological properties. Despite recent advances in the field, many mechanistic details of polyketide assembly by these megasynthases remain unknown. To expand our understanding of substrate loading, polyketide elongation, cyclization, and product release, active site occupancy and product output were explored by Fourier transform mass spectrometry using the norsolorinic acid anthrone-producing polyketide synthase, PksA, from the aflatoxin biosynthetic pathway in Aspergillus parasiticus. Here we report the simultaneous observation of covalent intermediates from all catalytic domains of PksA from in vitro reconstitution reactions. The data provide snapshots of iterative catalysis and reveal an underappreciated editing function for the C-terminal thioesterase domain beyond its recently established synthetic role in Claisen/Dieckmann cyclization and product release. The specificity of thioesterase catalyzed hydrolysis was explored using biosynthetically relevant protein-bound and small molecule acyl substrates and demonstrated activity against hexanoyl and acetyl, but not malonyl. Processivity of polyketide extension was supported by the inability of a nonhydrolyzable malonyl analog to trap products of intermediate chain lengths and by the detection of only fully extended species observed covalently bound to, and as the predominant products released by, PksA. High occupancy of the malonyl transacylase domain and fast relative rate of malonyl transfer compared to starter unit transfer indicate that rapid loading of extension units onto the carrier domain facilitates efficient chain extension in a manner kinetically favorable to ultimate product formation.