Rapid and efficient room-temperature serial synchrotron crystallography using the CFEL TapeDrive.

Serial crystallography at conventional synchrotron light sources (SSX) offers the possibility to routinely collect data at room temperature using micrometre-sized crystals of biological macromolecules. However, SSX data collection is not yet as routine and currently takes significantly longer than the standard rotation series cryo-crystallography. Thus, its use for high-throughput approaches, such as fragment-based drug screening, where the possibility to measure at physio-logical temperatures would be a great benefit, is impaired. On the way to high-throughput SSX using a conveyor belt based sample delivery system - the CFEL TapeDrive - with three different proteins of biological relevance (Klebsiella pneumoniae CTX-M-14 ß-lactamase, Nectria haematococca xylanase GH11 and Aspergillus flavus urate oxidase), it is shown here that complete datasets can be collected in less than a minute and only minimal amounts of sample are required.

Antiviral activity of natural phenolic compounds in complex at an allosteric site of SARS-CoV-2 papain-like protease.

SARS-CoV-2 papain-like protease (PLpro) covers multiple functions. Beside the cysteine-protease activity, facilitating cleavage of the viral polypeptide chain, PLpro has the additional and vital function of removing ubiquitin and ISG15 (Interferon-stimulated gene 15) from host-cell proteins to support coronaviruses in evading the host's innate immune responses. We identified three phenolic compounds bound to PLpro, preventing essential molecular interactions to ISG15 by screening a natural compound library. The compounds identified by X-ray screening and complexed to PLpro demonstrate clear inhibition of PLpro in a deISGylation activity assay. Two compounds exhibit distinct antiviral activity in Vero cell line assays and one inhibited a cytopathic effect in non-cytotoxic concentration ranges. In the context of increasing PLpro mutations in the evolving new variants of SARS-CoV-2, the natural compounds we identified may also reinstate the antiviral immune response processes of the host that are down-regulated in COVID-19 infections.

Crystal structure of an extracellular superoxide dismutase from Onchocerca volvulus and implications for parasite-specific drug development.

Superoxide dismutases (SODs) are metalloproteins that are responsible for the dismutation of superoxide anion radicals. SODs are consequently protective against oxidative damage to cellular components. Among other protective mechanisms, the filarial parasite Onchocerca volvulus has a well developed defense system to scavenge toxic free radicals using SODs during migration and sojourning of the microfilariae and adult worms in the human body. O. volvulus is responsible for the neglected disease onchocerciasis or `river blindness'. In the present study, an extracellular Cu/Zn-SOD from O. volvulus (OvEC-SOD) was cloned, purified and crystallized to obtain structural insight into an attractive drug target with the potential to combat onchocerciasis. The recombinant OvEC-SOD forms a dimer and the protein structure was solved and refined to 1.55 Šresolution by X-ray crystallography. Interestingly, a sulfate ion supports the coordination of the conserved copper ion. The overall protein shape was verified by small-angle X-ray scattering. The enzyme shows a different surface charge distribution and different termini when compared with the homologous human SOD. A distinct hydrophobic cleft to which both protomers of the dimer contribute was utilized for a docking approach with compounds that have previously been identified as SOD inhibitors to highlight the potential for individual structure-based drug development.

Structural basis to repurpose boron-based proteasome inhibitors Bortezomib and Ixazomib as ß-lactamase inhibitors.

ß-lactamases are a major cause of rapidly emerging and spreading antibiotic resistance. Currently ß-lactamase inhibitors (BLIs) in clinical use act only on Ambler Class A, C and some class D lactamases. The urgent need to identify new BLIs recently lead to FDA approval of boron-based compounds BLIs, e.g. Vaborbactam. The boron-based proteasome inhibitors Bortezomib and Ixazomib are used in cancer therapy as multiple myeloma drugs but they also bind to Ser-/Thr- proteases. In this study we show the crystal structures of the ß-lactamase CTX-M-14 with covalently bound Bortezomib and Ixazomib at high resolutions of 1.3 and 1.1 Å, respectively. Ixazomib is well defined in electron density whereas Bortezomib show some disorder which corresponds to weaker inhibition efficiency observed for Ixazomib. Both inhibitors mimic the deacylation transition state of ß-lactam hydrolysis, because they replace the deacylating water molecule. We further investigate differences in binding of Bortezomib/Ixazomib to CTX-M-14 and its target proteases as well as known ß-lactamase drugs. Our findings can help to use Bortezomib/Ixazomib as lead compounds for development of new BLIs.

Structure and activity of the DHNA Coenzyme-A Thioesterase from Staphylococcus aureus providing insights for innovative drug development.

Humanity is facing an increasing health threat caused by a variety of multidrug resistant bacteria. Within this scenario, Staphylococcus aureus, in particular methicillin resistant S. aureus (MRSA), is responsible for a number of hospital-acquired bacterial infections. The emergence of microbial antibiotic resistance urgently requires the identification of new and innovative strategies to treat antibiotic resistant microorganisms. In this context, structure and function analysis of potential drug targets in metabolic pathways vital for bacteria endurance, such as the vitamin K2 synthesis pathway, becomes interesting. We have solved and refined the crystal structure of the S. aureus DHNA thioesterase (SaDHNA), a key enzyme in the vitamin K2 pathway. The crystallographic structure in combination with small angle X-ray solution scattering data revealed a functional tetramer of SaDHNA. Complementary activity assays of SaDHNA indicated a preference for hydrolysing long acyl chains. Site-directed mutagenesis of SaDHNA confirmed the functional importance of Asp16 and Glu31 for thioesterase activity and substrate binding at the putative active site, respectively. Docking studies were performed and rational designed peptides were synthesized and tested for SaDHNA inhibition activity. The high-resolution structure of SaDHNA and complementary information about substrate binding will support future drug discovery and design investigations to inhibit the vitamin K2 synthesis pathway.

Structure and dynamics of UBA5-UFM1 complex formation showing new insights in the UBA5 activation mechanism.

Ubiquitin fold modifier 1 (UFM1) is an ubiquitin-like protein (Ubl) involved especially in endoplasmic stress response. Activation occurs via a three-step mechanism like other Ubls. Data obtained reveal that UFM1 regulates the oligomeric state of ubiquitin activating enzyme 5 (UBA5) to initiate the activation step. Mixtures of homodimers and heterotrimers are observed in solution at the equilibrium state, demonstrating that the UBA5-UFM1 complex undergoes several concentration dependent oligomeric translational states to form a final functional complex. The oligomerization state of unbound UBA5 is also concentration dependent and shifts from the monomeric to the dimeric state. Data describing different oligomeric states are complemented with binding studies that reveal a negative cooperativity for the complex formation and thereby provide more detailed insights into the complex formation mechanism.

The Komagataeibacter europaeus GqqA is the prototype of a novel bifunctional N-Acyl-homoserine lactone acylase with prephenate dehydratase activity.

Previously, we reported the isolation of a quorum quenching protein (QQ), designated GqqA, from Komagataeibacter europaeus CECT 8546 that is highly homologous to prephenate dehydratases (PDT) (Valera et al. in Microb Cell Fact 15, 88. https://doi.org/10.1186/s12934-016-0482-y , 2016). GqqA strongly interfered with N-acyl-homoserine lactone (AHL) quorum sensing signals from Gram-negative bacteria and affected biofilm formation in its native host strain Komagataeibacter europaeus. Here we present and discuss data identifying GqqA as a novel acylase. ESI-MS-MS data showed unambiguously that GqqA hydrolyzes the amide bond of the acyl side-chain of AHL molecules, but not the lactone ring. Consistent with this observation the protein sequence does not carry a conserved Zn2+ binding motif, known to be essential for metal-dependent lactonases, but in fact harboring the typical periplasmatic binding protein domain (PBP domain), acting as catalytic domain. We report structural details for the native structure at 2.5 Å resolution and for a truncated GqqA structure at 1.7 Å. The structures obtained highlight that GqqA acts as a dimer and complementary docking studies indicate that the lactone ring of the substrate binds within a cleft of the PBP domain and interacts with polar residues Y16, S17 and T174. The biochemical and phylogenetic analyses imply that GqqA represents the first member of a novel type of QQ family enzymes.

A Giant Extracellular Matrix Binding Protein of Staphylococcus epidermidis Binds Surface-Immobilized Fibronectin via a Novel Mechanism.

Although it is normally an innocuous part of the human skin microbiota, Staphylococcus epidermidis has emerged as a major nosocomial pathogen, and implanted foreign materials are an essential risk factor for the development of an infection. The extraordinary efficiency of S. epidermidis to colonize artificial surfaces is particularly related to the ability to form biofilms. Biofilm formation itself critically depends on stable pathogen binding to extracellular host matrix components, e.g. fibronectin (Fn), covering inserted devices in vast amounts. Extracellular matrix binding protein (Embp) and its subdomains referred to as the F-repeat and the FG-repeat are critical for adherence of S. epidermidis to surface-immobilized Fn. Embp-Fn interactions preferentially occur with surface-bound, but not folded, globular Fn via binding to the F3 domain. High-resolution structure analysis of F- and FG-repeats revealed that both repeats are composed of two tightly connected triple α-helix bundles, exhibiting an elongated but rather rigid structural organization in solution. Both F- and FG-repeat possess Fn-binding capacity via interactions with type III subdomain FN12, involving residues within the C and F ß-sheet. FN12 essentially supports stability of the globular Fn state, and thus these findings reasonably explain why Embp-mediated interaction of S. epidermidis necessitates Fn surface immobilization. Thus, Embp employs an uncharacterized bacterial Fn-binding mechanism to promote staphylococcal adherence.IMPORTANCEStaphylococcus epidermidis is a leading pathogen in implant-associated hospital infections. The pathogenesis critically depends on bacterial binding to ECM components, specifically fibronectin (Fn). The cell surface-localized, 1-MDa extracellular matrix binding protein (Embp) is essentially characterized by 10 F- and 40 FG-repeats. These repetitive units, each characterized by two α-helical bundles, organize themselves in a rigid, elongated form. Embp binds preferentially to surface-localized but not soluble Fn, with both F- and FG-repeats being sufficient for Fn binding and resulting bacterial adherence. Binding preferentially involves Fn type III domain, specifically residues of FN12 ß-sheets C and F. Both play key role in stabilizing the globular Fn conformation, explaining the necessity of Fn surface immobilization for a subsequent interaction with Embp. In comparison to many other bacterial Fn-binding proteins using the Fn N terminus, Embp employs a previously undescribed mechanism supporting the adhesion of S. epidermidis to surface-immobilized Fn.

High-resolution crystal structure and biochemical characterization of a GH11 endoxylanase from Nectria haematococca.

Enzymatic degradation of vegetal biomass offers versatile procedures to improve the production of alternative fuels and other biomass-based products. Here we present the three-dimensional structure of a xylanase from Nectria haematococca (NhGH11) at 1.0 Å resolution and its functional properties. The atomic resolution structure provides details and insights about the complex hydrogen bonding network of the active site region and allowed a detailed comparison with homologous structures. Complementary biochemical studies showed that the xylanase can catalyze the hydrolysis of complex xylan into simple xylose aldopentose subunits of different lengths. NhGH11 can catalyze the efficient breakdown of beechwood xylan, xylan polysaccharide, and wheat arabinoxylan with turnover numbers of 1730.6 ± 318.1 min-1, 1648.2 ± 249.3 min-1 and 2410.8 ± 517.5 min-1 respectively. NhGH11 showed maximum catalytic activity at pH 6.0 and 45 °C. The mesophilic character of NhGH11 can be explained by distinct structural features in comparison to thermophilic GH11 enzymes, including the number of hydrogen bonds, side chain interactions and number of buried water molecules. The enzymatic activity of NhGH11 is not very sensitive to metal ions and chemical reagents that are typically present in associated industrial production processes. The data we present highlights the potential of NhGH11 to be applied in industrial biomass degradation processes.

Structure of GTP cyclohydrolase I from Listeria monocytogenes, a potential anti-infective drug target.

A putative open reading frame encoding GTP cyclohydrolase I from Listeria monocytogenes was expressed in a recombinant Escherichia coli strain. The recombinant protein was purified and was confirmed to convert GTP to dihydroneopterin triphosphate (Km = 53 µM; vmax = 180 nmol mg-1 min-1). The protein was crystallized from 1.3 M sodium citrate pH 7.3 and the crystal structure was solved at a resolution of 2.4 Š(Rfree = 0.226) by molecular replacement using human GTP cyclohydrolase I as a template. The protein is a D5-symmetric decamer with ten topologically equivalent active sites. Screening a small library of about 9000 compounds afforded several inhibitors with IC50 values in the low-micromolar range. Several inhibitors had significant selectivity with regard to human GTP cyclohydrolase I. Hence, GTP cyclohydrolase I may be a potential target for novel drugs directed at microbial infections, including listeriosis, a rare disease with high mortality.

Wucherria bancrofti glutathione S-Transferase: Insights into the 2.3 Šresolution X-ray structure and function, a therapeutic target for human lymphatic filariasis.

The notoriety of parasitic nematode survival is directly related to chronic pathogenicity, which is evident in human lymphatic filariasis. It is a disease of poverty which causes severe disability affecting more than 120 million people worldwide. These nematodes down-regulate host immune system through a myriad of strategies that includes secretion of antioxidant and detoxification enzymes like glutathione-S-transferases (GSTs). Earlier studies have shown Wuchereria bancrofti GST to be a potential therapeutic target. Parasite GSTs catalyse the conjugation of glutathione to xenobiotic and other endogenous electrophiles and are essential for their long-term survival in lymph tissues. Hence, the crystal structure of WbGST along with its cofactor GSH at 2.3 Šresolution was determined. Structural comparisons against host GST reveal distinct differences in the substrate binding sites. The parasite xenobiotic binding site is more substrate/solvent accessible. The structure also suggests the presence of putative non-catalytic binding sites that may permit sequestration of endogenous and exogenous ligands. The structure of WbGST also provides a case for the role of the π-cation interaction in stabilizing catalytic Tyr compared to stabilization interactions described for other GSTs. Hence, the obtained information regarding crucial differences in the active sites will support future design of parasite specific inhibitors. Further, the study also evaluates the inhibition of WbGST and its variants by antifilarial diethylcarbamazine through kinetic assays.

Growing Protein Crystals with Distinct Dimensions Using Automated Crystallization Coupled with In Situ Dynamic Light Scattering.

The automated crystallization device is a patented technique1 especially developed for monitoring protein crystallization experiments with the aim to precisely maneuver the nucleation and crystal growth towards desired sizes of protein crystals. The controlled crystallization is based on sample investigation with in situ Dynamic Light Scattering (DLS) while all visual changes in the droplet are monitored online with the help of a microscope coupled to a CCD camera, thus enabling a full investigation of the protein droplet during all stages of crystallization. The use of in situ DLS measurements throughout the entire experiment allows a precise identification of the highly supersaturated protein solution transitioning to a new phase - the formation of crystal nuclei. By identifying the protein nucleation stage, the crystallization can be optimized from large protein crystals to the production of protein microcrystals. The experimental protocol shows an interactive crystallization approach based on precise automated steps such as precipitant addition, water evaporation for inducing high supersaturation, and sample dilution for slowing induced homogeneous nucleation or reversing phase transitions.

Microfluidic Chips for In Situ Crystal X-ray Diffraction and In Situ Dynamic Light Scattering for Serial Crystallography.

This protocol describes fabricating microfluidic devices with low X-ray background optimized for goniometer based fixed target serial crystallography. The devices are patterned from epoxy glue using soft lithography and are suitable for in situ X-ray diffraction experiments at room temperature. The sample wells are lidded on both sides with polymeric polyimide foil windows that allow diffraction data collection with low X-ray background. This fabrication method is undemanding and inexpensive. After the sourcing of a SU-8 master wafer, all fabrication can be completed outside of a cleanroom in a typical research lab environment. The chip design and fabrication protocol utilize capillary valving to microfluidically split an aqueous reaction into defined nanoliter sized droplets. This loading mechanism avoids the sample loss from channel dead-volume and can easily be performed manually without using pumps or other equipment for fluid actuation. We describe how isolated nanoliter sized drops of protein solution can be monitored in situ by dynamic light scattering to control protein crystal nucleation and growth. After suitable crystals are grown, complete X-ray diffraction datasets can be collected using goniometer based in situ fixed target serial X-ray crystallography at room temperature. The protocol provides custom scripts to process diffraction datasets using a suite of software tools to solve and refine the protein crystal structure. This approach avoids the artefacts possibly induced during cryo-preservation or manual crystal handling in conventional crystallography experiments. We present and compare three protein structures that were solved using small crystals with dimensions of approximately 10-20 µm grown in chip. By crystallizing and diffracting in situ, handling and hence mechanical disturbances of fragile crystals is minimized. The protocol details how to fabricate a custom X-ray transparent microfluidic chip suitable for in situ serial crystallography. As almost every crystal can be used for diffraction data collection, these microfluidic chips are a very efficient crystal delivery method.

Crystal Structure of Mistletoe Lectin I (ML-I) from Viscum album in Complex with 4-N-Furfurylcytosine at 2.85 Å Resolution.

BACKGROUND: Viscum album (the European mistletoe) is a semi-parasitic plant, which is of high medical interest. It is widely found in Europe, Asia, and North America. It contains at least three distinct lectins (i.e. ML-I, II, and III), varying in molecular mass and specificity. Among them, ML-I is in focus of medical research for various activities, including anti-cancer activities. To understand the molecular basis for such medical applications, a few studies have already addressed the structural and functional analysis of ML-I in complex with ligands. In continuation of these efforts, we are reporting the crystal structure of ML from Viscum album in complex with the nucleic acid oxidation product 4-N-furfurylcytosine (FC) refined to 2.85 Å resolution. FC is known to be involved in different metabolic pathways related to oxidative stress and DNA modification. METHODS: X-ray suitable hexagonal crystals of the ML-I/FC complex were grown within four days at 294 K using the hanging drop vapor diffusion method. Diffraction data were collected up to a resolution of 2.85 Å. The ligand affinity was verified by in-silico docking. RESULTS: The high-resolution structure was refined subsequently to analyze particularly the active site conformation and a binding epitope of 4-N-furfurylcytosine. A distinct 2Fo-Fc electron density at the active site was interpreted as a single FC molecule. The specific binding of FC is achieved also through hydrophobic interactions involving Tyr76A, Tyr115A, Glu165A, and Leu157A of the ML-I A-chain. The binding energy of FC to the active site of ML-I was calculated as well to be -6.03 kcal mol-1. CONCLUSION: In comparison to other reported ML-I complexes, we observed distinct differences in the vicinity of the nucleic acid base binding site upon interaction with FC. Therefore, data obtained will provide new insights in understanding the specificity, inhibition, and cytotoxicity of the ML-I A-chain, and related RIPs.

Emergence of ceftazidime/avibactam non-susceptibility in an MDR Klebsiella pneumoniae isolate.

Background: Avibactam is a novel broad-range ß-lactamase inhibitor active against Ambler class A (including ESBL and KPC) and some Ambler class C and D (e.g. OXA-48) enzymes. We here report on the emergence of ceftazidime/avibactam resistance in clinical, multiresistant, OXA-48 and CTX-M-14-producing Klebsiella pneumoniae isolate DT12 during ceftazidime/avibactam treatment. Methods and results: Comparative whole-genome sequence analysis identified two SNPs in the CTX-M-14-encoding gene leading to two amino acid changes (P170S and T264I). Compared with WT CTX-M-14, expression of the CTX-M-14Δ170Δ264 isoform in Escherichia coli led to a >64- and 16-fold increase in ceftazidime and ceftazidime/avibactam MICs, respectively, functionally linking the observed SNPs and elevated MICs. The mutated CTX-M-14 isoform exhibited augmented ceftazidime hydrolytic activity, which was a reasonable cause for impaired susceptibility to avibactam inhibition. The P170S exchange in CTX-M-14 was found in association with elevated ceftazidime/avibactam MICs for independent K. pneumoniae isolates, but was not sufficient for full resistance. Apparently, additional CTX-M-independent mechanisms contribute to ceftazidime/avibactam resistance in K. pneumoniae DT12. Conclusions: This study on the molecular basis of ceftazidime/avibactam resistance in clinical K. pneumoniae emerging in vivo underscores the need for continuous monitoring of ceftazidime/avibactam susceptibility during therapy. Despite sustained inhibition of OXA-48, rapid development of CTX-M-14 isoforms exhibiting augmented ceftazidime hydrolytic activity may limit the usefulness of ceftazidime/avibactam monotherapies in infections caused by isolates carrying blaCTX-M-14 and blaOXA-48.

A multicrystal diffraction data-collection approach for studying structural dynamics with millisecond temporal resolution.

Many biochemical processes take place on timescales ranging from femto-seconds to seconds. Accordingly, any time-resolved experiment must be matched to the speed of the structural changes of interest. Therefore, the timescale of interest defines the requirements of the X-ray source, instrumentation and data-collection strategy. In this study, a minimalistic approach for in situ crystallization is presented that requires only a few microlitres of sample solution containing a few hundred crystals. It is demonstrated that complete diffraction data sets, merged from multiple crystals, can be recorded within only a few minutes of beamtime and allow high-resolution structural information of high quality to be obtained with a temporal resolution of 40 ms. Global and site-specific radiation damage can be avoided by limiting the maximal dose per crystal to 400 kGy. Moreover, analysis of the data collected at higher doses allows the time-resolved observation of site-specific radiation damage. Therefore, our approach is well suited to observe structural changes and possibly enzymatic reactions in the low-millisecond regime.

Immunosuppressive Yersinia Effector YopM Binds DEAD Box Helicase DDX3 to Control Ribosomal S6 Kinase in the Nucleus of Host Cells.

Yersinia outer protein M (YopM) is a crucial immunosuppressive effector of the plaque agent Yersinia pestis and other pathogenic Yersinia species. YopM enters the nucleus of host cells but neither the mechanisms governing its nucleocytoplasmic shuttling nor its intranuclear activities are known. Here we identify the DEAD-box helicase 3 (DDX3) as a novel interaction partner of Y. enterocolitica YopM and present the three-dimensional structure of a YopM:DDX3 complex. Knockdown of DDX3 or inhibition of the exportin chromosomal maintenance 1 (CRM1) increased the nuclear level of YopM suggesting that YopM exploits DDX3 to exit the nucleus via the CRM1 export pathway. Increased nuclear YopM levels caused enhanced phosphorylation of Ribosomal S6 Kinase 1 (RSK1) in the nucleus. In Y. enterocolitica infected primary human macrophages YopM increased the level of Interleukin-10 (IL-10) mRNA and this effect required interaction of YopM with RSK and was enhanced by blocking YopM's nuclear export. We propose that the DDX3/CRM1 mediated nucleocytoplasmic shuttling of YopM determines the extent of phosphorylation of RSK in the nucleus to control transcription of immunosuppressive cytokines.

Structure of ThiM from Vitamin B1 biosynthetic pathway of Staphylococcus aureus - Insights into a novel pro-drug approach addressing MRSA infections.

Infections caused by the methicillin-resistant Staphylococcus aureus (MRSA) are today known to be a substantial threat for global health. Emerging multi-drug resistant bacteria have created a substantial need to identify and discover new drug targets and to develop novel strategies to treat bacterial infections. A promising and so far untapped antibiotic target is the biosynthesis of vitamin B1 (thiamin). Thiamin in its activated form, thiamin pyrophosphate, is an essential co-factor for all organisms. Therefore, thiamin analogous compounds, when introduced into the vitamin B1 biosynthetic pathway and further converted into non-functional co-factors by the bacterium can function as pro-drugs which thus block various co-factor dependent pathways. We characterized one of the key enzymes within the S. aureus vitamin B1 biosynthetic pathway, 5-(hydroxyethyl)-4-methylthiazole kinase (SaThiM; EC 2.7.1.50), a potential target for pro-drug compounds and analyzed the native structure of SaThiM and complexes with the natural substrate 5-(hydroxyethyl)-4-methylthiazole (THZ) and two selected substrate analogues.

High resolution structures of Plasmodium falciparum GST complexes provide novel insights into the dimer-tetramer transition and a novel ligand-binding site.

Protection from oxidative stress and efficient redox regulation are essential for malarial parasites which have to grow and multiply rapidly in pro-oxidant rich environments. Therefore, redox active proteins currently belong to the most attractive antimalarial drug targets. The glutathione S-transferase from Plasmodium falciparum (PfGST) is a redox active protein displaying a peculiar dimer-tetramer transition that causes full enzyme-inactivation. This distinct structural feature is absent in mammalian GST isoenzyme counterparts. A flexible loop between residues 113-119 has been reported to be necessary for this tetramerization process. However, here we present structural data of a modified PfGST lacking loop 113-119 at 1.9 Å resolution. Our results clearly show that this loop is not essential for the formation of stable tetramers. Moreover we present for the first time the structures of both, the inactive and tetrameric state at 1.7 Å and the active dimeric state in complex with reduced glutathione at 2.4 Å resolution. Surprisingly, the structure of the inactive tetrameric state reveals a novel non-substrate binding-site occupied by a 2-(N-morpholino) ethane sulfonic acid (MES) molecule in each monomer. Although it is known that the PfGST has the ability to bind lipophilic anionic ligands, the location of the PfGST ligand-binding site remained unclear up to now.