The Structure and Rearrangements of a Complete Polyketide Synthase Module During its Catalytic Cycle

Abstract

Polyketide natural products constitute a broad class of compounds with diverse structural features and biological activities. Their biosynthetic machinery, represented by type I polyketide synthases, has an architecture in which successive modules catalyze two-carbon linear extensions and keto group processing reactions on intermediates covalently tethered to carrier domains. We employed electron cryo-microscopy to visualize a full-length module and determine high-resolution 3D reconstructions that revealed a totally unexpected architecture compared to the homologous dimeric mammalian fatty acid synthase. A single reaction chamber provides access to all catalytic sites for the intra-module carrier domain. In contrast, the carrier from the preceding module uses a separate entrance outside the reaction chamber to deliver the upstream polyketide intermediate for subsequent extension and modification. This study reveals for the first time the structural basis for both intra-modular and inter-modular substrate transfer in polyketide synthases, and establishes a new model for molecular dissection of these multifunctional enzyme systems.