Structural insights into terminal arabinosylation biosynthesis of the mycobacterial cell wall arabinan.

The emergence of drug-resistant strains exacerbates the global challenge of tuberculosis caused by Mycobacterium tuberculosis (Mtb). Central to the pathogenicity of Mtb is its complex cell envelope, which serves as a barrier against both immune system and pharmacological attacks. Two key components of this envelope, arabinogalactan (AG) and lipoarabinomannan (LAM) are complex polysaccharides that contain integral arabinan domains important for cell wall structural and functional integrity. The arabinofuranosyltransferase AftB terminates the synthesis of these arabinan domains by catalyzing the addition of the addition of ß-(1→2)-linked terminal arabinofuranose residues. Here, we present the cryo-EM structures of Mycobacterium chubuense AftB in its apo and donor substrate analog-bound form, determined to 2.9 Å and 3.4 Å resolution, respectively. Our structures reveal that AftB has a GT-C fold transmembrane (TM) domain comprised of eleven TM helices and a periplasmic cap domain. AftB has an irregular tube-shaped cavity that bridges the two proposed substrate binding sites. By integrating structural analysis, biochemical assays, and molecular dynamics simulations, we elucidate the molecular basis of the reaction mechanism of AftB and propose a model for catalysis.

Structure and function of Campylobacter jejuni polynucleotide phosphorylase (PNPase): Insights into the role of this RNase in pathogenicity.

Ribonucleases are in charge of the processing, degradation and quality control of all cellular transcripts, which makes them crucial factors in RNA regulation. This post-transcriptional regulation allows bacteria to promptly react to different stress conditions and growth phase transitions, and also to produce the required virulence factors in pathogenic bacteria. Campylobacter jejuni is the main responsible for human gastroenteritis in the world. In this foodborne pathogen, exoribonuclease PNPase (CjPNP) is essential for low-temperature cell survival, affects the synthesis of proteins involved in virulence and has an important role in swimming, cell adhesion/invasion ability, and chick colonization. Here we report the crystallographic structure of CjPNP, complemented with SAXS, which confirms the characteristic doughnut-shaped trimeric arrangement and evaluates domain arrangement and flexibility. Mutations in highly conserved residues were constructed to access their role in RNA degradation and polymerization. Surprisingly, we found two mutations that altered CjPNP into a protein that is only capable of degrading RNA even in conditions that favour polymerization. These findings will be important to develop new strategies to combat C. jejuni infections.

Studies on the Interaction between Model Proteins and Fluorinated Ionic Liquids.

Proteins are inherently unstable, which limits their use as therapeutic agents. However, the use of biocompatible cosolvents or surfactants can help to circumvent this problem through the stabilization of intramolecular and solvent-mediated interactions. Ionic liquids (ILs) have been known to act as cosolvents or surface-active compounds. In the presence of proteins, ILs can have a beneficial effect on their refolding, shelf life, stability, and enzymatic activities. In the work described herein, we used small-angle X-ray scattering (SAXS) to monitor the aggregation of different concentrations of ILs with protein models, lysozyme (Lys) and bovine serum albumin (BSA), and fluorescence microscopy to assess micelle formation of fluorinated ILs (FILs) with Lys. Furthermore, coarse-grained molecular dynamics (CG-MD) simulations provided a better understanding of Lys-FIL interactions. The results showed that the proteins maintain their globular structures in the presence of FILs, with signs of partial unfolding for Lys and compaction for BSA with increased flexibility at higher FIL concentrations. Lys was encapsulated by FIL, thus reinforcing the potential of ILs to be used in the formulation of protein-based pharmaceuticals.

Production, homology modeling and mutagenesis studies on GlcH glucose transporter from Prochlorococcus sp. strain SS120.

The marine cyanobacterium Prochlorococcus is one of the main primary producers on Earth, which can take up glucose by using the high affinity, multiphasic transporter GlcH. We report here the overexpression of glcH from Prochlorococcus marinus strain SS120 in Escherichia coli. Modeling studies of GlcH using the homologous MelB melibiose transporter from Salmonella enterica serovar Typhimurium showed high conservation at the overall fold. We observed that an important structural interaction, mediated by a strong hydrogen bond between D8 and R141, is conserved in Prochlorococcus, although the corresponding amino acids in MelB from Salmonella are different. Biased docking studies suggested that when glucose reaches the pocket of the transporter and interacts with D8 and R141, the hydrogen bond network in which these residues are involved could be disrupted, favoring a conformational change with the subsequent translocation of the glucose molecule towards the cytoplasmic region of the pmGlcH structure. Based on these theoretical predictions and on the conservation of N117 and W348 in other MelB structures, D8, N117, R141 and W348 were mutated to glycine residues. Their key role in glucose transport was evaluated by glucose uptake assays. N117G and W348G mutations led to 17 % decrease in glucose uptake, while D8G and R141G decreased the glucose transport by 66 % and 92 % respectively. Overall, our studies provide insights into the Prochlorococcus 3D-structure of GlcH, paving the way for further analysis to understand the features which are involved in the high affinity and multiphasic kinetics of this transporter.

Impact of Fluorinated Ionic Liquids on Human Phenylalanine Hydroxylase-A Potential Drug Delivery System.

Phenylketonuria (PKU) is an autosomal recessive disease caused by deficient activity of human phenylalanine hydroxylase (hPAH), which can lead to neurologic impairments in untreated patients. Although some therapies are already available for PKU, these are not without drawbacks. Enzyme-replacement therapy through the delivery of functional hPAH could be a promising strategy. In this work, biophysical methods were used to evaluate the potential of [N1112(OH)][C4F9SO3], a biocompatible fluorinated ionic liquid (FIL), as a delivery system of hPAH. The results herein presented show that [N1112(OH)][C4F9SO3] spontaneously forms micelles in a solution that can encapsulate hPAH. This FIL has no significant effect on the secondary structure of hPAH and is able to increase its enzymatic activity, despite the negative impact on protein thermostability. The influence of [N1112(OH)][C4F9SO3] on the complex oligomerization equilibrium of hPAH was also assessed.

Secretory IgA and T cells targeting SARS-CoV-2 spike protein are transferred to the breastmilk upon mRNA vaccination.

In view of the scarcity of data to guide decision making, we evaluated how BNT162b2 and mRNA-1273 vaccines affect the immune response in lactating women and the protective profile of breastmilk. Compared with controls, lactating women had a higher frequency of circulating RBD memory B cells and higher anti-RBD antibody titers but similar neutralizing capacity. We show that upon vaccination, immune transfer to breastmilk occurs through a combination of anti-spike secretory IgA (SIgA) antibodies and spike-reactive T cells. Although we found that the concentration of anti-spike IgA in breastmilk might not be sufficient to directly neutralize SARS-CoV-2, our data suggest that cumulative transfer of IgA might provide the infant with effective neutralization capacity. Our findings put forward the possibility that breastmilk might convey both immediate (through anti-spike SIgA) and long-lived (via spike-reactive T cells) immune protection to the infant. Further studies are needed to address this possibility and to determine the functional profile of spike T cells.

Crystal Structure of Mannose Specific IIA Subunit of Phosphotransferase System from Streptococcus pneumoniae.

Streptococcus pneumoniae is a frequent bacterial pathogen of the human respiratory tract causing pneumonia, meningitis and sepsis, a serious healthcare burden in all age groups. S. pneumoniae lacks complete respiratory chain and relies on carbohydrate fermentation for energy generation. One of the essential components for this includes the mannose phosphotransferase system (Man-PTS), which plays a central role in glucose transport and exhibits a broad specificity for a range of hexoses. Importantly, Man-PTS is involved in the global regulation of gene expression for virulence determinants. We herein report the three-dimensional structure of the EIIA domain of S. pneumoniae mannose phosphotransferase system (SpEIIA-Man). Our structure shows a dimeric arrangement of EIIA and reveals a detailed molecular description of the active site. Since PTS transporters are exclusively present in microbes and sugar transporters have already been suggested as valid targets for antistreptococcal antibiotics, our work sets foundation for the future development of antimicrobial strategies against Streptococcus pneumoniae.

Cryo-EM structure of arabinosyltransferase EmbB from Mycobacterium smegmatis.

Arabinosyltransferase B (EmbB) belongs to a family of membrane-bound glycosyltransferases that build the lipidated polysaccharides of the mycobacterial cell envelope, and are targets of anti-tuberculosis drug ethambutol. We present the 3.3 Å resolution single-particle cryo-electron microscopy structure of Mycobacterium smegmatis EmbB, providing insights on substrate binding and reaction mechanism. Mutations that confer ethambutol resistance map mostly around the putative active site, suggesting this to be the location of drug binding.

Cryo-EM Structures and Regulation of Arabinofuranosyltransferase AftD from Mycobacteria.

Mycobacterium tuberculosis causes tuberculosis, a disease that kills over 1 million people each year. Its cell envelope is a common antibiotic target and has a unique structure due, in part, to two lipidated polysaccharides-arabinogalactan and lipoarabinomannan. Arabinofuranosyltransferase D (AftD) is an essential enzyme involved in assembling these glycolipids. We present the 2.9-Å resolution structure of M. abscessus AftD, determined by single-particle cryo-electron microscopy. AftD has a conserved GT-C glycosyltransferase fold and three carbohydrate-binding modules. Glycan array analysis shows that AftD binds complex arabinose glycans. Additionally, AftD is non-covalently complexed with an acyl carrier protein (ACP). 3.4- and 3.5-Å structures of a mutant with impaired ACP binding reveal a conformational change, suggesting that ACP may regulate AftD function. Mutagenesis experiments using a conditional knockout constructed in M. smegmatis confirm the essentiality of the putative active site and the ACP binding for AftD function.

3-Oxo-ß-sultam as a Sulfonylating Chemotype for Inhibition of Serine Hydrolases and Activity-Based Protein Profiling.

3-Oxo-ß-sultams are four-membered ring ambident electrophiles that can react with nucleophiles either at the carbonyl carbon or at the sulfonyl sulfur atoms, and that have been reported to inhibit serine hydrolases via acylation of the active-site serine residue. We have developed a panel of 3-oxo-ß-sultam inhibitors and show, through crystallographic data, that they are regioselective sulfonylating electrophiles, covalently binding to the catalytic serine of human and porcine elastases through the sulfur atom. Application of 3-oxo-ß-sultam-derived activity-based probes in a human proteome revealed their potential to label disease-related serine hydrolases and proteasome subunits. Activity-based protein profiling applications of 3-oxo-ß-sultams should open up new opportunities to investigate these classes of enzymes in complex proteomes and expand the toolbox of available sulfur-based covalent protein modifiers in chemical biology.

Syntheses of the plant auxin conjugate 2-O-(indole-3-acetyl)-myo-inositol IAInos.

The plant hormone conjugate 2-O-(indole-3-acetyl)-myo-inositol (IAInos) has been selectively prepared for the first time by two routes from myo-inositol. One of the syntheses depended upon the construction of the 3-indoleacetyl group by a Fischer indole synthesis on an unreactive axial hydroxyl group, while the other via a direct acylation of the equatorially orientated hydroxy group created by conformational constraint of the cyclohexane ring. The latter synthesis produced IAInos in 5 steps and 29% overall yield.

The key role of glutamate 172 in the mechanism of type II NADH:quinone oxidoreductase of Staphylococcus aureus.

Type II NADH:quinone oxidoreductases (NDH-2s) are membrane bound enzymes that deliver electrons to the respiratory chain by oxidation of NADH and reduction of quinones. In this way, these enzymes also contribute to the regeneration of NAD+, allowing several metabolic pathways to proceed. As for the other members of the two-Dinucleotide Binding Domains Flavoprotein (tDBDF) superfamily, the enzymatic mechanism of NDH-2s is still little explored and elusive. In this work we addressed the role of the conserved glutamate 172 (E172) residue in the enzymatic mechanism of NDH-2 from Staphylococcus aureus. We aimed to test our earlier hypothesis that E172 plays a key role in proton transfer to allow the protonation of the quinone. For this we performed a complete biochemical characterization of the enzyme's variants E172A, E172Q and E172S. Our steady state kinetic measurements show a clear decrease in the overall reaction rate, and our substrate interaction studies indicate the binding of the two substrates is also affected by these mutations. Interestingly our fast kinetic results show quinone reduction is more affected than NADH oxidation. We have also determined the X-ray crystal structure of the E172S mutant (2.55Ǻ) and compared it with the structure of the wild type (2.32Ǻ). Together these results support our hypothesis for E172 being of central importance in the catalytic mechanism of NDH-2, which may be extended to other members of the tDBDF superfamily.

Fluorinated ionic liquids for protein drug delivery systems: Investigating their impact on the structure and function of lysozyme.

Since the approval of recombinant human insulin by FDA in 1982, more than 200 proteins are currently available for pharmaceutical use to treat a wide range of diseases. However, innovation is still required to develop effective approaches for drug delivery. Our aim is to investigate the potential use of fluorinated ionic liquids (FILs) as drug delivery systems (DDS) for therapeutic proteins. Some initial parameters need to be assessed before further studies can proceed. This work evaluates the impact of FILs on the stability, function, structure and aggregation state of lysozyme. Different techniques were used for this purpose, which included differential scanning fluorimetry (DSF), spectrophotometric assays, circular dichroism (CD), dynamic light scattering (DLS), and scanning and transmission electron microscopy (SEM/TEM). Ionic liquids composed of cholinium-, imidazolium- or pyridinium- derivatives were combined with different anions and analysed at different concentrations in aqueous solutions (below and above the critical aggregation concentration, CAC). The results herein presented show that the addition of ionic liquids had no significant effect on the stability and hydrolytic activity of lysozyme. Moreover, a distinct behaviour was observed in DLS experiments for non-surfactant and surfactant ionic liquids, with the latter encapsulating the protein at concentrations above the CAC. These results encourage us to further study ionic liquids as promising tools for DDS of protein drugs.

Electron Accepting Units of the Diheme Cytochrome c TsdA, a Bifunctional Thiosulfate Dehydrogenase/Tetrathionate Reductase.

The enzymes of the thiosulfate dehydrogenase (TsdA) family are wide-spread diheme c-type cytochromes. Here, redox carriers were studied mediating the flow of electrons arising from thiosulfate oxidation into respiratory or photosynthetic electron chains. In a number of organisms, including Thiomonas intermedia and Sideroxydans lithotrophicus, the tsdA gene is immediately preceded by tsdB encoding for another diheme cytochrome. Spectrophotometric experiments in combination with enzymatic assays in solution showed that TsdB acts as an effective electron acceptor of TsdA in vitro when TsdA and TsdB originate from the same source organism. Although TsdA covers a range from -300 to +150 mV, TsdB is redox active between -100 and +300 mV, thus enabling electron transfer between these hemoproteins. The three-dimensional structure of the TsdB-TsdA fusion protein from the purple sulfur bacterium Marichromatium purpuratum was solved by X-ray crystallography to 2.75 Å resolution providing insights into internal electron transfer. In the oxidized state, this tetraheme cytochrome c contains three hemes with axial His/Met ligation, whereas heme 3 exhibits the His/Cys coordination typical for TsdA active sites. Interestingly, thiosulfate is covalently bound to Cys330 on heme 3. In several bacteria, including Allochromatium vinosum, TsdB is not present, precluding a general and essential role for electron flow. Both AvTsdA and the MpTsdBA fusion react efficiently in vitro with high potential iron-sulfur protein from A. vinosum (Em +350 mV). High potential iron-sulfur protein not only acts as direct electron donor to the reaction center in anoxygenic phototrophs but can also be involved in aerobic respiratory chains.

Clickable 4-Oxo-ß-lactam-Based Selective Probing for Human Neutrophil Elastase Related Proteomes.

Human neutrophil elastase (HNE) is a serine protease associated with several inflammatory processes such as chronic obstructive pulmonary disease (COPD). The precise involvement of HNE in COPD and other inflammatory disease mechanisms has yet to be clarified. Herein we report a copper-catalyzed alkyne-azide 1,3-dipolar cycloaddition (CuAAC, or 'click' chemistry) approach based on the 4-oxo-ß-lactam warhead that yielded potent HNE inhibitors containing a triazole moiety. The resulting structure-activity relationships set the basis to develop fluorescent and biotinylated activity-based probes as tools for molecular functional analysis. Attaching the tags to the 4-oxo-ß-lactam scaffold did not affect HNE inhibitory activity, as revealed by the IC50 values in the nanomolar range (56-118 nm) displayed by the probes. The nitrobenzoxadiazole (NBD)-based probe presented the best binding properties (ligand efficiency (LE)=0.31) combined with an excellent lipophilic ligand efficiency (LLE=4.7). Moreover, the probes showed adequate fluorescence properties, internalization in human neutrophils, and suitable detection of HNE in the presence of a large excess of cell lysate proteins. This allows the development of activity-based probes with promising applications in target validation and identification, as well as diagnostic tools.

Stabilization of porcine pancreatic elastase crystals by glutaraldehyde cross-linking.

Elastase is a serine protease from the chymotrypsin family of enzymes with the ability to degrade elastin, an important component of connective tissues. Excessive elastin proteolysis leads to a number of pathological diseases. Porcine pancreatic elastase (PPE) is often used for drug development as a model for human leukocyte elastase (HLE), with which it shares high sequence identity. Crystals of PPE were grown overnight using sodium sulfate and sodium acetate at acidic pH. Cross-linking the crystals with glutaraldehyde was needed to resist the soaking procedure with a diethyl N-(methyl)pyridinyl-substituted oxo-ß-lactam inhibitor. Crystals of PPE bound to the inhibitor belonged to the orthorhombic space group P212121, with unit-cell parameters a = 51.0, b = 58.3, c = 74.9 Å, and diffracted to 1.8 Šresolution using an in-house X-ray source.

Structural and Functional Characterization of an Ancient Bacterial Transglutaminase Sheds Light on the Minimal Requirements for Protein Cross-Linking.

Transglutaminases are best known for their ability to catalyze protein cross-linking reactions that impart chemical and physical resilience to cellular structures. Here, we report the crystal structure and characterization of Tgl, a transglutaminase from the bacterium Bacillus subtilis. Tgl is produced during sporulation and cross-links the surface of the highly resilient spore. Tgl-like proteins are found only in spore-forming bacteria of the Bacillus and Clostridia classes, indicating an ancient origin. Tgl is a single-domain protein, produced in active form, and the smallest transglutaminase characterized to date. We show that Tgl is structurally similar to bacterial cell wall endopeptidases and has an NlpC/P60 catalytic core, thought to represent the ancestral unit of the cysteine protease fold. We show that Tgl functions through a unique partially redundant catalytic dyad formed by Cys116 and Glu187 or Glu115. Strikingly, the catalytic Cys is insulated within a hydrophobic tunnel that traverses the molecule from side to side. The lack of similarity of Tgl to other transglutaminases together with its small size suggests that an NlpC/P60 catalytic core and insulation of the active site during catalysis may be essential requirements for protein cross-linking.

Type-II NADH:quinone oxidoreductase from Staphylococcus aureus has two distinct binding sites and is rate limited by quinone reduction.

A prerequisite for any rational drug design strategy is understanding the mode of protein-ligand interaction. This motivated us to explore protein-substrate interaction in Type-II NADH:quinone oxidoreductase (NDH-2) from Staphylococcus aureus, a worldwide problem in clinical medicine due to its multiple drug resistant forms. NDHs-2 are involved in respiratory chains and recognized as suitable targets for novel antimicrobial therapies, as these are the only enzymes with NADH:quinone oxidoreductase activity expressed in many pathogenic organisms. We obtained crystal and solution structures of NDH-2 from S. aureus, showing that it is a dimer in solution. We report fast kinetic analyses of the protein and detected a charge-transfer complex formed between NAD(+) and the reduced flavin, which is dissociated by the quinone. We observed that the quinone reduction is the rate limiting step and also the only half-reaction affected by the presence of HQNO, an inhibitor. We analyzed protein-substrate interactions by fluorescence and STD-NMR spectroscopies, which indicate that NADH and the quinone bind to different sites. In summary, our combined results show the presence of distinct binding sites for the two substrates, identified quinone reduction as the rate limiting step and indicate the establishment of a NAD(+)-protein complex, which is released by the quinone.

Expression, purification, crystallization and preliminary X-ray diffraction analysis of a type II NADH:quinone oxidoreductase from the human pathogen Staphylococcus aureus.

In recent years, type II NADH dehydrogenases (NDH-IIs) have emerged as potential drug targets for a wide range of human disease causative agents. In this work, the NDH-II enzyme from the Gram-positive human pathogen Staphylococcus aureus was recombinantly expressed in Escherichia coli, purified, crystallized and a crystallographic data set was collected at a wavelength of 0.873 Å. The crystals belonged to the orthorhombic space group P212121, with unit-cell parameters a = 81.8, b = 86.0, c = 269.9 Å, contained four monomers per asymmetric unit and diffracted to a resolution of 3.32 Å. A molecular-replacement solution was obtained and model building and refinement are currently under way.

Thiosulfate dehydrogenase (TsdA) from Allochromatium vinosum: structural and functional insights into thiosulfate oxidation.

Although the oxidative condensation of two thiosulfate anions to tetrathionate constitutes a well documented and significant part of the natural sulfur cycle, little is known about the enzymes catalyzing this reaction. In the purple sulfur bacterium Allochromatium vinosum, the reaction is catalyzed by the periplasmic diheme c-type cytochrome thiosulfate dehydrogenase (TsdA). Here, we report the crystal structure of the "as isolated" form of A. vinosum TsdA to 1.98 Šresolution and those of several redox states of the enzyme to different resolutions. The protein contains two typical class I c-type cytochrome domains wrapped around two hemes axially coordinated by His(53)/Cys(96) and His(164)/Lys(208). These domains are very similar, suggesting a gene duplication event during evolution. A ligand switch from Lys(208) to Met(209) is observed upon reduction of the enzyme. Cys(96) is an essential residue for catalysis, with the specific activity of the enzyme being completely abolished in several TsdA-Cys(96) variants. TsdA-K208N, K208G, and M209G variants were catalytically active in thiosulfate oxidation as well as in tetrathionate reduction, pointing to heme 2 as the electron exit point. In this study, we provide spectroscopic and structural evidence that the TsdA reaction cycle involves the transient presence of heme 1 in the high-spin state caused by movement of the Sγ atom of Cys(96) out of the iron coordination sphere. Based on the presented data, we draw important conclusions about the enzyme and propose a possible reaction mechanism for TsdA.