Architecture of the polyketide synthase module: surprises from electron cryo-microscopy

Abstract

Modular polyketide synthases produce a vast array of bioactive molecules that are the basis of many highly valued pharmaceuticals. The biosynthesis of these compounds is based on ordered assembly lines of multi-domain modules, each extending and modifying a specific chain-elongation intermediate before transfer to the next module for further processing. The first 3D structures of a full polyketide synthase module in different functional states were obtained recently by electron cryo-microscopy. The unexpected module architecture revealed a striking evolutionary divergence of the polyketide synthase compared to its metazoan fatty acid synthase homolog, as well as remarkable conformational rearrangements dependent on its biochemical state during the full catalytic cycle. The design and dynamics of the module are highly optimized for both catalysis and fidelity in the construction of complex, biologically active natural products.

Solution Structure of a Nonribosomal Peptide Synthetase Carrier Protein Loaded with Its Substrate Reveals Transient, Well-Defined Contacts

“…Multiple modules are arranged in an assembly-line fashion to comprise the full synthetase, an organization also encountered in related fatty acid synthases (FASs) and modular polyketide synthases (PKSs). 1–9 Each module within an NRPS is comprised of at least three core domains whose combined action leads to the selection, activation, and incorporation of a single small molecule into the growing peptide. 10–12 NRPS modules select starting materials from a pool of hundreds of small molecules including the 20 standard L-amino acids, aryl acids, and D-amino acids.…”

Active Multienzyme Assemblies for Long-Chain Olefinic Hydrocarbon Biosynthesis

“…In other cases, individually expressed proteins self-assemble in the cytosol, for example, the lumazine synthase-riboflavin synthase protein complex (16,17). The structure and function of a number of polyketide synthase and fatty acid synthase systems have been studied in significant detail (18)(19)(20). In contrast, the higher-order structure of hydrocarbon biosynthetic machinery has been far less characterized.…”

The structural biology of biosynthetic megaenzymes

“…93). The lack of success (at least so far) with X-ray crystallography most likely reflects the conformational heterogeneity of the PKS systems arising from their high flexibility.…”

“…The major discrepancy between the SAXS-and cryo-EM-derived models concerns the KS-AT portion of the module. In the SAXS case, this region was treated as a rigid body during modeling and refinement against the SAXS data, and so it necessarily retains its extended ' AT-out' 93 configuration. Whether such an arrangement is actually adopted by the module or whether it is an artifact of the data treatment remains to be determined.…”