Structure and dynamics of the staphylococcal pyridoxal 5-phosphate synthase complex reveal transient interactions at the enzyme interface.

Infectious diseases are a significant cause of death, and recent studies estimate that common bacterial infectious diseases were responsible for 13.6% of all global deaths in 2019. Among the most significant bacterial pathogens is Staphylococcus aureus, accounting for more than 1.1 million deaths worldwide in 2019. Vitamin biosynthesis has been proposed as a promising target for antibacterial therapy. Here, we investigated the biochemical, structural, and dynamic properties of the enzyme complex responsible for vitamin B6 (pyridoxal 5-phosphate, PLP) biosynthesis in S. aureus, which comprises enzymes SaPdx1 and SaPdx2. The crystal structure of the 24-mer complex of SaPdx1-SaPdx2 enzymes indicated that the S. aureus PLP synthase complex forms a highly dynamic assembly with transient interaction between the enzymes. Solution scattering data indicated that SaPdx2 typically binds to SaPdx1 at a substoichiometric ratio. We propose a structure-based view of the PLP synthesis mechanism initiated with the assembly of SaPLP synthase complex that proceeds in a highly dynamic interaction between Pdx1 and Pdx2. This interface interaction can be further explored as a potentially druggable site for the design of new antibiotics.

Energy landscape of the domain movement in Staphylococcus aureus UDP-N-acetylglucosamine 2-epimerase.

Staphylococcus aureus is an important cause of resistant healthcare-associated infections. It has been shown that the wall teichoic acid (WTA) may be an important drug target acting on antibiotic-resistant cells. The UDP-N-acetylglucosamine 2-epimerase, MnaA, is one of the first enzymes on the pathway for the biosynthesis of the WTA. Here, detailed molecular dynamics simulations of S. aureus MnaA were used to characterize the conformational changes that occur in the presence of UDP and UDP-GlcNac and also the energetic landscape associated with these changes. Using different simulation techniques, such as ABMD and GAMD, it was possible to assess the energetic profile for the protein with and without ligands in its active site. We found that there is a dynamic energy landscape that has its minimum changed by the presence of the ligands, with a closed structure of the enzyme being more frequently observed for the bound state while the unbound enzyme favors an opened conformation. Further structural analysis indicated that positively charged amino acids associated with UDP and UDP-GlcNac interactions play a major role in the enzyme opening movement. Finally, the energy landscape profiled in this work provides important conclusions for the design of inhibitor candidates targeting S. aureus MnaA.

Ligand- and receptor-based docking with LiBELa.

Methodologies on molecular docking are constantly improving. The problem consists on finding an optimal interplay between the computational cost and a satisfactory physical description of ligand-receptor interaction. In pursuit of an advance in current methods we developed a mixed docking approach combining ligand- and receptor-based strategies in a docking engine, where tridimensional descriptors for shape and charge distribution of a reference ligand guide the initial placement of the docking molecule and an interaction energy-based global minimization follows. This hybrid docking was evaluated with soft-core and force field potentials taking into account ligand pose and scoring. Our approach was found to be competitive to a purely receptor-based dock resulting in improved logAUC values when evaluated with DUD and DUD-E. Furthermore, the smoothed potential as evaluated here, was not advantageous when ligand binding poses were compared to experimentally determined conformations. In conclusion we show that a combination of ligand- and receptor-based strategy docking with a force field energy model results in good reproduction of binding poses and enrichment of active molecules against decoys. This strategy is implemented in our tool, LiBELa, available to the scientific community.

Structure-based identification of novel PPAR gamma ligands.

Peroxisome proliferator-activated receptor γ (PPARγ) is a nuclear receptor with an important role in the glucose metabolism and a target for type 2 diabetes mellitus therapy. The recent findings relating the use of the receptor full agonist rosiglitazone and the incidence of myocardial infarction raised concerns regarding whether receptor activation can actually be useful for diabetes management. The discovery of MRL-24 and GQ-16, ligands that can partially activate PPARγ and prevent weight gain and fluid retention, showed that a submaximal receptor activation can be a goal in the development of new ligands for PPARγ. Additionally, two previously described receptor antagonists, SR-202 and BADGE, were also shown to improve insulin sensitivity and decrease TNF-α level, revealing that receptor antagonism may also be an approach to pursue. Here, we used a structure-based approach to screen the subset 'Drugs-Now' of ZINC database. Fifteen ligands were selected after visual inspection and tested for their ability to bind to PPARγ. A benzoimidazol acetate, a bromobenzyl-thio-tetrazol benzoate and a [[2-[(1,3-dioxoinden-2-ylidene)methyl]phenoxy]methyl]benzoate were identified as PPARγ ligands, with IC50 values smaller than 10µM. Molecular dynamic simulations showed that the residues H323, H449, Y327, Y473, K367 and S289 are key structural elements for the molecular recognition of these ligands and the polar arm of PPARγ binding pocket.

2-({1-[2-(Methyl-sulfan-yl)phen-yl]-1H-tetra-zol-5-yl}sulfan-yl)acetic acid.

In the title compound, C10H10N4O2S2, the tetra-zole and benzene rings are almost normal to one another, with a dihedral angle between their planes of 84.33 (9)°. In the crystal, mol-ecules are linked via pairs of bifurcated O-H⋯(N,N) hydrogen bonds, forming inversion dimers with graph-set motif R 4 (4)(12). The dimers are linked by significant π-π inter-actions involving inversion-related tetra-zole rings and inversion-related benzene rings, with centroid-centroid distances of 3.7376 (14) and 3.8444 (15) Å, respectively.

4-{5-[(2-Bromo-benz-yl)sulfan-yl]-1H-tetra-zol-1-yl}benzoic acid.

In the title compound, C15H11BrN4O2S, the tetra-zole ring makes dihedral angles of 45.97 (10) and 75.41 (1)°, respectively, with the benzoyl and bromo-benzene rings while the dihedral angle between the benzene rings is 73.77 (1)°. In the crystal, mol-ecules are linked through O-H⋯ N and C-H⋯ O hydrogen bonds, giving infinite chains in both the [110] and [1-10] directions. These chains are further connected by C-Br⋯π and C-O⋯π inter-actions and also by π-π stacking between tetra-zole rings [centroid-centroid distance = 3.312 (1) Å], generating a three-dimensional network.

Ligand binding free energy evaluation by Monte Carlo Recursion.

The correct evaluation of ligand binding free energies by computational methods is still a very challenging active area of research. The most employed methods for these calculations can be roughly classified into four groups: (i) the fastest and less accurate methods, such as molecular docking, designed to sample a large number of molecules and rapidly rank them according to the potential binding energy; (ii) the second class of methods use a thermodynamic ensemble, typically generated by molecular dynamics, to analyze the endpoints of the thermodynamic cycle for binding and extract differences, in the so-called 'end-point' methods; (iii) the third class of methods is based on the Zwanzig relationship and computes the free energy difference after a chemical change of the system (alchemical methods); and (iv) methods based on biased simulations, such as metadynamics, for example. These methods require increased computational power and as expected, result in increased accuracy for the determination of the strength of binding. Here, we describe an intermediate approach, based on the Monte Carlo Recursion (MCR) method first developed by Harold Scheraga. In this method, the system is sampled at increasing effective temperatures, and the free energy of the system is assessed from a series of terms W(b,T), computed from Monte Carlo (MC) averages at each iteration. We show the application of the MCR for ligand binding with datasets of guest-hosts systems (N = 75) and we observed that a good correlation is obtained between experimental data and the binding energies computed with MCR. We also compared the experimental data with an end-point calculation from equilibrium Monte Carlo calculations that allowed us to conclude that the lower-energy (lower-temperature) terms in the calculation are the most relevant to the estimation of the binding energies, resulting in similar correlations between MCR and MC data and the experimental values. On the other hand, the MCR method provides a reasonable view of the binding energy funnel, with possible connections with the ligand binding kinetics, as well. The codes developed for this analysis are publicly available on GitHub as a part of the LiBELa/MCLiBELa project (https://github.com/alessandronascimento/LiBELa).

Conservation and divergence of the G-interfaces of Drosophila melanogaster septins.

Septins possess a conserved guanine nucleotide-binding (G) domain that participates in the stabilization of organized hetero-oligomeric complexes which assemble into filaments, rings and network-like structures. The fruit fly, Drosophila melanogaster, has five such septin genes encoding Sep1, Sep2, Sep4, Sep5 and Pnut. Here, we report the crystal structure of the heterodimer formed between the G-domains of Sep1 and Sep2, the first from an insect to be described to date. A G-interface stabilizes the dimer (in agreement with the expected arrangement for the Drosophila hexameric particle) and this bears significant resemblance to its human counterparts, even down to the level of individual amino acid interactions. On the other hand, a model for the G-interface formed between the two copies of Pnut which occupy the centre of the hexamer, shows important structural differences, including the loss of a highly favourable bifurcated salt-bridge network. Whereas wild-type Pnut purifies as a monomer, the reintroduction of the salt-bridge network results in stabilizing the dimeric interface in solution as shown by size exclusion chromatography and thermal stability measurements. Adaptive steered molecular dynamics reveals an unzipping mechanism for dimer dissociation which initiates at a point of electrostatic repulsion within the switch II region. Overall, the data contribute to a better understanding of the molecular interactions involved in septin assembly/disassembly.

Gaining ligand selectivity in thyroid hormone receptors via entropy.

Nuclear receptors are important targets for pharmaceuticals, but similarities between family members cause difficulties in obtaining highly selective compounds. Synthetic ligands that are selective for thyroid hormone (TH) receptor beta (TRbeta) vs. TRalpha reduce cholesterol and fat without effects on heart rate; thus, it is important to understand TRbeta-selective binding. Binding of 3 selective ligands (GC-1, KB141, and GC-24) is characterized at the atomic level; preferential binding depends on a nonconserved residue (Asn-331beta) in the TRbeta ligand-binding cavity (LBC), and GC-24 gains extra selectivity from insertion of a bulky side group into an extension of the LBC that only opens up with this ligand. Here we report that the natural TH 3,5,3'-triodothyroacetic acid (Triac) exhibits a previously unrecognized mechanism of TRbeta selectivity. TR x-ray structures reveal better fit of ligand with the TRalpha LBC. The TRbeta LBC, however, expands relative to TRalpha in the presence of Triac (549 A(3) vs. 461 A(3)), and molecular dynamics simulations reveal that water occupies the extra space. Increased solvation compensates for weaker interactions of ligand with TRbeta and permits greater flexibility of the Triac carboxylate group in TRbeta than in TRalpha. We propose that this effect results in lower entropic restraint and decreases free energy of interactions between Triac and TRbeta, explaining subtype-selective binding. Similar effects could potentially be exploited in nuclear receptor drug design.

The ß-lactam ticarcillin is a Staphylococcus aureus UDP-N-acetylglucosamine 2-epimerase binder.

Infectious diseases account for 25% of the causes of death worldwide and this rate is expected to increase due to antibiotic resistance. Among the bacteria associated with healthcare infections, Staphylococcus aureus is a prevalent pathogen and about 50% of the isolates are found to be methicillin-resistant. Here we describe the identification of ticarcillin as a weak binder of the S. aureus UDP-N-acetylglucosamine 2-epimerase. After a docking screening, ticarcillin was identified as a ligand in using the recently proposed isothermal analysis of differential scanning fluorimetry data. Finally, an equilibrium MD simulation confirmed the docking binding mode as a stable pose, with large contributions to the binding energy coming from interactions between Arg206 and Arg207 and the carboxylate groups in ticarcillin.

Modulation of beta-amyloid aggregation using ascorbic acid.

Studies have shown that the level of ascorbic acid (AA) is reduced in the brain of Alzheimer's disease (AD) patients. However, its effect on amyloid-ß 1-42 (Aß42) aggregation has not yet been elucidated. Here we investigated for the first time the effect of AA on Aß42 aggregation using fluorescence assay, circular dichroism, atomic force microscopy, isothermal titration calorimetry, ligand docking, and molecular dynamics. Our results showed that the fibril content decreases in the growth phase when the peptides are co-incubated with AA. AA molecules bind to Aß42 peptides with high binding affinity and a binding site for AA between the ß-strands of Aß42 oligomers prevents the stack of adjacent strands. We demonstrate the inhibitory effect of AA on the aggregation of Aß42 and its molecular interactions, which can contribute to the development of an accessible therapy for AD and also to the design of novel drugs for other amyloidogenic diseases.

A scalable screening of E. coli strains for recombinant protein expression.

Structural biology projects are highly dependent on the large-scale expression of soluble protein and, for this purpose, heterologous expression using bacteria or yeast as host systems is usually employed. In this scenario, some of the parameters to be optimized include (i) those related to the protein construct, such as the use of a fusion protein, the choice of an N-terminus fusion/tag or a C-terminus fusion/tag; (ii) those related to the expression stage, such as the concentration and selection of inducer agent and temperature expression and (iii) the choice of the host system, which includes the selection of a prokaryotic or eukaryotic cell and the adoption of a strain. The optimization of some of the parameters related to protein expression, stage (ii), is straightforward. On the other hand, the determination of the most suitable parameters related to protein construction requires a new cycle of gene cloning, while the optimization of the host cell is less straightforward. Here, we evaluated a scalable approach for the screening of host cells for protein expression in a structural biology pipeline. We evaluated four Escherichia coli strains looking for the best yield of soluble heterologous protein expression using the same strategy for protein construction and gene cloning and comparing it to our standard strain, Rosetta 2 (DE3). Using a liquid handling device (robot), E. coli pT-GroE, Lemo21(DE3), Arctic Express (DE3), and Rosetta Gami 2 (DE3) strains were screened for the maximal yield of soluble heterologous protein recovery. For the genes used in this experiment, the Arctic Express (DE3) strain resulted in better yields of soluble heterologous proteins. We propose that screening of host cell/strain is feasible, even for smaller laboratories and the experiment as proposed can easily be scalable to a high-throughput approach.

Tetrazoles as PPARγ ligands: A structural and computational investigation.

Diabetes is an important chronic disease affecting about 10% of the adult population in the US and over 420 million people worldwide, resulting in 1.6 million deaths every year, according to the World Health Organization. The most common type of the disease, type 2 diabetes, can be pharmacologically managed using oral hypoglycemic agents or thiazolidinediones (TZDs), such as pioglitazone, which act by activating the Peroxisome Proliferated-Activated Receptor γ. Despite their beneficial effects in diabetes treatment, TZDs like rosiglitazone and troglitazone were withdrawn due to safety reasons, creating a void in the pharmacological options for the treatment of this important disease. Here, we explored a structure-based approach in the screening for new chemical probes for a deeper investigation of the effects of PPARγ activation. A class of tetrazole compounds was identified and the compounds named T1, T2 and T3 were purchased and evaluated for their ability to interact with the PPARγ ligand binding domain (LBD). The compounds were binders with micromolar range affinity, as determined by their IC50 values. A Monte Carlo simulation of the compound T2 revealed that the tetrazole ring makes favorable interaction with the polar arm of the receptor binding pocket. Finally, the crystal structure of the PPARγ-LBD-T2 complex was solved at 2.3 Å, confirming the binding mode for this compound. The structure also revealed that, when the helix H12 is mispositioned, an alternative binding conformation is observed for the ligand suggesting an H12-dependent binding conformation for the tetrazole compound.

Structural and molecular dynamics investigations of ligand stabilization via secondary binding site interactions in Paenibacillus xylanivorans GH11 xylanase.

Glycoside hydrolases (GHs) are essential for plant biomass deconstruction. GH11 family consist of endo-ß-1,4-xylanases which hydrolyze xylan, the second most abundant cell wall biopolymer after cellulose, into small bioavailable oligomers. Structural requirements for enzymatic mechanism of xylan hydrolysis is well described for GH11 members. However, over the last years, it has been discovered that some enzymes from GH11 family have a secondary binding sites (SBS), which modulate the enzymes activities, but mechanistic details of the molecular communication between the active site and SBS of the enzymes remain a conundrum. In the present work we structurally characterized GH11 xylanase from Paenibacillus xylanivorans A57 (PxXyn11B), a microorganism of agricultural importance, using protein crystallography and molecular dynamics simulations. The PxXyn11B structure was solved to 2.5 Å resolution and different substrates (xylo-oligosaccharides from X3 to X6), were modelled in its active and SBS sites. Molecular Dynamics (MD) simulations revealed an important role of SBS in the activity and conformational mobility of PxXyn11B, demonstrating that binding of the reaction products to the SBS of the enzyme stabilizes the N-terminal region and, consequently, the active site. Furthermore, MD simulations showed that the longer the ligand, the better is the stabilization within active site, and the positive subsites contribute less to the stabilization of the substrates than the negative ones. These findings provide rationale for the observed enzyme kinetics, shedding light on the conformational modulation of the GH11 enzymes via their SBS mediated by the positive molecular feedback loop which involve the products of the enzymatic reaction.

Structural Dynamics and Perspectives of Vitamin B6 Biosynthesis Enzymes in Plasmodium: Advances and Open Questions.

Malaria is still today one of the most concerning diseases, with 219 million infections in 2019, most of them in Sub-Saharan Africa and Latin America, causing approx. 409,000 deaths per year. Despite the tremendous advances in malaria treatment and prevention, there is still no vaccine for this disease yet available and the increasing parasite resistance to already existing drugs is becoming an alarming issue globally. In this context, several potential targets for the development of new drug candidates have been proposed and, among those, the de novo biosynthesis pathway for the B6 vitamin was identified to be a promising candidate. The reason behind its significance is the absence of the pathway in humans and its essential presence in the metabolism of major pathogenic organisms. The pathway consists of two enzymes i.e. Pdx1 (PLP synthase domain) and Pdx2 (glutaminase domain), the last constituting a transient and dynamic complex with Pdx1 as the prime player and harboring the catalytic center. In this review, we discuss the structural biology of Pdx1 and Pdx2, together with and the understanding of the PLP biosynthesis provided by the crystallographic data. We also highlight the existing evidence of the effect of PLP synthesis inhibition on parasite proliferation. The existing data provide a flourishing environment for the structure-based design and optimization of new substrate analogs that could serve as inhibitors or even suicide inhibitors.

Crystal structure and statistical coupling analysis of highly glycosylated peroxidase from royal palm tree (Roystonea regia).

Royal palm tree peroxidase (RPTP) is a very stable enzyme in regards to acidity, temperature, H(2)O(2), and organic solvents. Thus, RPTP is a promising candidate for developing H(2)O(2)-sensitive biosensors for diverse applications in industry and analytical chemistry. RPTP belongs to the family of class III secretory plant peroxidases, which include horseradish peroxidase isozyme C, soybean and peanut peroxidases. Here we report the X-ray structure of native RPTP isolated from royal palm tree (Roystonea regia) refined to a resolution of 1.85A. RPTP has the same overall folding pattern of the plant peroxidase superfamily, and it contains one heme group and two calcium-binding sites in similar locations. The three-dimensional structure of RPTP was solved for a hydroperoxide complex state, and it revealed a bound 2-(N-morpholino) ethanesulfonic acid molecule (MES) positioned at a putative substrate-binding secondary site. Nine N-glycosylation sites are clearly defined in the RPTP electron-density maps, revealing for the first time conformations of the glycan chains of this highly glycosylated enzyme. Furthermore, statistical coupling analysis (SCA) of the plant peroxidase superfamily was performed. This sequence-based method identified a set of evolutionarily conserved sites that mapped to regions surrounding the heme prosthetic group. The SCA matrix also predicted a set of energetically coupled residues that are involved in the maintenance of the structural folding of plant peroxidases. The combination of crystallographic data and SCA analysis provides information about the key structural elements that could contribute to explaining the unique stability of RPTP.

Essential Metabolic Routes as a Way to ESKAPE From Antibiotic Resistance.

Antibiotic resistance is a worldwide concern that requires a concerted action from physicians, patients, governmental agencies, and academia to prevent infections and the spread of resistance, track resistant bacteria, improve the use of current antibiotics, and develop new antibiotics. Despite the efforts spent so far, the current antibiotics in the market are restricted to only five general targets/pathways highlighting the need for basic research focusing on the discovery and evaluation of new potential targets. Here we interrogate two biosynthetic pathways as potentially druggable pathways in bacteria. The biosynthesis pathway for thiamine (vitamin B1), absent in humans, but found in many bacteria, including organisms in the group of the ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumanii, Pseudomonas aeruginosa, and Enterobacter sp.) and the biosynthesis pathway for pyridoxal 5'-phosphate and its vitamers (vitamin B6), found in S. aureus. Using current genomic data, we discuss the possibilities of inhibition of enzymes in the pathway and review the current state of the art in the scientific literature.

Comparative Analysis of Electrostatic Models for Ligand Docking.

The precise modeling of molecular interactions remains an important goal among molecular modeling techniques. Some of the challenges in the field include the precise definition of a Hamiltonian for biomolecular systems, together with precise parameters derived from Molecular Mechanics Force Fields, for example. The problem is even more challenging when interaction energies from different species are computed, such as the interaction energy involving a ligand and a protein, given that small differences must be computed from large energies. Here we evaluated the effects of the electrostatic model for ligand binding energy evaluation in the context of ligand docking. For this purpose, a classical Coulomb potential with distance-dependent dielectrics was compared with a Poisson-Boltzmann (PB) model for electrostatic potential computation, based on DelPhi calculations. We found that, although the electrostatic energies were highly correlated for the Coulomb and PB models, the ligand pose and the enrichment of actual ligands against decoy compounds, were improved when binding energies were computed using PB as compared to the Coulomb model. We observed that the electrostatic energies computed with the Coulomb model were, on average, ten times larger than the energies computed with the PB model, suggesting a strong overestimation of the polar interactions in the Coulomb model. We also found that a slightly smoothed Lennard-Jones potential combined with the PB model resulted in a good compromise between ligand sampling and energetic scoring.

Structure and dynamics of Trichoderma harzianum Cel7B suggest molecular architecture adaptations required for a wide spectrum of activities on plant cell wall polysaccharides.

Cellulases from glycoside hydrolase family 7 (GH7) play crucial roles in plant lignocellulose deconstruction by fungi, but structural information available for GH7 fungal endoglucanases is limited when compared to the number of known sequences in the family. Here, we report the X-ray structure of the glycosylated catalytic domain (CD) of Trichoderma harzianum endoglucanase, ThCel7B, solved and refined at 2.9 Šresolution. Additionally, our extensive molecular dynamics simulations of this enzyme in complex with a variety of oligosaccharides provide a better understanding of its promiscuous hydrolytic activities on plant cell wall polysaccharides. The simulations demonstrate the importance of the hydrogen bond between substrate O2 hydroxyl in the subsite -1 and a side chain of catalytic Glu196 which renders ThCel7B capable to catalytically cleave cello and xylooligosaccharides, but not mannooligosaccharides. Moreover, detailed structural analyses and MD simulations revealed an additional binding pocket, suitable for accommodation of oligosaccharide decorations and/or substrates with mixed glycoside bonds that abuts onto the binding cleft close to subsite +2.

Interleukin-22 forms dimers that are recognized by two interleukin-22R1 receptor chains.

Interleukin-22 (IL-22) is a class 2 cytokine whose primary structure is similar to that of interleukin 10 (IL-10) and interferon-gamma (IFN-gamma). IL-22 induction during acute phase immune response indicates its involvement in mechanisms of inflammation. Structurally different from IL-10 and a number of other members of IL-10 family, which form intertwined inseparable V-shaped dimers of two identical polypeptide chains, a single polypeptide chain of IL-22 folds on itself in a relatively globular structure. Here we present evidence, based on native gel electrophoresis, glutaraldehyde cross-linking, dynamic light scattering, and small angle x-ray scattering experiments, that human IL-22 forms dimers and tetramers in solution under protein concentrations assessable by these experiments. Unexpectedly, low-resolution molecular shape of IL-22 dimers is strikingly similar to that of IL-10 and other intertwined cytokine dimeric forms. Furthermore, we determine an ab initio molecular shape of the IL-22/IL-22R1 complex which reveals the V-shaped IL-22 dimer interacting with two cognate IL-22R1 molecules. Based on this collective evidence, we argue that dimerization might be a common mechanism of all class 2 cytokines for the molecular recognition with their respective membrane receptor. We also speculate that the IL-22 tetramer formation could represent a way to store the cytokine in nonactive form at high concentrations that could be readily converted into functionally active monomers and dimers upon interaction with the cognate cellular receptors.