Structures of Pseudomonas aeruginosa β-ketoacyl-(acyl-carrier-protein) synthase II (FabF) and a C164Q mutant provide templates for antibacterial drug discovery and identify a buried potassium ion and a ligand-binding site that is an artefact of the crystal form

Towards new antibiotics: P. aeruginosa FabF ligands discovered by crystallographic fragment screening followed by hit expansion

There is an urgent need for new antibiotics. FabF (3-oxoacyl-[acyl-carrier-protein] synthase 2), which catalyses the rate limiting condensation reaction in the fatty acid synthesis II pathway, is an attractive target. Very few inhibitors of FabF are known and most are derived from natural products. In an effort to further explore the chemical space of FabF ligands, we have carried out fragment screening by X-ray crystallography against an intermediated state-mimicking variant of P. aeruginosa FabF (PaFabF C164Q). This screen has resulted in 48 hits out of which 16 bind in or close to the malonyl-CoA or fatty acid binding site or an adjacent dimer interface. None of the closer investigated fragments were active in a binding assay, but the same was the case for fragments derived from a potent FabF inhibitor. For hit optimization, we focused on the two fragments binding close to the catalytic residues of FabF. Different strategies were followed in the optimization process: exploration of commercially available analogues, fragment merging, virtual screening of a combinatorial make-on-demand space, and design and in-house synthesis of analogues. In total, more than 90 analogues of the hit compounds were explored, and for 10 of those co-crystal structures could be determined. The most potent ligand was discovered using manual structure-based design and has a binding affinity of 65 μM. This data package forms a strong foundation for the development of more potent and diverse FabF inhibitors.

Crystal structure of Pseudomonas aeruginosa FabB C161A, a template for structure-based design for new antibiotics

Background: FabB (3-oxoacyl-[acyl-carrier-protein] synthase 1) is part of the fatty acid synthesis II pathway found in bacteria and a potential target for antibiotics. The enzyme catalyses the Claisen condensation of malonyl-ACP (acyl carrier protein) with acyl-ACP via an acyl-enzyme intermediate. Here, we report the crystal structure of the intermediate-mimicking Pseudomonas aeruginosa FabB ( PaFabB) C161A variant.

Methods: His-tagged PaFabB C161A was expressed in E. coli Rosetta DE3 pLysS cells, cleaved by TEV protease and purified using affinity and size exclusion chromatography. Commercial screens were used to identify suitable crystallization conditions which were subsequently improved to obtain well diffracting crystals.

Results: We developed a robust and efficient system for recombinant expression of PaFabB C161A. Conditions to obtain well diffracting crystals were established. The crystal structure of PaFabB C161A was solved by molecular replacement at 1.3 Å resolution. Binding site comparison between PaFabB and PaFabF revealed a conserved malonyl binding site but differences in the fatty acid binding channel.

Conclusions: The PaFabB C161A crystal structure can be used as a template to facilitate the design of FabB inhibitors.

An Experimental Toolbox for Structure-Based Hit Discovery for P. aeruginosa FabF, a Promising Target for Antibiotics

FabF (3-oxoacyl-[acyl-carrier-protein] synthase 2), which catalyses the rate limiting condensation reaction in the fatty acid synthesis II pathway, is an attractive target for new antibiotics. Here, we focus on FabF from P. aeruginosa (PaFabF) as antibiotics against this pathogen are urgently needed. To facilitate exploration of this target we have set up an experimental toolbox consisting of binding assays using bio-layer interferometry (BLI) as well as saturation transfer difference (STD) and WaterLOGSY NMR in addition to robust conditions for structure determination. The suitability of the toolbox to support structure-based design of FabF inhibitors was demonstrated through the validation of hits obtained from virtual screening. Screening a library of almost 5 million compounds resulted in 6 compounds for which binding into the malonyl-binding site of FabF was shown. For one of the hits, the crystal structure in complex with PaFabF was determined. Based on the obtained binding mode, analogues were designed and synthesised, but affinity could not be improved. This work has laid the foundation for structure-based exploration of PaFabF.

Crystal structure of MBP-PigG fusion protein and the essential function of PigG in the prodigiosin biosynthetic pathway in Serratia marcescens FS14

Prodigiosin, a tripyrrole red pigment is synthesized by Serratia and some other microbes through a bifurcated biosynthesis pathway; MBC (4-methoxy-2,2'-bipyrrole-5-carbaldehyde) and MAP (2-methyl-3-n-amyl-pyrrole) are synthesized separately and then condensed by PigC to form prodigiosin. PigI, PigG and PigA have been shown to be involved in the first steps of MBC biosynthesis (proline incorporation). The crystal structure of PigG was resolved to elucidate its function and mechanism. PigG, an acyl carrier protein (ACP), features the ACP architecture:, a helical bundle fold containing three major helices and a minor distorted helix together with a conserved "S" motif. An in-frame deletion mutation of the pigG gene abolished the synthesis of prodigiosin in Serratia marcescens FS14. The production of prodigiosin was fully restored by complementation of intact pigG; however the S36A mutant was not able to restore function in the in-frame deletion pigG mutant, indicating that PigG and the conserved serine residue (S36) of PigG are essential for the synthesis of prodigiosin.