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Alkyl branching at the β position of a polyketide intermediate is an important variation on canonical polyketide natural product biosynthesis. The branching enzyme, 3-hydroxy-3-methylglutaryl synthase (HMGS), catalyzes the aldol addition of an acyl donor to a β-keto-polyketide intermediate acceptor. HMGS is highly selective for two specialized acyl carrier proteins (ACPs) that deliver the donor and acceptor substrates. The HMGS from the curacin A biosynthetic pathway (CurD) was examined to establish the basis for ACP selectivity. The donor ACP (CurB) had high affinity for the enzyme (K d = 0.5 μM) and could not be substituted by the acceptor ACP. Highresolution crystal structures of HMGS alone and in complex with its donor ACP reveal a tight interaction that depends on exquisite surface shape and charge complementarity between the proteins. Selectivity is explained by HMGS binding to an unusual surface cleft on the donor ACP, in a manner that would exclude the acceptor ACP. Within the active site, HMGS discriminates between pre-and postreaction states of the donor ACP. The free phosphopantetheine (Ppant) cofactor of ACP occupies a conserved pocket that excludes the acetyl-Ppant substrate. In comparison with HMG-CoA (CoA) synthase, the homologous enzyme from primary metabolism, HMGS has several differences at the active site entrance, including a flexible-loop insertion, which may account for the specificity of one enzyme for substrates delivered by ACP and the other by CoA.natural products | polyketide synthase | curacin | HMG synthase | acyl carrier protein P olyketides are a large and chemically diverse group of natural products that includes many pharmaceuticals with a broad range of biological activities and applications as antibiotics, antifungals, antiinflammatory drugs, and cancer chemotherapeutic agents (1). Polyketide synthase (PKS) biosynthetic pathways are subjects of efforts to engineer diversification of natural products in pursuit of pharmaceutical leads and compounds of industrial importance (2). They are rich sources for development of chemoenzymatic catalysts based on PKS enzymes with unusual catalytic activities.Modular type I PKS pathways, among the most versatile of nature's systems, are biosynthetic assembly lines composed of modules that act in a defined sequence to produce complex products with a variety of functional groups and chiral centers. Each module is a set of fused catalytic domains that extend and modify a polyketide intermediate. Biosynthesis proceeds from intermediates tethered to acyl carrier protein (ACP) domains via a thioester link to a phosphopantetheine (Ppant) cofactor. A ketosynthase (KS) domain catalyzes extension of the intermediate, and subsequent modification domains typically catalyze reduction and/or methylation of the β-keto (3-keto) extension product. Beyond the enzymes for these core reactions, many PKS pathways also include other catalytic functionality. Among the most interesting of these noncanonical capabilities is alkylation at the β position by a set of β...

Covering: up to the end of 2018 Polyketides are a valuable source of bioactive and clinically important molecules. The biosynthesis of these chemically complex molecules has led to the discovery of equally complex polyketide synthase (PKS) pathways. Crystallography has yielded snapshots of individual catalytic domains, di-domains, and multi-domains from a variety of PKS megasynthases, and cryo-EM studies have provided initial views of a PKS module in a series of defined biochemical states. Here, we review the structural and biochemical results that shed light on the protein-protein interactions critical to catalysis by PKS systems with an embedded acyltransferase. Interactions include those that occur both within and between PKS modules, as well as with accessory enzymes.

We show here that computer game players can build high-quality crystal structures. Introduction of a new feature into the computer game Foldit allows players to build and real-space refine structures into electron density maps. To assess the usefulness of this feature, we held a crystallographic model-building competition between trained crystallographers, undergraduate students, Foldit players and automatic model-building algorithms. After removal of disordered residues, a team of Foldit players achieved the most accurate structure. Analysing the target protein of the competition, YPL067C, uncovered a new family of histidine triad proteins apparently involved in the prevention of amyloid toxicity. From this study, we conclude that crystallographers can utilize crowdsourcing to interpret electron density information and to produce structure solutions of the highest quality.