Structural and functional characterization of Pseudomonas aeruginosa AlgX: role of AlgX in alginate acetylation

Applying a polysaccharide lyase from Stenotrophomonas maltophilia to disrupt alginate exopolysaccharide produced by Pseudomonas aeruginosa clinical isolates

Pseudomonas aeruginosa is considered one of the most challenging, drug-resistant, opportunistic pathogens partly due to its ability to synthesize robust biofilms. Biofilm is a mixture of extracellular polymeric substances (EPS) that encapsulates microbial cells, leading to immune evasion, antibiotic resistance, and thus higher risk of infection. In the cystic fibrosis lung environment, P. aeruginosa undergoes a mucoid transition, defined by overproduction of the exopolysaccharide alginate. Alginate encapsulation results in bacterial resistance to antibiotics and the host immune system. Given its role in airway inflammation and chronic infection, alginate is an obvious target to improve treatment for P. aeruginosa infection. Previously, we demonstrated polysaccharide lyase Smlt1473 from Stenotrophomonas maltophilia strain k279a can catalyze the degradation of multiple polyuronides in vitro, including D-mannuronic acid (poly-ManA). Poly-ManA is a major constituent of P. aeruginosa alginate, suggesting that Smlt1473 could have potential application against multidrug-resistant P. aeruginosa and perhaps other microbes with related biofilm composition. In this study, we demonstrate that Smlt1473 can inhibit and degrade alginate from P. aeruginosa. Additionally, we show that tested P. aeruginosa strains are dominant in acetylated alginate and that all but one have similar M-to-G ratios. These results indicate that variation in enzyme efficacy among the isolates is not primarily due to differences in total EPS or alginate chemical composition. Overall, these results demonstrate Smlt1473 can inhibit and degrade P. aeruginosa alginate and suggest that other factors including rate of EPS production, alginate sequence/chain length, or non-EPS components may explain differences in enzyme efficacy.

IMPORTANCE

Pseudomonas aeruginosa is a major opportunistic human pathogen in part due to its ability to synthesize biofilms that confer antibiotic resistance. Biofilm is a mixture of polysaccharides, DNA, and proteins that encapsulate cells, protecting them from antibiotics, disinfectants, and other cleaning agents. Due to its ability to increase antibiotic and immune resistance, the exopolysaccharide alginate plays a large role in airway inflammation and chronic P. aeruginosa infection. As a result, colonization with P. aeruginosa is the leading cause of morbidity and mortality in CF patients. Thus, it is an obvious target to improve the treatment regimen for P. aeruginosa infection. In this study, we demonstrate that polysaccharide lyase, Smlt1473, inhibits alginate secretion and degrades established alginate from a variety of mucoid P. aeruginosa clinical isolates. Additionally, Smlt1473 differs from other alginate lyases in that it is active against acetylated alginate, which is secreted during chronic lung infection. These results suggest that Smlt1473 may be useful in treating infections associated with alginate-producing P. aeruginosa, as well as have the potential to reduce P. aeruginosa EPS in non-clinical settings.

Plant Cell Wall Polysaccharide O-Acetyltransferases

Plant cell walls are largely composed of polysaccharide polymers, including cellulose, hemicelluloses (xyloglucan, xylan, mannan, and mixed-linkage β-1,3/1,4-glucan), and pectins. Among these cell wall polysaccharides, xyloglucan, xylan, mannan, and pectins are often O-acetylated, and polysaccharide O-acetylation plays important roles in cell wall assembly and disease resistance. Genetic and biochemical analyses have implicated the involvement of three groups of proteins in plant cell wall polysaccharide O-acetylation: trichome birefringence-like (TBL)/domain of unknown function 231 (DUF231), reduced wall acetylation (RWA), and altered xyloglucan 9 (AXY9). Although the exact roles of RWAs and AXY9 are yet to be identified, members of the TBL/DUF231 family have been found to be O-acetyltransferases responsible for the O-acetylation of xyloglucan, xylan, mannan, and pectins. Here, we provide a comprehensive overview of the occurrence of O-acetylated cell wall polysaccharides, the biochemical properties, structural features, and evolution of cell wall polysaccharide O-acetyltransferases, and the potential biotechnological applications of manipulations of cell wall polysaccharide acetylation. Further in-depth studies of the biochemical mechanisms of cell wall polysaccharide O-acetylation will not only enrich our understanding of cell wall biology, but also have important implications in engineering plants with increased disease resistance and reduced recalcitrance for biofuel production.

Acetylation of homogalacturonan and rhamnogalacturonan-I is catalyzed by a suite of trichome birefringence-like proteins

Plant cell wall polysaccharides, including xylan, mannan, xyloglucan, and pectins, are often acetylated and members of the domain of unknown function 231 (DUF231)/trichome birefringence-like (TBL) family have been shown to be O-acetyltransferases mediating the acetylation of xylan, mannan, and xyloglucan. However, little is known about the O-acetyltransferases responsible for pectin acetylation. In this report, we biochemically characterized a suite of Arabidopsis DUF231/TBL proteins for their roles in pectin acetylation. We generated 24 TBL recombinant proteins in mammalian cells and demonstrated that 10 of them were able to transfer acetyl groups from acetyl-CoA onto the pectins homogalacturonan (HG) or rhamnogalacturonan-I (RG-I), and thus were named pectin O-acetyltransferase 1 to 10 (POAT1 to 10). It was found that POAT2,4,9,10 specifically acetylated HG and POAT5,6 acetylated RG-I, whereas POAT1,3,7,8 could act on both HG and RG-I. The acetylation of HG and RG-I by POATs was further corroborated by hydrolysis with pectin acetylesterases and by nuclear magnetic resonance spectroscopy. In addition, mutations of the conserved GDS and DXXH motifs in POAT3 and POAT8 were shown to lead to a loss of their ability to acetylate HG and RG-I. Furthermore, simultaneous RNA interference downregulation of POAT1,3,6,7,8 resulted in reduced cell expansion, impaired plant growth, and decreased pectin acetylation. Together, our findings indicate that these POATs are pectin O-acetyltransferases involved in acetylation of the pectin polysaccharides HG and RG-I.

Synthesis and application of bacterial exopolysaccharides

Exopolysaccharides are produced and excreted by bacteria in the generation of biofilms to provide a protective environment. These polysaccharides are generally generated as heterogeneous polymers of varying length, featuring diverse substitution patterns. To obtain well-defined fragments of these polysaccharides, organic synthesis often is the method of choice, as it allows for full control over chain length and the installation of a pre-determined substitution pattern. This review presents several recent syntheses of exopolysaccharide fragments of Pseudomonas aeruginosa and Staphylococcus aureus and illustrates how these have been used to study biosynthesis enzymes and generate synthetic glycoconjugate vaccines.

Pseudomonas aeruginosa biofilm exopolysaccharides: assembly, function, and degradation

The biofilm matrix is a fortress; sheltering bacteria in a protective and nourishing barrier that allows for growth and adaptation to various surroundings. A variety of different components are found within the matrix including water, lipids, proteins, extracellular DNA, RNA, membrane vesicles, phages, and exopolysaccharides. As part of its biofilm matrix, Pseudomonas aeruginosa is genetically capable of producing three chemically distinct exopolysaccharides - alginate, Pel, and Psl - each of which has a distinct role in biofilm formation and immune evasion during infection. The polymers are produced by highly conserved mechanisms of secretion, involving many proteins that span both the inner and outer bacterial membranes. Experimentally determined structures, predictive modelling of proteins whose structures are yet to be solved, and structural homology comparisons give us insight into the molecular mechanisms of these secretion systems, from polymer synthesis to modification and export. Here, we review recent advances that enhance our understanding of P. aeruginosa multiprotein exopolysaccharide biosynthetic complexes, and how the glycoside hydrolases/lyases within these systems have been commandeered for antimicrobial applications.

Pseudomonas aeruginosa AlgF is a protein-protein interaction mediator required for acetylation of the alginate exopolysaccharide

Enzymatic modifications of bacterial exopolysaccharides enhance immune evasion and persistence during infection. In the Gram-negative opportunistic pathogen Pseudomonas aeruginosa, acetylation of alginate reduces opsonic killing by phagocytes and improves reactive oxygen species scavenging. Although it is well known that alginate acetylation in P. aeruginosa requires AlgI, AlgJ, AlgF, and AlgX, how these proteins coordinate polymer modification at a molecular level remains unclear. Here, we describe the structural characterization of AlgF and its protein interaction network. We characterize direct interactions between AlgF and both AlgJ and AlgX in vitro and demonstrate an association between AlgF and AlgX, as well as AlgJ and AlgI, in P. aeruginosa. We determine that AlgF does not exhibit acetylesterase activity and is unable to bind to polymannuronate in vitro. Therefore, we propose that AlgF functions to mediate protein-protein interactions between alginate acetylation enzymes, forming the periplasmic AlgJFXK (AlgJ-AlgF-AlgX-AlgK) acetylation and export complex required for robust biofilm formation.

WssI from the Gram-Negative Bacterial Cellulose Synthase is an O-acetyltransferase that Acts on Cello-oligomers with Several Acetyl Donor Substrates

In microbial biofilms, bacterial cells are encased in a self-produced matrix of polymers (e.g., exopolysaccharides) that enable surface adherence and protect against environmental stressors. For example, the wrinkly spreader phenotype of Pseudomonas fluorescens colonizes food/water sources and human tissue to form robust biofilms that can spread across surfaces. This biofilm largely consists of bacterial cellulose produced by the cellulose synthase proteins encoded by the wss operon, which also occurs in other species, including pathogenic Achromobacter species. Although phenotypic mutant analysis of the wssFGHI genes has previously shown that they are responsible for acetylation of bacterial cellulose, their specific roles remain unknown and distinct from the recently identified cellulose phosphoethanolamine modification found in other species. Here we have purified the C-terminal soluble form of WssI from P. fluorescens and A. insuavis and demonstrated acetyl-esterase activity with chromogenic substrates. The kinetic parameters (k_cat/K_M values of 13 and 8.0 M^-1∙ s^-1, respectively) indicate that these enzymes are up to four times more catalytically efficient than the closest characterized homolog, AlgJ from the alginate synthase. Unlike AlgJ and its cognate alginate polymer, WssI also demonstrated acetyltransferase activity onto cellulose oligomers (e.g., cellotetraose to cellohexaose) with multiple acetyl-donor substrates (pNP-Ac, MU-Ac and acetyl-CoA). Finally, a high-throughput screen identified three low micromolar WssI inhibitors that may be useful for chemically interrogating cellulose acetylation and biofilm formation.

Structure of the AlgKX modification and secretion complex required for alginate production and biofilm attachment in Pseudomonas aeruginosa

Synthase-dependent secretion systems are a conserved mechanism for producing exopolysaccharides in Gram-negative bacteria. Although widely studied, it is not well understood how these systems are organized to coordinate polymer biosynthesis, modification, and export across both membranes and the peptidoglycan. To investigate how synthase-dependent secretion systems produce polymer at a molecular level, we determined the crystal structure of the AlgK-AlgX (AlgKX) complex involved in Pseudomonas aeruginosa alginate exopolysaccharide acetylation and export. We demonstrate that AlgKX directly binds alginate oligosaccharides and that formation of the complex is vital for polymer production and biofilm attachment. Finally, we propose a structural model for the AlgEKX outer membrane modification and secretion complex. Together, our study provides insight into how alginate biosynthesis proteins coordinate production of a key exopolysaccharide involved in establishing persistent Pseudomonas lung infections.

Outstanding questions on xylan biosynthesis

Xylan is the second most abundant polysaccharide in plant biomass. It is a crucial component of cell wall structure as well as a significant factor contributing to biomass recalcitrance. Xylan consists of a linear chain of β-1,4-linked xylosyl residues that are often substituted with glycosyl side chains, such as glucuronosyl/methylglucuronosyl and arabinofuranosyl residues, and acetylated at O-2 and/or O-3. Xylan from gymnosperms and dicots contains a unique reducing end tetrasaccharide sequence that is not detected in xylan from grasses, bryophytes and seedless vascular plants. Grass xylan is heavily decorated at O-3 with arabinofuranosyl residues that are frequently esterified with hydroxycinnamates. Genetic and biochemical studies have uncovered a number of genes involved in xylan backbone elongation and acetylation, xylan glycosyl substitutions and their modifications, and the synthesis of the unique xylan reducing end tetrasaccharide sequence, but some outstanding issues on the biosynthesis of xylan still remain unanswered. Here, we provide a brief overview of xylan structure and focus on discussion of the current understanding and open questions on xylan biosynthesis. Further elucidation of the biochemical mechanisms underlying xylan biosynthesis will not only shed new insights into cell wall biology but also provide molecular tools for genetic modification of biomass composition tailored for diverse end uses.

The SGNH hydrolase family: a template for carbohydrate diversity

The substitution and de-substitution of carbohydrate materials are important steps in the biosynthesis and/or breakdown of a wide variety of biologically important polymers. The SGNH hydrolase superfamily is a group of related and well-studied proteins with a highly conserved catalytic fold and mechanism composed of 16 member families. SGNH hydrolases can be found in vertebrates, plants, fungi, bacteria, and archaea, and play a variety of important biological roles related to biomass conversion, pathogenesis, and cell signaling. The SGNH hydrolase superfamily is chiefly composed of a diverse range of carbohydrate-modifying enzymes, including but not limited to the carbohydrate esterase families 2, 3, 6, 12 and 17 under the carbohydrate-active enzyme classification system and database (CAZy.org). In this review, we summarize the structural and functional features that delineate these subfamilies of SGNH hydrolases, and which generate the wide variety of substrate preferences and enzymatic activities observed of these proteins to date.

The Pseudomonas aeruginosa homeostasis enzyme AlgL clears the periplasmic space of accumulated alginate during polymer biosynthesis

Pseudomonas aeruginosa is an opportunistic human pathogen and a leading cause of chronic infection in the lungs of individuals with cystic fibrosis. After colonization, P. aeruginosa often undergoes a phenotypic conversion to mucoidy, characterized by overproduction of the alginate exopolysaccharide. This conversion is correlated with poorer patient prognoses. The majority of genes required for alginate synthesis, including the alginate lyase, algL, are located in a single operon. Previous investigations of AlgL have resulted in several divergent hypotheses regarding the protein’s role in alginate production. To address these discrepancies, we determined the structure of AlgL and, using multiple sequence alignments, identified key active site residues involved in alginate binding and catalysis. In vitro enzymatic analysis of active site mutants highlights R249 and Y256 as key residues required for alginate lyase activity. In a genetically engineered P. aeruginosa strain where alginate biosynthesis is under arabinose control, we found that AlgL is required for cell viability and maintaining membrane integrity during alginate production. We demonstrate that AlgL functions as a homeostasis enzyme to clear the periplasmic space of accumulated polymer. Constitutive expression of the AlgU/T sigma factor mitigates the effects of an algL deletion during alginate production, suggesting that an AlgU/T-regulated protein or proteins can compensate for an algL deletion. Together, our study demonstrates the role of AlgL in alginate biosynthesis, explains the discrepancies observed previously across other P. aeruginosa ΔalgL genetic backgrounds, and clarifies the existing divergent data regarding the function of AlgL as an alginate degrading enzyme.

Comparison of Biochemical Characteristics, Action Models, and Enzymatic Mechanisms of a Novel Exolytic and Two Endolytic Lyases with Mannuronate Preference

Recent explorations of tool-like alginate lyases have been focused on their oligosaccharide-yielding properties and corresponding mechanisms, whereas most were reported as endo-type with α-L-guluronate (G) preference. Less is known about the β-D-mannuronate (M) preference, whose commercial production and enzyme application is limited. In this study, we elucidated Aly6 of Flammeovirga sp. strain MY04 as a novel M-preferred exolytic bifunctional lyase and compared it with AlgLs of Pseudomonas aeruginosa (Pae-AlgL) and Azotobacter vinelandii (Avi-AlgL), two typical M-specific endolytic lyases. This study demonstrated that the AlgL and heparinase_II_III modules play indispensable roles in determining the characteristics of the recombinant exo-type enzyme rAly6, which is preferred to degrade M-enriched substrates by continuously cleaving various monosaccharide units from the nonreducing end, thus yielding various size-defined ΔG-terminated oligosaccharides as intermediate products. By contrast, the endolytic enzymes Pae-rAlgL and Avi-rAlgL varied their action modes specifically against M-enriched substrates and finally degraded associated substrate chains into various size-defined oligosaccharides with a succession rule, changing from ΔM to ΔG-terminus when the product size increased. Furthermore, site-directed mutations and further protein structure tests indicated that H¹⁹⁵NHSTW is an active, half-conserved, and essential enzyme motif. This study provided new insights into M-preferring lyases for novel resource discoveries, oligosaccharide preparations, and sequence determinations.

Investigation of core machinery for biosynthesis of Vi antigen capsular polysaccharides in Gram-negative bacteria

Salmonella enterica serovar Typhi causes typhoid fever. It possesses a Vi antigen capsular polysaccharide coat that is important for virulence and is the basis of a current glycoconjugate vaccine. Vi antigen is also produced by environmental Bordetella isolates, while mammal-adapted Bordetella species (such as Bordetella bronchiseptica) produce a capsule of undetermined structure that cross-reacts with antibodies recognizing Vi antigen. The Vi antigen backbone is composed of poly-α-(1→4)-linked N-acetylgalactosaminuronic acid, modified with O-acetyl residues that are necessary for vaccine efficacy. Despite its biological and biotechnological importance, some central aspects of Vi antigen production are poorly understood. Here we demonstrate that TviE and TviD, two proteins encoded in the viaB (Vi antigen production) locus, interact and are the Vi antigen polymerase and O-acetyltransferase, respectively. Structural modeling and site-directed mutagenesis reveal that TviE is a GT4-family glycosyltransferase. While TviD has no identifiable homologs beyond Vi antigen systems in other bacteria, structural modeling suggests that it belongs to the large SGNH hydrolase family, which contains other O-acetyltransferases. Although TviD possesses an atypical catalytic triad, its O-acetyltransferase function was verified by antibody reactivity and ¹³C NMR data for tviD-mutant polysaccharide. The B. bronchiseptica genetic locus predicts a mode of synthesis distinct from classical S. enterica Vi antigen production, but which still involves TviD and TviE homologs that are both active in a reconstituted S. Typhi system. These findings provide new insight into Vi antigen production and foundational information for the glycoengineering of Vi antigen production in heterologous bacteria.

Weaving of bacterial cellulose by the Bcs secretion systems

Cellulose is the most abundant biological compound on Earth and while it is the predominant building constituent of plants, it is also a key extracellular matrix component in many diverse bacterial species. While bacterial cellulose was first described in the 19th century, it was not until this last decade that a string of structural works provided insights into how the cellulose synthase BcsA, assisted by its inner-membrane partner BcsB, senses c-di-GMP to simultaneously polymerize its substrate and extrude the nascent polysaccharide across the inner bacterial membrane. It is now established that bacterial cellulose can be produced by several distinct types of cellulose secretion systems and that in addition to BcsAB, they can feature multiple accessory subunits, often indispensable for polysaccharide production. Importantly, the last years mark significant progress in our understanding not only of cellulose polymerization per se but also of the bigger picture of bacterial signaling, secretion system assembly, biofilm formation and host tissue colonization, as well as of structural and functional parallels of this dominant biosynthetic process between the bacterial and eukaryotic domains of life. Here, we review current mechanistic knowledge on bacterial cellulose secretion with focus on the structure, assembly and cooperativity of Bcs secretion system components.

Inhibition of Pseudomonas aeruginosa Alginate Synthesis by Ebselen Oxide and Its Analogues

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen that is frequently found in the airways of cystic fibrosis (CF) patients due to the dehydrated mucus that collapses the underlying cilia and prevents mucociliary clearance. During this life-long chronic infection, P. aeruginosa cell accumulates mutations that lead to inactivation of the mucA gene that results in the constitutive expression of algD-algA operon and the production of alginate exopolysaccharide. The viscous alginate polysaccharide further occludes the airways of CF patients and serves as a protective matrix to shield P. aeruginosa from host immune cells and antibiotic therapy. Development of inhibitors of alginate production by P. aeruginosa would reduce the negative impact from this viscous polysaccharide. In addition to transcriptional regulation, alginate biosynthesis requires allosteric activation by bis (3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP) binding to an Alg44 protein. Previously, we found that ebselen (Eb) and ebselen oxide (EbO) inhibited diguanylate cyclase from synthesizing c-di-GMP. In this study, we show that EbO, Eb, ebsulfur (EbS), and their analogues inhibit alginate production. Eb and EbS can covalently modify the cysteine 98 (C98) residue of Alg44 and prevent its ability to bind c-di-GMP. However, P. aeruginosa with Alg44 C98 substituted with alanine or serine was still inhibited for alginate production by Eb and EbS. Our results indicate that EbO, Eb, and EbS are lead compounds for reducing alginate production by P. aeruginosa. Future development of these inhibitors could provide a potential treatment for CF patients infected with mucoid P. aeruginosa.

Identification of the Clostridial cellulose synthase and characterization of the cognate glycosyl hydrolase, CcsZ

Biofilms are community structures of bacteria enmeshed in a self-produced matrix of exopolysaccharides. The biofilm matrix serves numerous roles, including resilience and persistence, making biofilms a subject of research interest among persistent clinical pathogens of global health importance. Our current understanding of the underlying biochemical pathways responsible for biosynthesis of these exopolysaccharides is largely limited to Gram-negative bacteria. Clostridia are a class of Gram-positive, anaerobic and spore-forming bacteria and include the important human pathogens Clostridium perfringens, Clostridium botulinum and Clostridioides difficile, among numerous others. Several species of Clostridia have been reported to produce a biofilm matrix that contains an acetylated glucan linked to a series of hypothetical genes. Here, we propose a model for the function of these hypothetical genes, which, using homology modelling, we show plausibly encode a synthase complex responsible for polymerization, modification and export of an O-acetylated cellulose exopolysaccharide. Specifically, the cellulose synthase is homologous to that of the known exopolysaccharide synthases in Gram-negative bacteria. The remaining proteins represent a mosaic of evolutionary lineages that differ from the described Gram-negative cellulose exopolysaccharide synthases, but their predicted functions satisfy all criteria required for a functional cellulose synthase operon. Accordingly, we named these hypothetical genes ccsZABHI, for the Clostridial cellulose synthase (Ccs), in keeping with naming conventions for exopolysaccharide synthase subunits and to distinguish it from the Gram-negative Bcs locus with which it shares only a single one-to-one ortholog. To test our model and assess the identity of the exopolysaccharide, we subcloned the putative glycoside hydrolase encoded by ccsZ and solved the X-ray crystal structure of both apo- and product-bound CcsZ, which belongs to glycoside hydrolase family 5 (GH-5). Although not homologous to the Gram-negative cellulose synthase, which instead encodes the structurally distinct BcsZ belonging to GH-8, we show CcsZ displays specificity for cellulosic materials. This specificity of the synthase-associated glycosyl hydrolase validates our proposal that these hypothetical genes are responsible for biosynthesis of a cellulose exopolysaccharide. The data we present here allowed us to propose a model for Clostridial cellulose synthesis and serves as an entry point to an understanding of cellulose biofilm formation among class Clostridia.

Structural and mechanistic basis of capsule O-acetylation in Neisseria meningitidis serogroup A

O-Acetylation of the capsular polysaccharide (CPS) of Neisseria meningitidis serogroup A (NmA) is critical for the induction of functional immune responses, making this modification mandatory for CPS-based anti-NmA vaccines. Using comprehensive NMR studies, we demonstrate that O-acetylation stabilizes the labile anomeric phosphodiester-linkages of the NmA-CPS and occurs in position C3 and C4 of the N-acetylmannosamine units due to enzymatic transfer and non-enzymatic ester migration, respectively. To shed light on the enzymatic transfer mechanism, we solved the crystal structure of the capsule O-acetyltransferase CsaC in its apo and acceptor-bound form and of the CsaC-H228A mutant as trapped acetyl-enzyme adduct in complex with CoA. Together with the results of a comprehensive mutagenesis study, the reported structures explain the strict regioselectivity of CsaC and provide insight into the catalytic mechanism, which relies on an unexpected Gln-extension of a classical Ser-His-Asp triad, embedded in an α/β-hydrolase fold.

Structural insights into the putative bacterial acetylcholinesterase ChoE and its substrate inhibition mechanism

Mammalian acetylcholinesterase (AChE) is well-studied, being important in both cholinergic brain synapses and the peripheral nervous systems and also a key drug target for many diseases. In contrast, little is known about the structures and molecular mechanism of prokaryotic acetylcholinesterases. We report here the structural and biochemical characterization of ChoE, a putative bacterial acetylcholinesterase from Pseudomonas aeruginosa Analysis of WT and mutant strains indicated that ChoE is indispensable for P. aeruginosa growth with acetylcholine as the sole carbon and nitrogen source. The crystal structure of ChoE at 1.35 Å resolution revealed that this enzyme adopts a typical fold of the SGNH hydrolase family. Although ChoE and eukaryotic AChEs catalyze the same reaction, their overall structures bear no similarities constituting an interesting example of convergent evolution. Among Ser-38, Asp-285, and His-288 of the catalytic triad residues, only Asp-285 was not essential for ChoE activity. Combined with kinetic analyses of WT and mutant proteins, multiple crystal structures of ChoE complexed with substrates, products, or reaction intermediate revealed the structural determinants for substrate recognition, snapshots of the various catalytic steps, and the molecular basis of substrate inhibition at high substrate concentrations. Our results indicate that substrate inhibition in ChoE is due to acetate release being blocked by the binding of a substrate molecule in a nonproductive mode. Because of the distinct overall folds and significant differences of the active site between ChoE and eukaryotic AChEs, these structures will serve as a prototype for other prokaryotic acetylcholinesterases.

Analysis of the alginate O-acetylation machinery in Pseudomonas aeruginosa

O-acetylation of alginate produced by the opportunistic human pathogen Pseudomonas aeruginosa significantly contributes to its pathogenesis. Three proteins, AlgI, AlgJ and AlgF have been implicated to form a complex and act together with AlgX for O-acetylation of alginate. AlgI was proposed to transfer the acetyl group across the cytoplasmic membrane, while periplasmic AlgJ was hypothesised to transfer the acetyl group to AlgX that acetylates alginate. To elucidate the proposed O-acetylation multiprotein complex, isogenic knockout mutants of algI, algJ and algF genes were generated in the constitutively alginate overproducing P. aeruginosa PDO300 to enable mutual stability studies. All knockout mutants were O-acetylation negative and complementation with the respective genes in cis or trans restored O-acetylation of alginate. Interestingly, only the AlgF deletion impaired alginate production suggesting a link to the alginate polymerisation/secretion multiprotein complex. Mutual stability experiments indicated that AlgI and AlgF interact independent of AlgJ as well as impact on stability of the alginate polymerisation/secretion multiprotein complex. Deletion of AlgJ did not destabilise AlgX and vice versa. When the alginate polymerase, Alg8, was absent, then AlgI and AlgF stability was strongly impaired supporting a link of the O-acetylation machinery with alginate polymerisation. Pull-down experiments suggested that AlgI interacts with AlgJ, while AlgF interacts with AlgJ and AlgI. Overall, these results suggested that AlgI-AlgJ-AlgF form a multiprotein complex linked via Alg8 to the envelope-spanning alginate polymerisation/secretion multiprotein complex to mediate O-acetylation of nascent alginate. Here, we provide the first insight on how the O-acetylation machinery is associated with alginate production.

Cytosolic Acetyl-CoA Generated by ATP-Citrate Lyase Is Essential for Acetylation of Cell Wall Polysaccharides

Plant cell wall polysaccharides, including xylan, glucomannan, xyloglucan and pectin, are often acetylated. Although a number of acetyltransferases responsible for the acetylation of some of these polysaccharides have been biochemically characterized, little is known about the source of acetyl donors and how acetyl donors are translocated into the Golgi, where these polysaccharides are synthesized. In this report, we investigated roles of ATP-citrate lyase (ACL) that generates cytosolic acetyl-CoA in cell wall polysaccharide acetylation and effects of simultaneous mutations of four Reduced Wall Acetylation (RWA) genes on acetyl-CoA transport into the Golgi in Arabidopsis thaliana. Expression analyses of genes involved in the generation of acetyl-CoA in different subcellular compartments showed that the expression of several ACL genes responsible for cytosolic acetyl-CoA synthesis was elevated in interfascicular fiber cells and induced by secondary wall-associated transcriptional activators. Simultaneous downregulation of the expression of ACL genes was demonstrated to result in a substantial decrease in the degree of xylan acetylation and a severe alteration in secondary wall structure in xylem vessels. In addition, the degree of acetylation of other cell wall polysaccharides, including glucomannan, xyloglucan and pectin, was also reduced. Moreover, Golgi-enriched membrane vesicles isolated from the rwa1/2/3/4 quadruple mutant were found to exhibit a drastic reduction in acetyl-CoA transport activity compared with the wild type. These findings indicate that cytosolic acetyl-CoA generated by ACL is essential for cell wall polysaccharide acetylation and RWAs are required for its transport from the cytosol into the Golgi.

A Bifunctional UDP-Sugar 4-Epimerase Supports Biosynthesis of Multiple Cell Surface Polysaccharides in Sinorhizobium meliloti

Sinorhizobium meliloti produces multiple extracellular glycans, including among others, lipopolysaccharides (LPS), and the exopolysaccharides (EPS) succinoglycan (SG) and galactoglucan (GG). These polysaccharides serve cell protective roles. Furthermore, SG and GG promote the interaction of S. meliloti with its host Medicago sativa in root nodule symbiosis. ExoB has been suggested to be the sole enzyme catalyzing synthesis of UDP-galactose in S. meliloti (A. M. Buendia, B. Enenkel, R. Köplin, K. Niehaus, et al. Mol Microbiol 5:1519-1530, 1991, https://doi.org/10.1111/j.1365-2958.1991.tb00799.x). Accordingly, exoB mutants were previously found to be affected in the synthesis of the galactose-containing glycans LPS, SG, and GG and consequently, in symbiosis. Here, we report that the S. meliloti Rm2011 uxs1-uxe-apsS-apsH1-apsE-apsH2 (SMb20458-63) gene cluster directs biosynthesis of an arabinose-containing polysaccharide (APS), which contributes to biofilm formation, and is solely or mainly composed of arabinose. Uxe has previously been identified as UDP-xylose 4-epimerase. Collectively, our data from mutational and overexpression analyses of the APS biosynthesis genes and in vitro enzymatic assays indicate that Uxe functions as UDP-xylose 4- and UDP-glucose 4-epimerase catalyzing UDP-xylose/UDP-arabinose and UDP-glucose/UDP-galactose interconversions, respectively. Overexpression of uxe suppressed the phenotypes of an exoB mutant, evidencing that Uxe can functionally replace ExoB. We suggest that under conditions stimulating expression of the APS biosynthesis operon, Uxe contributes to the synthesis of multiple glycans and thereby to cell protection, biofilm formation, and symbiosis. Furthermore, we show that the C₂H₂ zinc finger transcriptional regulator MucR counteracts the previously reported CuxR-c-di-GMP-mediated activation of the APS biosynthesis operon. This integrates the c-di-GMP-dependent control of APS production into the opposing regulation of EPS biosynthesis and swimming motility in S. meliloti IMPORTANCE Bacterial extracellular polysaccharides serve important cell protective, structural, and signaling roles. They have particularly attracted attention as adhesives and matrix components promoting biofilm formation, which significantly contributes to resistance against antibiotics. In the root nodule symbiosis between rhizobia and leguminous plants, extracellular polysaccharides have a signaling function. UDP-sugar 4-epimerases are important enzymes in the synthesis of the activated sugar substrates, which are frequently shared between multiple polysaccharide biosynthesis pathways. Thus, these enzymes are potential targets to interfere with these pathways. Our finding of a bifunctional UDP-sugar 4-epimerase in Sinorhizobium meliloti generally advances the knowledge of substrate promiscuity of such enzymes and specifically of the biosynthesis of extracellular polysaccharides involved in biofilm formation and symbiosis in this alphaproteobacterium.

Secondary cell wall biosynthesis

Contents Summary 1703 I. Introduction 1703 II. Cellulose biosynthesis 1705 III. Xylan biosynthesis 1709 IV. Glucomannan biosynthesis 1713 V. Lignin biosynthesis 1714 VI. Concluding remarks 1717 Acknowledgements 1717 References 1717 SUMMARY: Secondary walls are synthesized in specialized cells, such as tracheary elements and fibers, and their remarkable strength and rigidity provide strong mechanical support to the cells and the plant body. The main components of secondary walls are cellulose, xylan, glucomannan and lignin. Biochemical, molecular and genetic studies have led to the discovery of most of the genes involved in the biosynthesis of secondary wall components. Cellulose is synthesized by cellulose synthase complexes in the plasma membrane and the recent success of in vitro synthesis of cellulose microfibrils by a single recombinant cellulose synthase isoform reconstituted into proteoliposomes opens new doors to further investigate the structure and functions of cellulose synthase complexes. Most genes involved in the glycosyl backbone synthesis, glycosyl substitutions and acetylation of xylan and glucomannan have been genetically characterized and the biochemical properties of some of their encoded enzymes have been investigated. The genes and their encoded enzymes participating in monolignol biosynthesis and modification have been extensively studied both genetically and biochemically. A full understanding of how secondary wall components are synthesized will ultimately enable us to produce plants with custom-designed secondary wall composition tailored to diverse applications.

A partial reconstitution implicates DltD in catalyzing lipoteichoic acid d-alanylation

Modifications to the Gram-positive bacterial cell wall play important roles in antibiotic resistance and pathogenesis, but the pathway for the d-alanylation of teichoic acids (DLT pathway), a ubiquitous modification, is poorly understood. The d-alanylation machinery includes two membrane proteins of unclear function, DltB and DltD, which are somehow involved in transfer of d-alanine from a carrier protein inside the cell to teichoic acids on the cell surface. Here, we probed the role of DltD in the human pathogen Staphylococcus aureus using both cell-based and biochemical assays. We first exploited a known synthetic lethal interaction to establish the essentiality of each gene in the DLT pathway for d-alanylation of lipoteichoic acid (LTA) and confirmed this by directly detecting radiolabeled d-Ala-LTA both in cells and in vesicles prepared from mutant strains of S. aureus We developed a partial reconstitution of the pathway by using cell-derived vesicles containing DltB, but no other components of the d-alanylation pathway, and showed that d-alanylation of previously formed lipoteichoic acid in the DltB vesicles requires the presence of purified and reconstituted DltA, DltC, and DltD, but not of the LTA synthase LtaS. Finally, based on the activity of DltD mutants in cells and in our reconstituted system, we determined that Ser-70 and His-361 are essential for d-alanylation activity, and we propose that DltD uses a catalytic dyad to transfer d-alanine to LTA. In summary, we have developed a suite of assays for investigating the bacterial DLT pathway and uncovered a role for DltD in LTA d-alanylation.

Members of the DUF231 Family are O-Acetyltransferases Catalyzing 2-O- and 3-O-Acetylation of Mannan

Mannans are hemicellulosic polysaccharides commonly found in the primary and secondary cell walls of land plants, and their mannosyl residues are often acetylated at O-2 and O-3. Currently, little is known about the genes responsible for the acetylation of mannans. In this report, we investigated the roles of a subgroup of DUF231 proteins including 11 from Arabidopsis thaliana and one from Amorphophallus konjac in mannan acetylation. Acetyltransferase activity assays of their recombinant proteins revealed that four Arabidopsis DUF231 proteins possessed an enzymatic activity capable of transferring acetyl groups from acetyl-CoA onto the mannohexaose acceptor, and thus were named mannan O-acetyltransferases (MOAT1, MOAT2, MOAT3 and MOAT4). Their close homolog from A. konjac (named AkMOAT1) also exhibited mannan O-acetyltransferase activity. Structural analysis of the MOAT-catalyzed reaction products demonstrated that these MOATs catalyzed 2-O- and 3-O-monoacetylation of mannosyl residues, an acetyl substitution pattern similar to that of Arabidopsis glucomannan. Site-directed mutagenesis showed that mutations of the conserved residues in the GDS and DXXH motifs of MOAT3 abolished its acetyltransferase activity, indicating the essential roles of these motifs in its activity. In addition, simultaneous RNA interference (RNAi) inhibition of the expression of the four Arabidopsis MOAT genes led to a drastic reduction in the degree of acetyl substitutions in glucomannan, further corroborating their role in glucomannan acetylation. Together, these results present the first lines of biochemical and genetic evidence demonstrating that these four Arabidopsis DUF231 members and their close A. konjac homolog are mannan O-acetyltransferases.

Xyloglucan O-acetyltransferases from Arabidopsis thaliana and Populus trichocarpa catalyze acetylation of fucosylated galactose residues on xyloglucan side chains

AXY4/XGOAT1, AXY4L/XGOAT2 and PtrXGOATs are O-acetyltransferases acetylating fucosylated galactose residues on xyloglucan and AXY9 does not directly catalyze O-acetylation of xyloglucan but exhibits weak acetylesterase activity. Xyloglucan is a major hemicellulose that cross-links cellulose in the primary walls of dicot plants and the galactose (Gal) residues on its side chains can be mono- and di-O-acetylated. In Arabidopsis thaliana, mutations of three AXY (altered xyloglucan) genes, AXY4, AXY4L and AXY9, have previously been shown to cause a reduction in xyloglucan acetylation, but their biochemical functions remain to be investigated. In this report, we demonstrated that recombinant proteins of AXY4/XGOAT1 (xyloglucan O-acetyltransferase1), AXY4L/XGOAT2 and their close homologs from Populus trichocarpa, PtrXGOATs, displayed O-acetyltransferase activities transferring acetyl groups from acetyl CoA onto xyloglucan oligomers. Structural analysis of XGOAT-catalyzed reaction products revealed that XGOATs mediated predominantly 6-O-monoacetylation and a much lesser degree of 3-O and 4-O-monoacetylation and 4,6-di-O-acetylation of Gal residues on xyloglucan side chains. XGOATs appeared to preferentially acetylate fucosylated Gal residues with little activity toward non-fucosylated Gal residues. Mutations of the conserved amino acid residues in the GDS and DXXH motifs in AXY4/XGOAT1 resulted in a drastic reduction in its ability to transfer acetyl groups onto xyloglucan oligomers. In addition, although recombinant AXY9 was unable to transfer acetyl groups from acetyl CoA onto xyloglucan oligomers, it was catalytically active as demonstrated by its weak acetylesterase activity that was also exhibited by AXY4/XGOAT1 and AXY4L/XGOAT2. Furthermore, we showed that the AXY8 fucosidase was able to hydrolyze fucosyl residues from both non-acetylated and acetylated xyloglucan oligomers. These findings provide biochemical evidence that AXY4/XGOAT1, AXY4L/XGOAT2 and PtrXGOATs are xyloglucan O-acetyltransferases catalyzing acetyl transfer onto fucosylated Gal residues on xyloglucan side chains and the defucosylation of these acetylated side chains by apoplastic AXY8 generates side chains with acetylated, non-fucosylated Gal residues.

Mechanistic Pathways for Peptidoglycan O-Acetylation and De-O-Acetylation

The post-synthetic O-acetylation of the essential component of bacterial cell walls, peptidoglycan (PG), is performed by many pathogenic bacteria to help them evade the lytic action of innate immunity responses. Occurring at the C-6 hydroxyl of N-acetylmuramoyl residues, this modification to the glycan backbone of PG sterically blocks the activity of lysozymes. As such, the enzyme responsible for this modification in Gram-positive bacteria is recognized as a virulence factor. With Gram-negative bacteria, the O-acetylation of PG provides a means of control of their autolysins at the substrate level. In this review, we discuss the pathways for PG O-acetylation and de-O-acetylation and the structure and function relationship of the O-acetyltransferases and O-acetylesterases that catalyze these reactions. The current understanding of their mechanisms of action is presented and the prospects of targeting these systems for the development of novel therapeutics are explored.

Peptidoglycan Modification by the Catalytic Domain of Streptococcus pneumoniae OatA Follows a Ping-Pong Bi-Bi Mechanism of Action

Streptococcus pneumoniae among other Gram-positive pathogens produces O-acetylated peptidoglycan using the enzyme OatA. This process occurs through the transfer of an acetyl group from a donor to the hydroxyl group of an acceptor sugar. While it has been established that this process involves the extracellular, catalytic domain of OatA ( SpOatA_C), mechanistic insight is still unavailable. This study examined the enzymatic characteristics of SpOatA_C-catalyzed reactions through analysis of both pre-steady- and steady-state kinetics. Our findings clearly show that SpOatA_C follows a ping-pong bi-bi mechanism of action involving a covalent acetyl-enzyme intermediate. The modified residue was verified to be the catalytic nucleophile, Ser438. The pH dependence of the enzyme kinetics revealed that a single ionizable group is involved, which is consistent with the participation of a His residue. Single-turnover kinetics of esterase activity demonstrated that k₂ ≫ k₃, revealing that the rate-limiting step for the hydrolytic reaction was the breakdown of the acetyl-enzyme intermediate with a half-life of >1 min. The previous assignment of Asn491 as an oxyanion hole residue was also confirmed as its replacement with Ala resulted in a 50-fold decrease in catalytic efficiency relative to that of wild-type SpOatA_C. However, this loss of catalytic efficiency was mostly due to a large increase in K_M, suggesting that Asn491 contributes more to substrate binding.

Chemical synthesis of guanosine diphosphate mannuronic acid (GDP-ManA) and its C-4-O-methyl and C-4-deoxy congeners

Described is the first synthesis of guanosine diphosphate mannuronic acid (GDP-ManA), the sugar donor used by algae and bacteria for the production of alginate, an anionic polysaccharide composed of β-d-mannuronic acid (ManA) and α-l-guluronic acid (GulA). Understanding the biosynthesis of these polyanionic polysaccharides on the molecular level, opens up avenues to use and modulate the biosynthesis machinery for biotechnological and therapeutic applications. The synthesis reported here delivers multi-milligram amounts of the GDP-ManA donor that can be used to study the polymerase (Alg8 in Pseudomonas aeruginosa) that generates the poly-ManA chain. Also reported is the assembly of two close analogues of GDP-ManA: the first bears a C-4-O-methyl group, while the second has been deoxygenated at this position. Both molecules may be used as "chain stoppers" in future enzymatic ManA polymerisation reactions. The crucial pyrophosphate linkage of the GDP-mannuronic acids has been constructed by the phosphorylation of the appropriate ManA-1-phosphates with a guanosine phosphoramidite.

Biological function of a polysaccharide degrading enzyme in the periplasm

Carbohydrate polymers are industrially and medically important. For instance, a polysaccharide, alginate (from seaweed), is widely used in food, textile and pharmaceutical industries. Certain bacteria also produce alginate through membrane spanning multi-protein complexes. Using Pseudomonas aeruginosa as a model organism, we investigated the biological function of an alginate degrading enzyme, AlgL, in alginate production and biofilm formation. We showed that AlgL negatively impacts alginate production through its enzymatic activity. We also demonstrated that deletion of AlgL does not interfere with polymer length control, epimerization degree or stability of the biosynthesis complex, arguing that AlgL is a free periplasmic protein dispensable for alginate production. This was further supported by our protein-stability and interaction experiments. Interestingly, over-production of AlgL interfered with polymer length control, suggesting that AlgL could be loosely associated with the biosynthesis complex. In addition, chromosomal expression of algL enhanced alginate O-acetylation; both attachment and dispersal stages of the bacterial biofilm lifecycle were sensitive to the level of O-acetylation. Since this modification also protects the pathogen against host defences and enhances other virulence factors, chromosomal expression of algL could be important for the pathogenicity of this organism. Overall, this work improves our understanding of bacterial alginate production and provides new knowledge for alginate production and disease control.

Analysis of respiratory activity and carbon usage of a mutant of Azotobacter vinelandii impaired in poly-β-hydroxybutyrate synthesis

In this study, the respiratory activity and carbon usage of the mutant strain of A. vinelandii AT6, impaired in poly-β-hydroxybutyrate (PHB) production, and their relationship with the synthesis of alginate were evaluated. The alginate yield and the specific oxygen uptake rate were higher (2.5-fold and 62 %, respectively) for the AT6 strain, compared to the control strain (ATCC 9046), both in shake flasks cultures and in bioreactor, under fixed dissolved oxygen tension (1 %). In contrast, the degree of acetylation was similar in both strains. These results, together with the analysis of carbon usage (% C-mol), suggest that in the case of the AT6 strain, the flux of acetyl-CoA (precursor molecule for PHB biosynthesis and alginate acetylation) was diverted to the respiratory chain passing through the tricarboxylic acids cycle, and an important % C-mol was directed through alginate biosynthesis, up to 25.9 % and to a lesser extent, to biomass production (19.7 %).

Adhesins Involved in Attachment to Abiotic Surfaces by Gram-Negative Bacteria

During the first step of biofilm formation, initial attachment is dictated by physicochemical and electrostatic interactions between the surface and the bacterial envelope. Depending on the nature of these interactions, attachment can be transient or permanent. To achieve irreversible attachment, bacterial cells have developed a series of surface adhesins promoting specific or nonspecific adhesion under various environmental conditions. This article reviews the recent advances in our understanding of the secretion, assembly, and regulation of the bacterial adhesins during biofilm formation, with a particular emphasis on the fimbrial, nonfimbrial, and discrete polysaccharide adhesins in Gram-negative bacteria.

Alginate Biosynthesis inAzotobacter vinelandii: Overview of Molecular Mechanisms in Connection with the Oxygen Availability

The Gram-negative bacteriumAzotobacter vinelandiican synthetize the biopolymer alginate that has material properties appropriate for plenty of applications in industry as well as in medicine. In order to settle the foundation for improving alginate production without compromising its quality, a better understanding of the polymer biosynthesis and the mechanism of regulation during fermentation processes is necessary. This knowledge is crucial for the development of novel production strategies. Here, we highlight the key aspects of alginate biosynthesis that can lead to producing an alginate with specific material properties with particular focus on the role of oxygen availability linked with the molecular mechanisms involved in the alginate production.

EpsA is an essential gene in exopolysaccharide production in Lactobacillus johnsonii FI9785

Lactobacillus johnsonii FI9785 has an eps gene cluster which is required for the biosynthesis of homopolymeric exopolysaccharides (EPS)‐1 and heteropolymeric EPS‐2 as a capsular layer. The first gene of the cluster, epsA, is the putative transcriptional regulator. In this study we showed the crucial role of epsA in EPS biosynthesis by demonstrating that deletion of epsA resulted in complete loss of both EPS‐1 and EPS‐2 on the cell surface. Plasmid complementation of the epsA gene fully restored EPS production, as confirmed by transmission electron microscopy and nuclear magnetic resonance (NMR) analysis. Furthermore, this complementation resulted in a twofold increase in the expression levels of this gene, which almost doubled amounts of EPS production in comparison with the wild‐type strain. Analysis of EPS by NMR showed an increased ratio of the heteropolysaccharide to homopolysaccharide in the complemented strain and allowed identification of the acetylated residue in EPS‐2 as the (1,4)‐linked βGlcp unit, with the acetyl group located at O‐6. These findings indicate that epsA is a positive regulator of EPS production and that EPS production can be manipulated by altering its expression.

9-O-Acetylation of sialic acids is catalysed by CASD1 via a covalent acetyl-enzyme intermediate

Sialic acids, terminal sugars of glycoproteins and glycolipids, play important roles in development, cellular recognition processes and host–pathogen interactions. A common modification of sialic acids is 9-O-acetylation, which has been implicated in sialoglycan recognition, ganglioside biology, and the survival and drug resistance of acute lymphoblastic leukaemia cells. Despite many functional implications, the molecular basis of 9-O-acetylation has remained elusive thus far. Following cellular approaches, including selective gene knockout by CRISPR/Cas genome editing, we here show that CASD1—a previously identified human candidate gene—is essential for sialic acid 9-O-acetylation. In vitro assays with the purified N-terminal luminal domain of CASD1 demonstrate transfer of acetyl groups from acetyl-coenzyme A to CMP-activated sialic acid and formation of a covalent acetyl-enzyme intermediate. Our study provides direct evidence that CASD1 is a sialate O-acetyltransferase and serves as key enzyme in the biosynthesis of 9-O-acetylated sialoglycans.

9-O-Acetylation is one of the most common modifications of sialic acids, implicated in sialoglycan recognition and ganglioside biology. Here, the authors show that the key enzyme for the biosynthesis of 9-O-acetylated sialoglycans is CASD1, which uses CMP-activated sialic acid as acceptor substrate.

Alginate-modifying enzymes: biological roles and biotechnological uses

Alginate denotes a group of industrially important 1-4-linked biopolymers composed of the C-5-epimers β-D-mannuronic acid (M) and α-L-guluronic acid (G). The polysaccharide is manufactured from brown algae where it constitutes the main structural cell wall polymer. The physical properties of a given alginate molecule, e.g., gel-strength, water-binding capacity, viscosity and biocompatibility, are determined by polymer length, the relative amount and distribution of G residues and the acetyl content, all of which are controlled by alginate modifying enzymes. Alginate has also been isolated from some bacteria belonging to the genera Pseudomonas and Azotobacter, and bacterially synthesized alginate may be O-acetylated at O-2 and/or O-3. Initially, alginate is synthesized as polymannuronic acid, and some M residues are subsequently epimerized to G residues. In bacteria a mannuronan C-5-epimerase (AlgG) and an alginate acetylase (AlgX) are integral parts of the protein complex necessary for alginate polymerization and export. All alginate-producing bacteria use periplasmic alginate lyases to remove alginate molecules aberrantly released to the periplasm. Alginate lyases are also produced by organisms that utilize alginate as carbon source. Most alginate-producing organisms encode more than one mannuronan C-5 epimerase, each introducing its specific pattern of G residues. Acetylation protects against further epimerization and from most alginate lyases. An enzyme from Pseudomonas syringae with alginate deacetylase activity has been reported. Functional and structural studies reveal that alginate lyases and epimerases have related enzyme mechanisms and catalytic sites. Alginate lyases are now utilized as tools for alginate characterization. Secreted epimerases have been shown to function well in vitro, and have been engineered further in order to obtain enzymes that can provide alginates with new and desired properties for use in medical and pharmaceutical applications.

Enzymatic modifications of exopolysaccharides enhance bacterial persistence

Biofilms are surface-attached communities of bacterial cells embedded in a self-produced matrix that are found ubiquitously in nature. The biofilm matrix is composed of various extracellular polymeric substances, which confer advantages to the encapsulated bacteria by protecting them from eradication. The matrix composition varies between species and is dependent on the environmental niche that the bacteria inhabit. Exopolysaccharides (EPS) play a variety of important roles in biofilm formation in numerous bacterial species. The ability of bacteria to thrive in a broad range of environmental settings is reflected in part by the structural diversity of the EPS produced both within individual bacterial strains as well as by different species. This variability is achieved through polymerization of distinct sugar moieties into homo- or hetero-polymers, as well as post-polymerization modification of the polysaccharide. Specific enzymes that are unique to the production of each polymer can transfer or remove non-carbohydrate moieties, or in other cases, epimerize the sugar units. These modifications alter the physicochemical properties of the polymer, which in turn can affect bacterial pathogenicity, virulence, and environmental adaptability. Herein, we review the diversity of modifications that the EPS alginate, the Pel polysaccharide, Vibrio polysaccharide, cepacian, glycosaminoglycans, and poly-N-acetyl-glucosamine undergo during biosynthesis. These are EPS produced by human pathogenic bacteria for which studies have begun to unravel the effect modifications have on their physicochemical and biological properties. The biological advantages these polymer modifications confer to the bacteria that produce them will be discussed. The expanding list of identified modifications will allow future efforts to focus on linking these modifications to specific biosynthetic genes and biofilm phenotypes.

Alginate Polymerization and Modification Are Linked in Pseudomonas aeruginosa

The molecular mechanisms of alginate polymerization/modification/secretion by a proposed envelope-spanning multiprotein complex are unknown. Here, bacterial two-hybrid assays and pulldown experiments showed that the catalytic subunit Alg8 directly interacts with the proposed copolymerase Alg44 while embedded in the cytoplasmic membrane. Alg44 additionally interacts with the lipoprotein AlgK bridging the periplasmic space. Site-specific mutagenesis of Alg44 showed that protein-protein interactions and stability were independent of conserved amino acid residues R17 and R21, which are involved in c-di-GMP binding, the N-terminal PilZ domain, and the C-terminal 26 amino acids. Site-specific mutagenesis was employed to investigate the c-di-GMP-mediated activation of alginate polymerization by the PilZ_Alg44 domain and Alg8. Activation was found to be different from the proposed activation mechanism for cellulose synthesis. The interactive role of Alg8, Alg44, AlgG (epimerase), and AlgX (acetyltransferase) on alginate polymerization and modification was studied by using site-specific deletion mutants, inactive variants, and overproduction of subunits. The compositions, molecular masses, and material properties of resulting novel alginates were analyzed. The molecular mass was reduced by epimerization, while it was increased by acetylation. Interestingly, when overproduced, Alg44, AlgG, and the nonepimerizing variant AlgG(D324A) increased the degree of acetylation, while epimerization was enhanced by AlgX and its nonacetylating variant AlgX(S269A). Biofilm architecture analysis showed that acetyl groups promoted cell aggregation while nonacetylated polymannuronate alginate promoted stigmergy. Overall, this study sheds new light on the arrangement of the multiprotein complex involved in alginate production. Furthermore, the activation mechanism and the interplay between polymerization and modification of alginate were elucidated.

This study provides new insights into the molecular mechanisms of the synthesis of the unique polysaccharide, alginate, which not only is an important virulence factor of the opportunistic human pathogen Pseudomonas aeruginosa but also has, due to its material properties, many applications in medicine and industry. Unraveling the assembly and composition of the alginate-synthesizing and envelope-spanning multiprotein complex will be of tremendous significance for the scientific community. We identified a protein-protein interaction network inside the multiprotein complex and studied its relevance with respect to alginate polymerization/modification as well as the c-di-GMP-mediated activation mechanism. A relationship between alginate polymerization and modification was shown. Due to the role of alginate in pathogenesis as well as its unique material properties harnessed in numerous applications, results obtained in this study will aid the design and development of inhibitory drugs as well as the commercial bacterial production of tailor-made alginates.

A molecular description of cellulose biosynthesis

Cellulose is the most abundant biopolymer on Earth, and certain organisms from bacteria to plants and animals synthesize cellulose as an extracellular polymer for various biological functions. Humans have used cellulose for millennia as a material and an energy source, and the advent of a lignocellulosic fuel industry will elevate it to the primary carbon source for the burgeoning renewable energy sector. Despite the biological and societal importance of cellulose, the molecular mechanism by which it is synthesized is now only beginning to emerge. On the basis of recent advances in structural and molecular biology on bacterial cellulose synthases, we review emerging concepts of how the enzymes polymerize glucose molecules, how the nascent polymer is transported across the plasma membrane, and how bacterial cellulose biosynthesis is regulated during biofilm formation. Additionally, we review evolutionary commonalities and differences between cellulose synthases that modulate the nature of the cellulose product formed.

Mechanism of action of peptidoglycan O-acetyltransferase B involves a Ser-His-Asp catalytic triad

The O-acetylation of the essential cell wall polymer peptidoglycan is essential in many bacteria for their integrity and survival, and it is catalyzed by peptidoglycan O-acetlytransferase B (PatB). Using PatB from Neisseria gonorrhoeae as the model, we have shown previously that the enzyme has specificity for polymeric muropeptides that possess tri- and tetrapeptide stems and that rates of reaction increase with increasing degrees of polymerization. Here, we present the catalytic mechanism of action of PatB, the first to be described for an O-acetyltransferase of any bacterial exopolysaccharide. The influence of pH on PatB activity was investigated, and pKa values of 6.4-6.45 and 6.25-6.35 for the enzyme-substrate complex (kcat vs pH) and the free enzyme (kcat·KM(-1) vs pH), respectively, were determined for the respective cosubstrates. The enzyme is partially inactivated by sulfonyl fluorides but not by EDTA, suggesting the participation of a serine residue in its catalytic mechanism. Alignment of the known and hypothetical PatB amino acid sequences identified Ser133, Asp302, and His305 as three invariant amino acid residues that could potentially serve as a catalytic triad. Replacement of Asp302 with Ala resulted in an enzyme with less than 20% residual activity, whereas activity was barely detectable with (His305 → Ala)PatB and (Ser133 → Ala)PatB was totally inactive. The reaction intermediate of the transferase reaction involving acetyl- and propionyl-acyl donors was trapped on both the wild-type and (Asp302 → Ala) enzymes and LC-MS/MS analysis of tryptic peptides identified Ser133 as the catalytic nucleophile. A transacetylase mechanism is proposed based on the mechanism of action of serine esterases.

P. aeruginosa SGNH hydrolase-like proteins AlgJ and AlgX have similar topology but separate and distinct roles in alginate acetylation

The O-acetylation of polysaccharides is a common modification used by pathogenic organisms to protect against external forces. Pseudomonas aeruginosa secretes the anionic, O-acetylated exopolysaccharide alginate during chronic infection in the lungs of cystic fibrosis patients to form the major constituent of a protective biofilm matrix. Four proteins have been implicated in the O-acetylation of alginate, AlgIJF and AlgX. To probe the biological function of AlgJ, we determined its structure to 1.83 Å resolution. AlgJ is a SGNH hydrolase-like protein, which while structurally similar to the N-terminal domain of AlgX exhibits a distinctly different electrostatic surface potential. Consistent with other SGNH hydrolases, we identified a conserved catalytic triad composed of D190, H192 and S288 and demonstrated that AlgJ exhibits acetylesterase activity in vitro. Residues in the AlgJ signature motifs were found to form an extensive network of interactions that are critical for O-acetylation of alginate in vivo. Using two different electrospray ionization mass spectrometry (ESI-MS) assays we compared the abilities of AlgJ and AlgX to bind and acetylate alginate. Binding studies using defined length polymannuronic acid revealed that AlgJ exhibits either weak or no detectable polymer binding while AlgX binds polymannuronic acid specifically in a length-dependent manner. Additionally, AlgX was capable of utilizing the surrogate acetyl-donor 4-nitrophenyl acetate to catalyze the O-acetylation of polymannuronic acid. Our results, combined with previously published in vivo data, suggest that the annotated O-acetyltransferases AlgJ and AlgX have separate and distinct roles in O-acetylation. Our refined model for alginate acetylation places AlgX as the terminal acetlytransferase and provides a rationale for the variability in the number of proteins required for polysaccharide O-acetylation.

Bacteria utilize many defense strategies to protect themselves against external forces. One mechanism used by the bacterium Pseudomonas aeruginosa is the production of the long sugar polymer alginate. The bacteria use this polymer to form a biofilm – a barrier to protect against antibiotics and the host immune response. During its biosynthesis alginate undergoes a chemical modification whereby acetate is added to the polymer. Acetylation of alginate is important as this modification makes the bacterial biofilm less susceptible to recognition and clearance by the host immune system. In this paper we present the atomic structure of AlgJ; one of four proteins required for O-acetylation of the polymer. AlgJ is structurally similar to AlgX, which we have shown previously is also required for alginate acetylation. To understand why both enzymes are required for O-acetylation we functionally characterized the proteins and found that although AlgJ exhibits acetylesterase activity – catalyzing the removal of acetyl groups from a surrogate substrate – it does not bind to short mannuornic acid polymers. In contrast, AlgX bound alginate in a length-dependent manner and was capable of transfering acetate from a surrogate substrate onto alginate. This has allowed us to not only understand how acetate is added to alginate, but increases our understanding of how acetate is added to other bacterial sugar polymers.

Impact of alginate-producing Pseudomonas aeruginosa on alveolar macrophage apoptotic cell clearance

Pseudomonas aeruginosa infection is a hallmark of lung disease in cystic fibrosis. Acute infection with P. aeruginosa profoundly inhibits alveolar macrophage clearance of apoptotic cells (efferocytosis) via direct effect of virulence factors. During chronic infection, P. aeruginosa evades host defense by decreased virulence, which includes the production or, in the case of mucoidy, overproduction of alginate. The impact of alginate on innate immunity, in particular on macrophage clearance of apoptotic cells is not known. We hypothesized that P. aeruginosa strains that exhibit reduced virulence impair macrophage clearance of apoptotic cells and we investigated if the polysaccharide alginate produced by mucoid P. aeruginosa is sufficient to inhibit alveolar macrophage efferocytosis. Rat alveolar or human peripheral blood monocyte (THP-1)-derived macrophage cell lines were exposed in vitro to exogenous alginate or to wild type or alginate-overproducing mucoid P. aeruginosa prior to challenge with apoptotic human Jurkat T-lymphocytes. The importance of LPS contamination and that of structural integrity of alginate polymers was tested using alginate of different purities and alginate lyase, respectively. Alginate inhibited alveolar macrophage efferocytosis in a dose- and time-dependent manner. This effect was augmented but not exclusively attributed to lipopolysaccharide (LPS) present in alginates. Alginate-producing P. aeruginosa inhibited macrophage efferocytosis by more than 50%. A mannuronic-specific alginate lyase did not restore efferocytosis inhibited by exogenous guluronic-rich marine alginate, but had a marked beneficial effect on efferocytosis of alveolar macrophages exposed to mucoid P. aeruginosa. Despite decreased virulence, mucoid P. aeruginosa may contribute to chronic airway inflammation through significant inhibition of alveolar clearance of apoptotic cells and debris. The mechanism by which mucoid bacteria inhibit efferocytosis may involve alginate production and synergy with LPS, suggesting that alginate lyase may be an attractive therapeutic approach to airway inflammation in cystic fibrosis and other chronic obstructive pulmonary diseases characterized by P. aeruginosa colonization.

Microbial alginate production, modification and its applications

Alginate is an important polysaccharide used widely in the food, textile, printing and pharmaceutical industries for its viscosifying, and gelling properties. All commercially produced alginates are isolated from farmed brown seaweeds. These algal alginates suffer from heterogeneity in composition and material properties. Here, we will discuss alginates produced by bacteria; the molecular mechanisms involved in their biosynthesis; and the potential to utilize these bacterially produced or modified alginates for high-value applications where defined material properties are required.