Role of active-site residues of dispersin B, a biofilm-releasing beta-hexosaminidase from a periodontal pathogen, in substrate hydrolysis

Strategy to combat biofilms: a focus on biofilm dispersal enzymes

Bacterial biofilms, which consist of three-dimensional extracellular polymeric substance (EPS), not only function as signaling networks, provide nutritional support, and facilitate surface adhesion, but also serve as a protective shield for the residing bacterial inhabitants against external stress, such as antibiotics, antimicrobials, and host immune responses. Biofilm-associated infections account for 65-80% of all human microbial infections that lead to serious mortality and morbidity. Tremendous effort has been spent to address the problem by developing biofilm-dispersing agents to discharge colonized microbial cells to a more vulnerable planktonic state. Here, we discuss the recent progress of enzymatic eradicating strategies against medical biofilms, with a focus on dispersal mechanisms. Particularly, we review three enzyme classes that have been extensively investigated, namely glycoside hydrolases, proteases, and deoxyribonucleases.

Mapping protein-exopolysaccharide binding interaction in Staphylococcus epidermidis biofilms by live cell proximity labeling

Bacterial biofilms consist of cells encased in an extracellular polymeric substance (EPS) composed of exopolysaccharides, extracellular DNA, and proteins that are critical for cell-cell adhesion and protect the cells from environmental stress, antibiotic treatments, and the host immune response. Degrading EPS components or blocking their production have emerged as promising strategies for prevention or dispersal of bacterial biofilms, but we still have little information about the specific biomolecular interactions that occur between cells and EPS components and how those interactions contribute to biofilm production. Staphylococcus epidermidis is a leading cause of nosocomial infections as a result of producing biofilms that use the exopolysaccharide poly-(1→6)-β-N-acetylglucosamine (PNAG) as a major structural component. In this study, we have developed a live cell proximity labeling approach combined with quantitative mass spectrometry-based proteomics to map the PNAG interactome of live S. epidermidis biofilms. Through these measurements we discovered elastin-binding protein (EbpS) as a major PNAG-interacting protein. Using live cell binding measurements, we found that the lysin motif (LysM) domain of EbpS specifically binds to PNAG present in S. epidermidis biofilms. Our work provides a novel method for the rapid identification of exopolysaccharide-binding proteins in live biofilms that will help to extend our understanding of the biomolecular interactions that are required for bacterial biofilm formation.

The endoglycosidase activity of Dispersin B is mediated through electrostatic interactions with cationic poly-β-(1→6)-N-acetylglucosamine

Bacterial biofilms consist of bacterial cells embedded within a self-produced extracellular polymeric substance (EPS) composed of exopolysaccharides, extra cellular DNA, proteins and lipids. The enzyme Dispersin B (DspB) is a CAZy type 20 β-hexosaminidase enzyme that catalyses the hydrolysis of poly-N-acetylglucosamine (PNAG), a major biofilm polysaccharide produced by a wide variety of biofilm-forming bacteria. Native PNAG is partially de-N-acetylated, and the degree of deacetylation varies between species and dependent on the environment. We have previously shown that DspB is able to perform both endo- and exo-glycosidic bond cleavage of PNAG depending on the de-N-acetylation patterns present in the PNAG substrate. Here, we used a combination of synthetic PNAG substrate analogues, site-directed mutagenesis and in vitro biofilm dispersal assay to investigate the molecular basis for the endo-glycosidic cleavage activity of DspB and the importance of this activity for dispersal of PNAG-dependent Staphylococcus epidermidis biofilms. We found that D242 contributes to the endoglycosidase activity of DspB through electrostatic interactions with cationic substrates in the -2 binding site. A DspB_D242N mutant was highly deficient in endoglycosidase activity while maintaining exoglycosidase activity. When used to disperse S. epidermidis biofilms, this DspB_D242N mutant resulted in an increase in residual biofilm biomass after treatment when compared to wild-type DspB. These results suggest that the de-N-acetylation of PNAG in S. epidermidis biofilms is not uniformly distributed and that the endoglycosidase activity of DspB is required for efficient biofilm dispersal.

Termination of Poly-N-acetylglucosamine (PNAG) Polymerization with N-Acetylglucosamine Analogues

Bacteria require polysaccharides for structure, survival, and virulence. Despite their central role in microbiology, few tools are available to manipulate their production. In E. coli, the glycosyltransferase complex PgaCD produces poly-N-acetylglucosamine (PNAG), an extracellular matrix polysaccharide required for biofilm formation. We report that C6-substituted (H, F, N₃, SH, NH₂) UDP-GlcNAc substrate analogues are inhibitors of PgaCD. In vitro, the inhibitors cause PNAG chain termination, consistent with the mechanism of PNAG polymerization from the nonreducing terminus. In vivo, expression of the GlcNAc-1-kinase NahK in E. coli provided a non-native GlcNAc salvage pathway that produced the UDP-GlcNAc analogue inhibitors in situ. The 6-fluoro and 6-deoxy derivatives were potent inhibitors of biofilm formation in the transformed strain, providing a tool to manipulate this key exopolysaccharide. Characterization of the UDP-GlcNAc pool and quantification of PNAG generation support PNAG termination as the primary in vivo mechanism of biofilm inhibition by 6-fluoro UDP-GlcNAc.

Enzymatic Dispersion of Biofilms: An Emerging Biocatalytic Avenue to Combat Biofilm-Mediated Microbial Infections

Drug resistance by pathogenic microbes has emerged as a matter of great concern to mankind. Microorganisms such as bacteria and fungi employ multiple defense mechanisms against drugs and the host immune system. A major line of microbial defense is the biofilm, which comprises extracellular polymeric substances that are produced by the population of microorganisms. Around 80% of chronic bacterial infections are associated with biofilms. The presence of biofilms can increase the necessity of doses of certain antibiotics up to 1000-fold to combat infection. Thus, there is an urgent need for strategies to eradicate biofilms. Although a few physicochemical methods have been developed to prevent and treat biofilms, these methods have poor efficacy and biocompatibility. In this review, we discuss the existing strategies to combat biofilms and their challenges. Subsequently, we spotlight the potential of enzymes, in particular, polysaccharide degrading enzymes, for biofilm dispersion, which might lead to facile antimicrobial treatment of biofilm-associated infections.

Multifunctional fluorescent probes for high-throughput characterization of hexosaminidase enzyme activity

Microbial polysaccharides composed of N-acetylglucosamine (GlcNAc), such as chitin, peptidoglycan and poly-β-(1 → 6)-GlcNAc (dPNAG), play a critical role in maintaining cell integrity or in facilitating biofilm formation in numerous fungal and bacterial pathogens. Glycosyl hydrolase enzymes that catalyze the degradation of these β-GlcNAc containing polysaccharides play important roles in normal microbial cell physiology and can also be exploited as biocatalysts with applications as anti-fungal, anti-bacterial, or biofilm dispersal agents. Assays to rapidly detect and characterize the activity of such glycosyl hydrolase enzymes can facilitate their development as biocatalyst, however, currently available probes such as 4-methylumbelliferyl-β-GlcNAc (4MU-GlcNAc) are not universally accepted as substrates, and their fluorescent signal is sensitive to changes in pH. Here, we present the development of a new multifunctional fluorescent substrate analog for the detection and characterization of hexosaminidase enzyme activity containing a 7-amino-4-methyl coumarin (AMC) carbamate aglycone. This probe is widely tolerated as a substrate for exo-acting β-hexosaminidase, family 19 endo-chitinase, and the dPNAG hydrolase enzyme Dispersin B (DspB) and enables detection of hexosaminidase enzyme activity via either single wavelength fluorescent measurements or ratiometric fluorescent detection. We demonstrate the utility of this probe to screen for recombinant DspB activity in Escherichia coli cell lysates, and for the development of a high-throughput assay to screen for DspB inhibitors.

Structural basis of chitin utilization by a GH20 β-N-acetylglucosaminidase from Vibrio campbellii strain ATCC BAA-1116

Crystal structures of a GH20 β-N-acetylglucosaminidase from V. campbellii reveal substrate specificity in chitin utilization.

Vibrio species play a crucial role in maintaining the carbon and nitrogen balance between the oceans and the land through their ability to employ chitin as a sole source of energy. This study describes the structural basis for the action of the GH20 β-N-acetylglucosaminidase (VhGlcNAcase) in chitin metabolism by Vibrio campbellii (formerly V. harveyi) strain ATCC BAA-1116. Crystal structures of wild-type VhGlcNAcase in the absence and presence of the sugar ligand, and of the unliganded D437A mutant, were determined. VhGlcNAcase contains three distinct domains: an N-terminal carbohydrate-binding domain linked to a small α+β domain and a C-terminal (β/α)₈ catalytic domain. The active site of VhGlcNAcase has a narrow, shallow pocket that is suitable for accommodating a small chitooligosaccharide. VhGlcNAcase is a monomeric enzyme of 74 kDa, but its crystal structures show two molecules of enzyme per asymmetric unit, in which Gln16 at the dimeric interface of the first molecule partially blocks the entrance to the active site of the neighboring molecule. The GlcNAc unit observed in subsite −1 makes exclusive hydrogen bonds to the conserved residues Arg274, Tyr530, Asp532 and Glu584, while Trp487, Trp546, Trp582 and Trp505 form a hydrophobic wall around the −1 GlcNAc. The catalytic mutants D437A/N and E438A/Q exhibited a drastic loss of GlcNAcase activity, confirming the catalytic role of the acidic pair (Asp437–Glu438).

Anionic amino acids support hydrolysis of poly-β-(1,6)-N-acetylglucosamine exopolysaccharides by the biofilm dispersing glycosidase Dispersin B

The exopolysaccharide poly-β-(1→6)-N-acetylglucosamine (PNAG) is a major structural determinant of bacterial biofilms responsible for persistent and nosocomial infections. The enzymatic dispersal of biofilms by PNAG-hydrolyzing glycosidase enzymes, such as Dispersin B (DspB), is a possible approach to treat biofilm-dependent bacterial infections. The cationic charge resulting from partial de-N-acetylation of native PNAG is critical for PNAG-dependent biofilm formation. We recently demonstrated that DspB has increased catalytic activity on de-N-acetylated PNAG oligosaccharides, but the molecular basis for this increased activity is not known. Here, we analyze the role of anionic amino acids surrounding the catalytic pocket of DspB in PNAG substrate recognition and hydrolysis using a combination of site-directed mutagenesis, activity measurements using synthetic PNAG oligosaccharide analogs, and in vitro biofilm dispersal assays. The results of these studies support a model in which bound PNAG is weakly associated with a shallow anionic groove on the DspB protein surface with recognition driven by interactions with the -1 GlcNAc residue in the catalytic pocket. An increased rate of hydrolysis for cationic PNAG was driven, in part, by interaction with D147 on the anionic surface. Moreover, we identified that a DspB mutant with improved hydrolysis of fully acetylated PNAG oligosaccharides correlates with improved in vitro dispersal of PNAG-dependent Staphylococcus epidermidis biofilms. These results provide insight into the mechanism of substrate recognition by DspB and suggest a method to improve DspB biofilm dispersal activity by mutation of the amino acids within the anionic binding surface.

Mechanisms and Control Measures of Mature Biofilm Resistance to Antimicrobial Agents in the Clinical Context

Biofilms are the aggregation of micro-organisms, which are composed of extracellular polymeric substance (EPS) and many other biochemical components. Though they might be beneficial to some wastewater and soil treatment processes, they may expose chronic infection and risk to personal hygiene in the industrial as well as the clinical context. Despite having a well-established disinfection and hygiene monitoring program for the prevention of formation and growth, biofilm persistently remains in the medical settings because of its antibiotic resistance to antimicrobial agents and even the immune system. In this paper, the contributing factors of antibiotic resistance and the corresponding mechanisms, including heterogeneity inside biofilms, the roles of the EPS matrix, cell density, and quorum sensing, and cell mutability, are reviewed. Moreover, current clinical practice and strategic applications are also suggested to address the biofilm resistance issues.

A Colorimetric Assay to Enable High-Throughput Identification of Biofilm Exopolysaccharide-Hydrolyzing Enzymes

Glycosidase enzymes that hydrolyze the biofilm exopolysaccharide poly-β-(1→6)-N-acetylglucosamine (PNAG) are critical tools to study biofilm and potential therapeutic biofilm dispersal agents. Function-driven metagenomic screening is a powerful approach for the discovery of new glycosidase but requires sensitive assays capable of distinguishing between the desired enzyme and functionally related enzymes. Herein, we report the synthesis of a colorimetric PNAG disaccharide analogue whose hydrolysis by PNAG glycosidases results in production of para-nitroaniline that can be continuously monitored at 410 nm. The assay is specific for enzymes capable of hydrolyzing PNAG and not related β-hexosaminidase enzymes with alternative glycosidic linkage specificities. This analogue enabled development of a continuous colorimetric assay for detection of PNAG hydrolyzing enzyme activity in crude E. coli cell lysates and suggests that this disaccharide probe will be critical for establishing the functional screening of metagenomic DNA libraries.

An Inactive Dispersin B Probe for Monitoring PNAG Production in Biofilm Formation

The bacterial exopolysaccharide poly-β-1,6-N-acetylglucosamine is a major extracellular matrix component in biofilms of both Gram-positive and Gram-negative organisms. We have leveraged the specificity of the biofilm-dispersing glycoside hydrolase Dispersin B (DspB) to generate a probe (Dispersin B PNAG probe, DiPP) for monitoring PNAG production and localization during biofilm formation. Mutation of the active site of Dispersin B gave DiPP, which was an effective probe despite its low affinity for PNAG oligosaccharides (K_D ∼ 1-10 mM). Imaging of PNAG-dependent and -independent biofilms stained with a fluorescent-protein fusion of DiPP (GFP-DiPP) demonstrated the specificity of the probe for the structure of PNAG on both single-cell and biofilm levels, indicating a high local concentration of PNAG at the bacterial cell surface. Through quantitative bacterial cell binding assays and confocal microscopy analysis using GFP-DiPP, discrete areas of local high concentrations of PNAG were detected on the surface of early log phase cells. These distinct areas were seen to grow, slough from cells, and accumulate in interbacterial regions over the course of several cell divisions, showing the development of a PNAG-dependent biofilm. A potential helical distribution of staining was also noted, suggesting some degree of organization of PNAG production at the cell surface prior to cell aggregation. Together, these experiments shed light on the early stages of PNAG-dependent biofilm formation and demonstrate the value of a low-affinity-high-specificity probe for monitoring the production of bacterial exopolysaccharides.

Differential Recognition of Deacetylated PNAG Oligosaccharides by a Biofilm Degrading Glycosidase

Exopolysaccharides consisting of partially de-N-acetylated poly-β-d-(1→6)-N-acetyl-glucosamine (dPNAG) are key structural components of the biofilm extracellular polymeric substance of both Gram-positive and Gram-negative human pathogens. De-N-acetylation is required for the proper assembly and function of dPNAG in biofilm development suggesting that different patterns of deacetylation may be preferentially recognized by proteins that interact with dPNAG, such as Dispersin B (DspB). The enzymatic degradation of dPNAG by the Aggregatibacter actinomycetemcomitans native β-hexosaminidase enzyme DspB plays a role in biofilm dispersal. To test the role of substrate de-N-acetylation on substrate recognition by DspB, we applied an efficient preactivation-based one-pot glycosylation approach to prepare a panel of dPNAG trisaccharide analogs with defined acetylation patterns. These analogs served as effective DspB substrates, and the rate of hydrolysis was dependent on the specific substrate de-N-acetylation pattern, with glucosamine (GlcN) located +2 from the site of cleavage being preferentially hydrolyzed. The product distributions support a primarily exoglycosidic cleavage activity following a substrate assisted cleavage mechanism, with the exception of substrates containing a nonreducing GlcN that were cleaved endo leading to the exclusive formation of a nonreducing disaccharide product. These observations provide critical insight into the substrate specificity of dPNAG specific glycosidase that can help guide their design as biocatalysts.

PolyGlcNAc-containing exopolymers enable surface penetration by non-motile Enterococcus faecalis

Bacterial pathogens have evolved strategies that enable them to invade tissues and spread within the host. Enterococcus faecalis is a leading cause of local and disseminated multidrug-resistant hospital infections, but the molecular mechanisms used by this non-motile bacterium to penetrate surfaces and translocate through tissues remain largely unexplored. Here we present experimental evidence indicating that E. faecalis generates exopolysaccharides containing β-1,6-linked poly-N-acetylglucosamine (polyGlcNAc) as a mechanism to successfully penetrate semisolid surfaces and translocate through human epithelial cell monolayers. Genetic screening and molecular analyses of mutant strains identified glnA, rpiA and epaX as genes critically required for optimal E. faecalis penetration and translocation. Mechanistically, GlnA and RpiA cooperated to generate uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) that was utilized by EpaX to synthesize polyGlcNAc-containing polymers. Notably, exogenous supplementation with polymeric N-acetylglucosamine (PNAG) restored surface penetration by E. faecalis mutants devoid of EpaX. Our study uncovers an unexpected mechanism whereby the RpiA-GlnA-EpaX metabolic axis enables production of polyGlcNAc-containing polysaccharides that endow E. faecalis with the ability to penetrate surfaces. Hence, targeting carbohydrate metabolism or inhibiting biosynthesis of polyGlcNAc-containing exopolymers may represent a new strategy to more effectively confront enterococcal infections in the clinic.

Enterococcus faecalis is a microbial inhabitant of the human gastrointestinal tract that can cause lethal infections. Typically classified as a non-motile bacterium, E. faecalis can readily migrate and translocate across epithelial barriers to invade distant organs. Nevertheless, the molecular pathways driving enterococcal invasive attributes remain poorly understood. In this study, we uncover that E. faecalis produces a polyGlcNAc-containing extracellular glycopolymer to efficiently migrate into semisolid surfaces and translocate through human epithelial cell monolayers. Our work provides evidence that non-motile bacterial pathogens can exploit endogenous carbohydrate metabolic pathways to penetrate surfaces. Thus, targeting glycopolymer biosynthetic programs might be useful to control infections by Gram-positive cocci in the clinic.

Antibody to Poly-N-acetyl glucosamine provides protection against intracellular pathogens: Mechanism of action and validation in horse foals challenged with Rhodococcus equi

Immune correlates of protection against intracellular bacterial pathogens are largely thought to be cell-mediated, although a reasonable amount of data supports a role for antibody-mediated protection. To define a role for antibody-mediated immunity against an intracellular pathogen, Rhodococcus equi, that causes granulomatous pneumonia in horse foals, we devised and tested an experimental system relying solely on antibody-mediated protection against this host-specific etiologic agent. Immunity was induced by vaccinating pregnant mares 6 and 3 weeks prior to predicted parturition with a conjugate vaccine targeting the highly conserved microbial surface polysaccharide, poly-N-acetyl glucosamine (PNAG). We ascertained antibody was transferred to foals via colostrum, the only means for foals to acquire maternal antibody. Horses lack transplacental antibody transfer. Next, a randomized, controlled, blinded challenge was conducted by inoculating at ~4 weeks of age ~10⁶ cfu of R. equi via intrabronchial challenge. Eleven of 12 (91%) foals born to immune mares did not develop clinical R. equi pneumonia, whereas 6 of 7 (86%) foals born to unvaccinated controls developed pneumonia (P = 0.0017). In a confirmatory passive immunization study, infusion of PNAG-hyperimmune plasma protected 100% of 5 foals against R. equi pneumonia whereas all 4 recipients of normal horse plasma developed clinical disease (P = 0.0079). Antibodies to PNAG mediated killing of extracellular and intracellular R. equi and other intracellular pathogens. Killing of intracellular organisms depended on antibody recognition of surface expression of PNAG on infected cells, along with complement deposition and PMN-assisted lysis of infected macrophages. Peripheral blood mononuclear cells from immune and protected foals released higher levels of interferon-γ in response to PNAG compared to controls, indicating vaccination also induced an antibody-dependent cellular release of this critical immune cytokine. Overall, antibody-mediated opsonic killing and interferon-γ release in response to PNAG may protect against diseases caused by intracellular bacterial pathogens.

Development of effective vaccines for diseases such as tuberculosis, brucellosis and others caused by intracellular pathogens has proved challenging, as data exist supporting both antibody and cellular immune effectors as mediators of protection. To address this problem against an important, and representative, equine intracellular pathogen, we chose to test a vaccine candidate for the ability to protect horse foals challenged at 4 weeks of age with Rhodococcus equi. To make these foals immune, their pregnant mares were immunized with a vaccine targeting the conserved surface antigen found on many microbes, termed PNAG. Antibody in the pregnant mares was transferred to their foals and, after the foals were challenged, 91% of those born to vaccinated mares were protected against R. equi pneumonia. Meanwhile, 86% of the non-vaccinated controls developed pneumonia. We also showed antibody to PNAG could kill various bacteria that produce this antigen when residing inside of human macrophage cells, a new mechanism for antibody-mediated immunity to intracellular bacteria. These results support the development of PNAG as a vaccine for intracellular bacterial pathogens.

PgaB orthologues contain a glycoside hydrolase domain that cleaves deacetylated poly-β(1,6)-N-acetylglucosamine and can disrupt bacterial biofilms

Poly-β(1,6)-N-acetyl-D-glucosamine (PNAG) is a major biofilm component of many pathogenic bacteria. The production, modification, and export of PNAG in Escherichia coli and Bordetella species require the protein products encoded by the pgaABCD operon. PgaB is a two-domain periplasmic protein that contains an N-terminal deacetylase domain and a C-terminal PNAG binding domain that is critical for export. However, the exact function of the PgaB C-terminal domain remains unclear. Herein, we show that the C-terminal domains of Bordetella bronchiseptica PgaB (PgaB_Bb) and E. coli PgaB (PgaB_Ec) function as glycoside hydrolases. These enzymes hydrolyze purified deacetylated PNAG (dPNAG) from Staphylococcus aureus, disrupt PNAG-dependent biofilms formed by Bordetella pertussis, Staphylococcus carnosus, Staphylococcus epidermidis, and E. coli, and potentiate bacterial killing by gentamicin. Furthermore, we found that PgaB_Bb was only able to hydrolyze PNAG produced in situ by the E. coli PgaCD synthase complex when an active deacetylase domain was present. Mass spectrometry analysis of the PgaB-hydrolyzed dPNAG substrate showed a GlcN-GlcNAc-GlcNAc motif at the new reducing end of detected fragments. Our 1.76 Å structure of the C-terminal domain of PgaB_Bb reveals a central cavity within an elongated surface groove that appears ideally suited to recognize the GlcN-GlcNAc-GlcNAc motif. The structure, in conjunction with molecular modeling and site directed mutagenesis led to the identification of the dPNAG binding subsites and D474 as the probable catalytic acid. This work expands the role of PgaB within the PNAG biosynthesis machinery, defines a new glycoside hydrolase family GH153, and identifies PgaB as a possible therapeutic agent for treating PNAG-dependent biofilm infections.

From plaque on teeth to infections in the lungs of cystic fibrosis patients, biofilms are a serious health concern and difficult to eradicate. One of the key building blocks involved in biofilm formation are polymeric sugar compounds that are secreted by the bacteria. Our work focuses on the biopolymer poly-β(1,6)-N-acetyl-D-glucosamine (PNAG), which is produced by numerous pathogenic organisms. Deacetylation of PNAG by the N-terminal domain of PgaB is a critical step in polymer maturation and is required for the formation of robust biofilms. Herein, we show that the C-terminal domain of PgaB is a glycoside hydrolase active on partially deacetylated PNAG, and that the enzyme disrupts PNAG-dependent biofilms and potentiates killing by antibiotics. Only deacetylated PNAG could be cleaved, suggesting that PgaB deacetylates and hydrolyses the polymer in sequential order. Analyzing the chemical structure of the cleaved dPNAG fragments revealed a distinct motif of sugar units. Structural and functional studies identify key amino acids positioned in an elongated polymer-binding groove that potentially recognize the sugar motif during cleavage. Our study provides further insight into the mechanism of periplasmic PNAG modification, and suggests PgaB could be utilized as a therapeutic agent to eliminate biofilms.

Immunization with outer membrane vesicles displaying conserved surface polysaccharide antigen elicits broadly antimicrobial antibodies

Many microbial pathogens produce a β-(1→6)-linked poly-N-acetyl-d-glucosamine (PNAG) surface capsule, including bacterial, fungal, and protozoan cells. Broadly protective immune responses to this single conserved polysaccharide antigen in animals are possible but only when a deacetylated poly-N-acetyl-d-glucosamine (dPNAG; <30% acetate) glycoform is administered as a conjugate to a carrier protein. Unfortunately, conventional methods for natural extraction or chemical synthesis of dPNAG and its subsequent conjugation to protein carriers can be technically demanding and expensive. Here, we describe an alternative strategy for creating broadly protective vaccine candidates that involved coordinating recombinant poly-N-acetyl-d-glucosamine (rPNAG) biosynthesis with outer membrane vesicle (OMV) formation in laboratory strains of Escherichia coli The glycosylated outer membrane vesicles (glycOMVs) released by these engineered bacteria were decorated with the PNAG glycopolymer and induced high titers of PNAG-specific IgG antibodies after immunization in mice. When a Staphylococcus aureus enzyme responsible for PNAG deacetylation was additionally expressed in these cells, glycOMVs were generated that elicited antibodies to both highly acetylated PNAG (∼95-100% acetate) and a chemically deacetylated dPNAG derivative (∼15% acetate). These antibodies mediated efficient in vitro killing of two distinct PNAG-positive bacterial species, namely S. aureus and Francisella tularensis subsp. holarctica, and mice immunized with PNAG-containing glycOMVs developed protective immunity against these unrelated pathogens. Collectively, our results reveal the potential of glycOMVs for targeting this conserved polysaccharide antigen and engendering protective immunity against the broad range of pathogens that produce surface PNAG.

Nanomedicine and advanced technologies for burns: Preventing infection and facilitating wound healing

According to the latest report from the World Health Organization, an estimated 265,000 deaths still occur every year as a direct result of burn injuries. A widespread range of these deaths induced by burn wound happens in low- and middle-income countries, where survivors face a lifetime of morbidity. Most of the deaths occur due to infections when a high percentage of the external regions of the body area is affected. Microbial nutrient availability, skin barrier disruption, and vascular supply destruction in burn injuries as well as systemic immunosuppression are important parameters that cause burns to be susceptible to infections. Topical antimicrobials and dressings are generally employed to inhibit burn infections followed by a burn wound therapy, because systemic antibiotics have problems in reaching the infected site, coupled with increasing microbial drug resistance. Nanotechnology has provided a range of molecular designed nanostructures (NS) that can be used in both therapeutic and diagnostic applications in burns. These NSs can be divided into organic and non-organic (such as polymeric nanoparticles (NPs) and silver NPs, respectively), and many have been designed to display multifunctional activity. The present review covers the physiology of skin, burn classification, burn wound pathogenesis, animal models of burn wound infection, and various topical therapeutic approaches designed to combat infection and stimulate healing. These include biological based approaches (e.g. immune-based antimicrobial molecules, therapeutic microorganisms, antimicrobial agents, etc.), antimicrobial photo- and ultrasound-therapy, as well as nanotechnology-based wound healing approaches as a revolutionizing area. Thus, we focus on organic and non-organic NSs designed to deliver growth factors to burned skin, and scaffolds, dressings, etc. for exogenous stem cells to aid skin regeneration. Eventually, recent breakthroughs and technologies with substantial potentials in tissue regeneration and skin wound therapy (that are as the basis of burn wound therapies) are briefly taken into consideration including 3D-printing, cell-imprinted substrates, nano-architectured surfaces, and novel gene-editing tools such as CRISPR-Cas.

The molecular structure of the glycoside hydrolase domain of Cwp19 from Clostridium difficile

Clostridium difficile is a burden to healthcare systems around the world, causing tens of thousands of deaths annually. The S‐layer of the bacterium, a layer of protein found of the surface of cells, has received a significant amount of attention over the past two decades as a potential target to combat the growing threat presented by C. difficile infections. The S‐layer contains a wide range of proteins, each of which possesses three cell wall‐binding domains, while many also possess a “functional” region. Here, we present the high resolution structure of the functional region of one such protein, Cwp19 along with preliminary functional characterisation of the predicted glycoside hydrolase. Cwp19 has a TIM barrel fold and appears to possess a high degree of substrate selectivity. The protein also exhibits peptidoglycan hydrolase activity, an order of magnitude slower than that of lysozyme and is the first member of glycoside hydrolase‐like family 10 to be characterised. This research goes some way to understanding the role of Cwp19 in the S‐layer of C. difficile.

Database

Structural data are available in the PDB under the accession numbers 5OQ2 and 5OQ3.

Biofilm dispersal: multiple elaborate strategies for dissemination of bacteria with unique properties

In most environments, microorganisms evolve in a sessile mode of growth, designated as biofilm, which is characterized by cells embedded in a self-produced extracellular matrix. Although a biofilm is commonly described as a "cozy house" where resident bacteria are protected from aggression, bacteria are able to break their biofilm bonds and escape to colonize new environments. This regulated process is observed in a wide variety of species; it is referred to as biofilm dispersal, and is triggered in response to various environmental and biological signals. The first part of this review reports the main regulatory mechanisms and effectors involved in biofilm dispersal. There is some evidence that dispersal is a necessary step between the persistence of bacteria inside biofilm and their dissemination. In the second part, an overview of the main methods used so far to study the dispersal process and to harvest dispersed bacteria was provided. Then focus was on the properties of the biofilm-dispersed bacteria and the fundamental role of the dispersal process in pathogen dissemination within a host organism. In light of the current body of knowledge, it was suggested that dispersal acts as a potent means of disseminating bacteria with enhanced colonization properties in the surrounding environment.

EPS-Then and Now

“Slime” played a brief and spectacular role in the 19th century founded by the theory of primordial slime by Ernst Haeckel. However, that substance was never found and eventually abandoned. Further scientific attention slowly began in the 1930s referring to slime as a microbial product and then was inspired by “How bacteria stick” by Costerton et al. in 1978, and the matrix material was considered to be polysaccharides. Later, it turned out that proteins, nucleic acids and lipids were major other constituents of the extracellular polymeric substances (EPS), an acronym which was highly discussed. The role of the EPS matrix turns out to be fundamental for biofilms, in terms of keeping cells in proximity and allowing for extended interaction, resource capture, mechanical strength and other properties, which emerge from the life of biofilm organisms, including enhanced tolerance to antimicrobials and other stress. The EPS components are extremely complex and dynamic and fulfil many functional roles, turning biofilms into the most ubiquitous and successful form of life on Earth.

In Vitro Antimicrobial Efficacy of Tobramycin Against Staphylococcus aureus Biofilms in Combination With or Without DNase I and/or Dispersin B: A Preliminary Investigation

Staphylococcus aureus in biofilms is highly resistant to the treatment with antibiotics, to which the planktonic cells are susceptible. This is likely to be due to the biofilm creating a protective barrier that prevents antibiotics from accessing the live pathogens buried in the biofilm. S. aureus biofilms consist of an extracellular matrix comprising, but not limited to, extracellular bacterial DNA (eDNA) and poly-β-1, 6-N-acetyl-d-glucosamine (PNAG). Our study revealed that despite inferiority of dispersin B (an enzyme that degrades PNAG) to DNase I that cleaves eDNA, in dispersing the biofilm of S. aureus, both enzymes were equally efficient in enhancing the antibacterial efficiency of tobramycin, a relatively narrow-spectrum antibiotic against infections caused by gram-positive and gram-negative pathogens, including S. aureus, used in this investigation. However, a combination of these two biofilm-degrading enzymes was found to be significantly less effective in enhancing the antimicrobial efficacy of tobramycin than the individual application of the enzymes. These findings indicate that combinations of different biofilm-degrading enzymes may compromise the antimicrobial efficacy of antibiotics and need to be carefully assessed in vitro before being used for treating medical devices or in pharmaceutical formulations for use in the treatment of chronic ear or respiratory infections.

Surface display of Aggregatibacter actinomycetemcomitans autotransporter Aae and dispersin B hybrid act as antibiofilm agents

Among the various proteins expressed by the periodontopathogen Aggregatibacter actinomycetemcomitans, two proteins play important roles for survival in the oral cavity. The autotransporter Aae facilitates the attachment of the pathogen to oral epithelial cells, which act as a reservoir, while the biofilm-degrading glycoside hydrolase dispersin B facilitates the movement of daughter cells from the mature biofilm to a new site. The objective of this study was to use the potential of these two proteins to control biofilms. To this end, we generated a hybrid construct between the Aae C-terminal translocating domain and dispersin B, and mobilized it into Escherichia coli Rosetta (DE3) pLysS cells. Immunofluorescence analysis of the modified E. coli cells confirmed the presence of dispersin B on the surface. Further, the membrane localization of the displayed dispersin B was confirmed with Western blot analysis. The integrity of the E. coli cells displaying the dispersin B was confirmed through FACS analysis. The hydrolytic activity of the surface-displayed dispersin B was confirmed by using 4-methylumbelliferyl-β-d-glucopyranoside as the substrate. The detachment ability of the dispersin B surface-displaying E. coli cells was shown using Staphylococcus epidermidis and Actinobacillus pleuropneumoniae biofilms in a microtiter assay. We concluded that the Aae β-domain is sufficient to translocate foreign enzymes in the native folded form and that the method of Aae-mediated translocation of surface displayed enzymes might be useful for control of biofilms.

Physiological adaptations of key oral bacteria

Oral colonising bacteria are highly adapted to the various environmental niches harboured within the mouth, whether that means while contributing to one of the major oral diseases of caries, pulp infections, or gingival/periodontal disease or as part of a commensal lifestyle. Key to these infections is the ability to adhere to surfaces via a range of specialised adhesins targeted at both salivary and epithelial proteins, their glycans and to form biofilm. They must also resist the various physical stressors they are subjected to, including pH and oxidative stress. Possibly most strikingly, they have developed the ability to harvest both nutrient sources provided by the diet and those derived from the host, such as protein and surface glycans. We have attempted to review recent developments that have revealed much about the molecular mechanisms at work in shaping the physiology of oral bacteria and how we might use this information to design and implement new treatment strategies.

Challenges in predicting the evolutionary maintenance of a phage transgene

BACKGROUND

In prior work, a phage engineered with a biofilm-degrading enzyme (dispersin B) cleared artificial, short-term biofilms more fully than the phage lacking the enzyme. An unresolved question is whether the transgene will be lost or maintained during phage growth - its loss would limit the utility of the engineering. Broadly supported evolutionary theory suggests that transgenes will be lost through a 'tragedy of the commons' mechanism unless the ecology of growth in biofilms meets specific requirements. We test that theory here.

RESULTS

Functional properties of the transgenic phage were identified. Consistent with the previous study, the dispersin phage was superior to unmodified phage at clearing short term biofilms grown in broth, shown here to be an effect attributable to free enzyme. However, the dispersin phage was only marginally better than control phages on short term biofilms in minimal media and was no better than control phages in clearing long term biofilms. There was little empirical support for the tragedy of the commons framework despite a strong theoretical foundation for its supposed relevance. The framework requires that the transgene imposes an intrinsic cost, yet the transgene was intrinsically neutral or beneficial when expressed from one part of the phage genome. Expressed from a different part of the genome, the transgene did behave as if intrinsically costly, but its maintenance did not benefit from spatially structured growth per se - violating the tragedy framework.

CONCLUSIONS

Overall, the transgene was beneficial under many conditions, but no insight to its maintenance was attributable to the established evolutionary framework. The failure likely resides in system details that would be used to parameterize the models. Our study cautions against naive applications of evolutionary theory to synthetic biology, even qualitatively.

Modification and periplasmic translocation of the biofilm exopolysaccharide poly-β-1,6-N-acetyl-D-glucosamine

Poly-β-1,6-N-acetyl-D-glucosamine (PNAG) is an exopolysaccharide produced by a wide variety of medically important bacteria. Polyglucosamine subunit B (PgaB) is responsible for the de-N-acetylation of PNAG, a process required for polymer export and biofilm formation. PgaB is located in the periplasm and likely bridges the inner membrane synthesis and outer membrane export machinery. Here, we present structural, functional, and molecular simulation data that suggest PgaB associates with PNAG continuously during periplasmic transport. We show that the association of PgaB's N- and C-terminal domains forms a cleft required for the binding and de-N-acetylation of PNAG. Molecular dynamics (MD) simulations of PgaB show a binding preference for N-acetylglucosamine (GlcNAc) to the N-terminal domain and glucosammonium to the C-terminal domain. Continuous ligand binding density is observed that extends around PgaB from the N-terminal domain active site to an electronegative groove on the C-terminal domain that would allow for a processive mechanism. PgaB's C-terminal domain (PgaB310-672) directly binds PNAG oligomers with dissociation constants of ∼1-3 mM, and the structures of PgaB310-672 in complex with β-1,6-(GlcNAc)6, GlcNAc, and glucosamine reveal a unique binding mode suitable for interaction with de-N-acetylated PNAG (dPNAG). Furthermore, PgaB310-672 contains a β-hairpin loop (βHL) important for binding PNAG that was disordered in previous PgaB42-655 structures and is highly dynamic in the MD simulations. We propose that conformational changes in PgaB310-672 mediated by the βHL on binding of PNAG/dPNAG play an important role in the targeting of the polymer for export and its release.

Interactions in bacterial biofilm development: a structural perspective

A community-based life style is the normal mode of growth and survival for many bacterial species. These cellular accretions or biofilms are initiated upon recognition of solid phases by cell surface exposed adhesive moieties. Further cell-cell interactions, cell signalling and bacterial replication leads to the establishment of dense populations encapsulated in a mainly self-produced extracellular matrix; this comprises a complex mixture of macromolecules. These fascinating architectures protect the inhabitants from radiation damage, dehydration, pH fluctuations and antimicrobial compounds. As such they can cause bacterial persistence in disease and problems in industrial applications. In this review we discuss the current understandings of these initial biofilm-forming processes based on structural data. We also briefly describe latter biofilm maturation and dispersal events, which although lack high-resolution insights, are the present focus for many structural biologists working in this field. Finally we give an overview of modern techniques aimed at preventing and disrupting problem biofilms.

Antibody to a conserved antigenic target is protective against diverse prokaryotic and eukaryotic pathogens

Microbial capsular antigens are effective vaccines but are chemically and immunologically diverse, resulting in a major barrier to their use against multiple pathogens. A β-(1→6)-linked poly-N-acetyl-d-glucosamine (PNAG) surface capsule is synthesized by four proteins encoded in genetic loci designated intercellular adhesion in Staphylococcus aureus or polyglucosamine in selected Gram-negative bacterial pathogens. We report that many microbial pathogens lacking an identifiable intercellular adhesion or polyglucosamine locus produce PNAG, including Gram-positive, Gram-negative, and fungal pathogens, as well as protozoa, e.g., Trichomonas vaginalis, Plasmodium berghei, and sporozoites and blood-stage forms of Plasmodium falciparum. Natural antibody to PNAG is common in humans and animals and binds primarily to the highly acetylated glycoform of PNAG but is not protective against infection due to lack of deposition of complement opsonins. Polyclonal animal antibody raised to deacetylated glycoforms of PNAG and a fully human IgG1 monoclonal antibody that both bind to native and deacetylated glycoforms of PNAG mediated complement-dependent opsonic or bactericidal killing and protected mice against local and/or systemic infections by Streptococcus pyogenes, Streptococcus pneumoniae, Listeria monocytogenes, Neisseria meningitidis serogroup B, Candida albicans, and P. berghei ANKA, and against colonic pathology in a model of infectious colitis. PNAG is also a capsular polysaccharide for Neisseria gonorrhoeae and nontypable Hemophilus influenzae, and protects cells from environmental stress. Vaccination targeting PNAG could contribute to immunity against serious and diverse prokaryotic and eukaryotic pathogens, and the conserved production of PNAG suggests that it is a critical factor in microbial biology.

Microbial interactions in building of communities

Establishment of a community is considered to be essential for microbial growth and survival in the human oral cavity. Biofilm communities have increased resilience to physical forces, antimicrobial agents and nutritional variations. Specific cell-to-cell adherence processes, mediated by adhesin-receptor pairings on respective microbial surfaces, are able to direct community development. These interactions co-localize species in mutually beneficial relationships, such as streptococci, veillonellae, Porphyromonas gingivalis and Candida albicans. In transition from the planktonic mode of growth to a biofilm community, microorganisms undergo major transcriptional and proteomic changes. These occur in response to sensing of diffusible signals, such as autoinducer molecules, and to contact with host tissues or other microbial cells. Underpinning many of these processes are intracellular phosphorylation events that regulate a large number of microbial interactions relevant to community formation and development.

Insight into the composition of the intercellular matrix of Streptococcus pneumoniae biofilms

Biofilm matrices consist of a mixture of extracellular polymeric substances synthesized in large part by the biofilm-producing microorganisms themselves. These matrices are responsible for the cohesion and three-dimensional architecture of biofilms. The present study demonstrates the existence of a matrix composed of extracellular DNA, proteins and polysaccharides in the biofilm formed by the human pathogen Streptococcus pneumoniae. Extracellular DNA, visualized by fluorescent labelling, was an important component of this matrix. The existence of DNA-protein complexes associated with bacterial aggregates and other polymers was hypothesized based on the unexpected DNA binding activity of lysozyme LytC, a novel moonlighting protein. Actually, a 25-amino-acid-long peptide derived from LytC (positions 408 and 432 of the mature LytC) was also capable of efficiently binding to DNA. Moreover, the presence of intercellular DNA-LytC protein complexes in pneumococcal biofilms was demonstrated by confocal laser scanning microscopy. Evidence of extracellular polysaccharide different from the capsule was obtained by staining with Calcofluor dye and four types of lectin conjugated to Alexa fluorophores, and by incubation with glycoside hydrolases. The presence of residues of Glcp(1→4) and GlcNAc(1→4) (in its deacetylated form) in the pneumococcal biofilm was confirmed by GC-MS techniques.

Poly-N-acetylglucosamine expression by wild-type Yersinia pestis is maximal at mammalian, not flea, temperatures

Numerous bacteria, including Yersinia pestis, express the poly-N-acetylglucosamine (PNAG) surface carbohydrate, a major component of biofilms often associated with a specific appearance of colonies on Congo red agar. Biofilm formation and PNAG synthesis by Y. pestis have been reported to be maximal at 21 to 28°C or “flea temperatures,” facilitating the regurgitation of Y. pestis into a mammalian host during feeding, but production is diminished at 37°C and thus presumed to be decreased during mammalian infection. Most studies of PNAG expression and biofilm formation by Y. pestis have used a low-virulence derivative of strain KIM, designated KIM6+, that lacks the pCD1 virulence plasmid, and an isogenic mutant without the pigmentation locus, which contains the hemin storage genes that encode PNAG biosynthetic proteins. Using confocal microscopy, fluorescence-activated cell sorter analysis and growth on Congo red agar, we confirmed prior findings regarding PNAG production with the KIM6+ strain. However, we found that fully virulent wild-type (WT) strains KIM and CO92 had maximal PNAG expression at 37°C, with lower PNAG production at 28°C both in broth medium and on Congo red agar plates. Notably, the typical dark colony morphology appearing on Congo red agar was maintained at 28°C, indicating that this phenotype is not associated with PNAG expression in WT Y. pestis. Extracts of WT sylvatic Y. pestis strains from the Russian Federation confirmed the maximal expression of PNAG at 37°C. PNAG production by WT Y. pestis is maximal at mammalian and not insect vector temperatures, suggesting that this factor may have a role during mammalian infection.

Yersinia pestis transitions from low-temperature residence and replication in insect vectors to higher-temperature replication in mammalian hosts. Prior findings based primarily on an avirulent derivative of WT (wild-type) KIM, named KIM6+, showed that biofilm formation associated with synthesis of poly-N-acetylglucosamine (PNAG) is maximal at 21 to 28°C and decreased at 37°C. Biofilm formation was purported to facilitate the transmission of Y. pestis from fleas to mammals while having little importance in mammalian infection. Here we found that for WT strains KIM and CO92, maximal PNAG production occurs at 37°C, indicating that temperature regulation of PNAG production in WT Y. pestis is not mimicked by strain KIM6+. Additionally, we found that Congo red binding does not always correlate with PNAG production, despite its widespread use as an indicator of biofilm production. Taken together, the findings show that a role for PNAG in WT Y. pestis infection should not be disregarded and warrants further study.

Targeting pan-resistant bacteria with antibodies to a broadly conserved surface polysaccharide expressed during infection

BACKGROUND

New therapeutic targets for antibiotic-resistant bacterial pathogens are desperately needed. The bacterial surface polysaccharide poly-β-(1-6)-N-acetyl-glucosamine (PNAG) mediates biofilm formation by some bacterial species, and antibodies to PNAG can confer protective immunity. By analyzing sequenced genomes, we found that potentially multidrug-resistant bacterial species such as Klebsiella pneumoniae, Enterobacter cloacae, Stenotrophomonas maltophilia, and the Burkholderia cepacia complex (BCC) may be able to produce PNAG. Among patients with cystic fibrosis patients, highly antibiotic-resistant bacteria in the BCC have emerged as problematic pathogens, providing an impetus to study the potential of PNAG to be targeted for immunotherapy against pan-resistant bacterial pathogens.

METHODS

The presence of PNAG on BCC was assessed using a combination of bacterial genetics, microscopy, and immunochemical approaches. Antibodies to PNAG were tested using opsonophagocytic assays and for protective efficacy against lethal peritonitis in mice.

RESULTS

PNAG is expressed in vitro and in vivo by the BCC, and cystic fibrosis patients infected by the BCC species B. dolosa mounted a PNAG-specific opsonophagocytic antibody response. Antisera to PNAG mediated opsonophagocytic killing of BCC and were protective against lethal BCC peritonitis even during coinfection with methicillin-resistant Staphylococcus aureus.

CONCLUSIONS

Our findings raise potential new therapeutic options against PNAG-producing bacteria, including even pan-resistant pathogens.

Should we stay or should we go: mechanisms and ecological consequences for biofilm dispersal

In most environments, bacteria reside primarily in biofilms, which are social consortia of cells that are embedded in an extracellular matrix and undergo developmental programmes resulting in a predictable biofilm 'life cycle'. Recent research on many different bacterial species has now shown that the final stage in this life cycle includes the production and release of differentiated dispersal cells. The formation of these cells and their eventual dispersal is initiated through diverse and remarkably sophisticated mechanisms, suggesting that there are strong evolutionary pressures for dispersal from an otherwise largely sessile biofilm. The evolutionary aspect of biofilm dispersal is now being explored through the integration of molecular microbiology with eukaryotic ecological and evolutionary theory, which provides a broad conceptual framework for the diversity of specific mechanisms underlying biofilm dispersal. Here, we review recent progress in this emerging field and suggest that the merging of detailed molecular mechanisms with ecological theory will significantly advance our understanding of biofilm biology and ecology.

Synthesis of β-(1→6)-linked N-acetyl-D-glucosamine oligosaccharide substrates and their hydrolysis by Dispersin B

Dispersin B (DspB) from Aggregatibacter actinomycetemcomitans is a β-hexosaminidase exhibiting biofilm detachment activity. A series of β-(1→6)-linked N-acetyl-D-glucosamine thiophenyl glycosides with degree of polymerisation (DP) of 2, 3, 4 and 5 were synthesized, and substrate specificity of DspB was studied on the obtained oligosaccharides. For oligomer synthesis a 1+2, 2+2, 1+4 coupling strategy was applied, using bromo-sugars as glycosyl donors. The formation of 1,2-trans interglycosidic bond has been ensured by 2-phtalimido protecting group; chloroacetyl group was installed to mask temporarily the 6-hydroxyl and acetate esters were applied as permanent protecting groups. Enzymatic studies revealed that DP of the GlcNAc oligomers strongly affected the hydrolysis rate, and the hydrolytic activity of DspB on the tetramer and pentamer have been found to be approximately 10-fold higher than that of the dimer. This fact indicates that four units are required for a strong binding at the active centre of DspB. The role of aromatic amino acids W237, Y187 and Y278 in substrate specificity and catalysis was also examined using mutant enzymes.

Molecular cloning and crystal structural analysis of a novel beta-N-acetylhexosaminidase from Paenibacillus sp. TS12 capable of degrading glycosphingolipids

We report the molecular cloning and characterization of two novel beta-N-acetylhexosaminidases (beta-HEX, EC 3.2.1.52) from Paenibacillus sp. strain TS12. The two beta-HEXs (Hex1 and Hex2) were 70% identical in primary structure, and the N-terminal region of both enzymes showed significant similarity with beta-HEXs belonging to glycoside hydrolase family 20 (GH20). Interestingly, however, the C-terminal region of Hex1 and Hex2 shared no sequence similarity with the GH20 beta-HEXs or other known proteins. Both recombinant enzymes, expressed in Escherichia coli BL21(DE3), hydrolyzed the beta-N-acetylhexosamine linkage of chitooligosaccharides and glycosphingolipids such as asialo GM2 and Gb4Cer in the absence of detergent. However, the enzyme was not able to hydrolyze GM2 ganglioside in the presence or in the absence of detergent. We determined three crystal structures of Hex1; the Hex1 deletion mutant Hex1-DeltaC at a resolution of 1.8 A; Hex1-DeltaC in complex with beta-N-acetylglucosamine at 1.6 A; and Hex1-DeltaC in complex with beta-N-acetylgalactosamine at 1.9 A. We made a docking model of Hex1-DeltaC with GM2 oligosaccharide, revealing that the sialic acid residue of GM2 could hinder access of the substrate to the active site cavity. This is the first report describing the molecular cloning, characterization and X-ray structure of a procaryotic beta-HEX capable of hydrolyzing glycosphingolipids.

Amino acid residues crucial in pH regulation and proteolytic activation of N-acylethanolamine-hydrolyzing acid amidase

N-Acylethanolamine-hydrolyzing acid amidase (NAAA) is a lysosomal enzyme which hydrolyzes bioactive N-acylethanolamines, including anandamide and N-palmitoylethanolamine. NAAA shows acidic pH optimum in terms of both catalytic activity and maturation by specific proteolysis. However, molecular mechanism involved in this characteristic pH dependency remained unclear. Here we report the important role of Glu-195 of human NAAA by analyzing the mutants E195A and E195Q overexpressed in human embryonic kidney 293 cells. Concanamycin A, raising lysosomal pH, inhibited maturation of the wild-type, but not of the Glu-195 mutants. The purified precursors of the mutants, but not the wild-type, were proteolytically cleaved at pH 7.4 during 24-h incubation. Furthermore, when assayed for N-palmitoylethanolamine-hydrolyzing activity at different pH, the mutants did not exhibit a sharp peak around pH 4.5 in the pH-dependent activity profile. Mutants of other seven glutamic acid residues did not show such an abnormality. These results suggested a unique role of Glu-195 in the pH-dependent activity and proteolytic maturation. Moreover, Arg-142, Asp-145, and Asn-287 as well as previously identified Cys-126 were shown to be essential for the proteolytic activation. Since these residues were predicted to be catalytically important, the results strongly suggested that the proteolysis occurs through an autocatalytic mechanism.