Characterization of the ß-KDO Transferase KpsS, the Initiating Enzyme in the Biosynthesis of the Lipid Acceptor for Escherichia coli Polysialic Acid.

Antiphagocytic capsular polysaccharides are key components of effective vaccines against pathogenic bacteria. Neisseria meningitidis groups B and C, as well as Escherichia coli serogroups K1 and K92, are coated with polysialic acid capsules. Although the chemical structure of these polysaccharides and the organization of the associated gene clusters have been described for many years, only recently have the details of the biosynthetic pathways been discovered. The polysialic acid chains are synthesized by polysialyltransferases on a proposed phosphatidylglycerol lipid acceptor with a poly keto-deoxyoctulosonate (KDO) linker. Synthesis of this acceptor requires at least three enzymes in E. coli K1: KpsS, KpsC, and NeuE. In this report, we have characterized the ß-KDO glycosyltransferase KpsS, the first enzyme in the pathway for lipid acceptor synthesis. After purification of KpsS in a soluble active form, we investigated its function and substrate specificity and showed that KpsS can transfer a KDO residue to a fluorescently labeled phosphatidylglycerol lipid. The enzyme tolerated various lengths of fatty acid acyl chains on the phosphatidylglycerol, including fluorescent tags, but exhibited a preference for phosphatidylglycerol diacylated with longer fatty acid chains as indicated by the smaller Kd and Km values for substrates with chains with more than 14 members. Additional structural analysis of the KpsS product confirmed that KpsS transfers KDO from CMP-KDO to the 1-hydroxyl of phosphatidylglycerol to form a ß-KDO linkage.

Effect of Acceptor Chain Length and Hydrophobicity on Polymerization Kinetics of the Neisseria meningitidis Group C Polysialyltransferase.

Polysialic acids (PSA) are important extracellular virulence factors of the human pathogens Neisseria meningitidis and Escherichia coli. The importance of these polysaccharides in virulence make the polysialyltransferases (PST) targets for therapeutic drugs and protein engineering to facilitate efficient vaccine production. Here, we have generated recombinant bovine nucleotide monophosphate kinase to facilitate steady state kinetic assays of the PST. We have characterized the N. meningitidis group C (NmC) PST kinetically, using substrate analogues to describe the polymerization reaction. We observed a decrease in Km as the length of the oligo-sialic acid acceptor was increased, indicating a tighter binding of longer oligomers. In addition, we observed a biphasic relationship between kcat and chain length, which can be attributed to a switch in the mechanism of transfer of sialic acid from distributive to processive as the chain length increased above six sialic acid units. Substitution of donor substrate with the analogue CMP-9-F-sialic acid had minimal effect on acceptor Km, but it decreased kcat 6-fold. We propose that this decrease in kcat is caused by a destabilization of the transition state and/or an increase affinity of the product due to presence of the fluoro substituent. The acceptor's hydrophobicity also plays a role in catalysis. The kinetic analysis of the NmC PST with hydrophobic aglycon acceptor substrates indicated that they bind tighter and are turned over at a faster rate than the α-2,9 polysialic acid substrates lacking the hydrophobic end. This finding suggests the presence of a secondary ligand binding site that tethers the acceptor substrate to the enzyme active site.

Interaction of Neisseria meningitidis Group X N-acetylglucosamine-1-phosphotransferase with its donor substrate.

Neisseria meningitidis Group X is an emerging cause of bacterial meningitis in Sub-Saharan Africa. The capsular polysaccharide of Group X is a homopolymer of N-acetylglucosamine α(1-4) phosphate and is a vaccine target for prevention of disease associated with this meningococcal serogroup. We have demonstrated previously that the formation of the polymer is catalyzed by a phosphotransferase which transfers N-acetylglucosamine-1-phosphate from UDP-N-acetylglucosamine to the 4-hydroxyl of the N-acetylglucosamine on the nonreducing end of the growing chain. In this study, we use substrate analogs of UDP-GlcNAc to define the enzyme/donor substrate interactions critical for catalysis. Our kinetic analysis of the phosphotransferase reaction is consistent with a sequential mechanism of substrate addition and product release. The use of novel uracil modified analogs designed by Wagner et al. enabled us to assess whether the CsxA-catalyzed reaction is consistent with a donor dependent conformational change. As expected with this model for glycosyltransferases, UDP-GlcNAc analogs with bulky uracil modifications are not substrates but are inhibitors. An analog with a smaller iodo uracil substitution is a substrate and a less potent inhibitor. Moreover, our survey of analogs with modifications on the N-acetylglucosamine residue of the sugar nucleotide donor highlights the importance of substituents at C2 and C4 of the sugar residue. The hydroxyl group at C4 and the structure of the acyl group at C2 are very important for specificity and substrate interactions during the polymerization reaction. While most analogs modified at C2 were inhibitors, acetamido analogs were also substrates suggesting the importance of the carbonyl group.

Conjugate Vaccines from Bacterial Antigens by Squaric Acid Chemistry: A Closer Look.

By using O-SP-core (O-SPcNH2 ) polysaccharide, isolated from Vibrio cholera O1 lipopolysaccharide (LPS) and related synthetic substances, a detailed study of factors that affect conjugation of bacterial polysaccharides to protein carriers through squaric acid chemistry to form conjugate vaccines has been carried out. Several previously unrecognized processes that take place during the squarate labeling of the O-SPcNH2 and subsequent conjugation of the formed squarate (O-SPcNH-SqOMe) have been identified. The efficiency of conjugation at pH 8.5, 9.0, and 9.5 to bovine serum albumin (BSA) and to the recombinant tetanus toxin fragment C (rTT-Hc) has been determined. The study led to a protocol for more efficient labeling of O-SPcNH2 antigen with the methyl squarate group, to yield a higher-quality, more potent squarate conjugation reagent. Its use resulted in about twofold increases in conjugation efficiency (from 23-26 % on BSA to 51 % on BSA and 55 % on rTT-Hc). The spent conjugation reagent could be recovered and regenerated by treatment with MeI in the absence of additional base. The immunological properties of the experimental vaccine made from the regenerated conjugation reagent were comparable with those of the immunogen made from the parent O-SPcNH-SqOMe.

Characterization of the meningococcal serogroup X capsule N-acetylglucosamine-1-phosphotransferase.

Neisseria meningitidis serogroups A, B, C, Y, W135 and X are responsible for most cases of meningococcal meningitis. Neisseria meningitidis serogroup X has recently emerged as a contributor to outbreaks of disease in Africa, but there is currently no vaccine against serogroup X. Understanding of the biosynthesis of the serogroup X capsular polysaccharide would provide useful tools for vaccine production. The serogroup X polysaccharide is a homopolymer of (α1→4)-linked N-acetylglucosamine (GlcNAc)-1-phosphate. It has been shown that the gene cluster xcbABC encodes synthesis of this polysaccharide. The xcbA gene product has significant homology with sacB, which is responsible for synthesis of the Neisseria serogroup A capsular polysaccharide, an (α1→6)-N-acetylmannosamine-1-phosphate homopolymer. The xcbA protein also shares homology with the catalytic domain of human N-acetylglucosamine-1-phosphoryltransferase, a key enzyme in the mannose-6-phosphate receptor pathway. In this study, we show that xcbA in the appropriate background is sufficient for the synthesis of N. meningitidis serogroup X polysaccharide. By ELISA we detected polysaccharide in fractions of Escherichia coli expressing the xcbA gene. We isolated polysaccharide from an E. coli strain expressing XcbA and demonstrated that this polysaccharide has a (13)C-NMR spectrum identical to that of polysaccharide isolated from N. meningitidis Group X. We also demonstrate that the purified XcbA protein is an N-acetylglucosamine-1-phosphotransferase that transfers N-acetylglucosamine-1-phosphate from UDP-GlcNAc to the 4-hydroxyl of an N-acetylglucosamine-1-phosphate oligosaccharide. Oligosaccharides fluorescently labeled at the aglycon are extended by XcbA only after the 4-phosphate occupying the non-reducing GlcNAc has been removed. The minimum size of fluorescent acceptors is a trisaccharide.

Chemoenzymatic synthesis of immunogenic meningococcal group C polysialic acid-tetanus Hc fragment glycoconjugates.

Vaccination with meningococcal glycoconjugate vaccines has decreased the incidence of invasive meningitis worldwide. These vaccines contain purified capsular polysaccharides attached to a carrier protein. Because of derivatization chemistries used in the process, conjugation of polysaccharide to protein often results in heterogeneous mixtures. Well-defined vaccines are needed to determine the relationship between vaccine structure and generated immune response. Here, we describe efforts to produce well-defined vaccine candidates by chemoenzymatic synthesis. Chemically synthesized lactosides were substrates for recombinant sialyltransferase enzymes from Camplyobacter jejuni and Neisseria meningitidis serogroup C. These resulting oligosialic acids have the same α(2-9) sialic acid repeat structure as Neisseria polysaccharide capsule with the addition of a conjugatable azide aglycon. The degree of polymerization (DP) of carbohydrate products was controlled by inclusion of the inhibitor CMP-9-deoxy-NeuNAc. Polymers with estimated DP < 47 (median DP 25) and DP < 100 (median DP 51) were produced. The receptor binding domain of the tetanus toxin protein (TetHc) was coupled as a carrier to the enzymatically synthesized oligosialic acids. Recombinant TetHc was derivatized with an alkyne squarate. Protein modification sites were determined by trypsin proteolysis followed by LC/MS-MS(E) analysis of peptides. Oligosialic acid azides were conjugated to modified TetHc via click chemistry. These chemoenzymatically prepared glycoconjugates were reactive in immunoassays with specific antibodies against either group C polysaccharide or TetHc. Sera of mice immunized with oligosialic acid-TetHc glycoconjugates contained much greater levels of polysaccharide-reactive IgG than the sera of control mice receiving unconjugated oligosialic acids. There was no apparent difference between glycoconjugates containing oligosaccharides of DP < 47 and DP < 100. These results suggest that chemoenzymatic synthesis may provide a viable method for making defined meningococcal vaccine candidates.

Development of Shigella conjugate vaccines targeting Shigella flexneri 2a and S. flexneri 3a using a simple platform-approach conjugation by squaric acid chemistry.

There is a need for vaccines effective against shigella infection in young children in resource-limited areas. Protective immunity against shigella infection targets the O-specific polysaccharide (OSP) component of lipopolysaccharide. Inducing immune responses to polysaccharides in young children can be problematic, but high level and durable responses can be induced by presenting polysaccharides conjugated to carrier proteins. An effective shigella vaccine will need to be multivalent, targeting the most common global species and serotypes such as Shigella flexneri 2a, S. flexneri 3a, S. flexneri 6, and S. sonnei. Here we report the development of shigella conjugate vaccines (SCV) targeting S. flexneri 2a (SCV-Sf2a) and 3a (SCV-Sf3a) using squaric acid chemistry to result in single point sun-burst type display of OSP from carrier protein rTTHc, a 52 kDa recombinant protein fragment of the heavy chain of tetanus toxoid. We confirmed structure and demonstrated that these conjugates were recognized by serotype-specific monoclonal antibodies and convalescent sera of humans recovering from shigellosis in Bangladesh, suggesting correct immunological display of OSP. We vaccinated mice and found induction of serotype-specific OSP and LPS IgG responses, as well as rTTHc-specific IgG responses. Vaccination induced serotype-specific bactericidal antibody responses against S. flexneri, and vaccinated animals were protected against keratoconjunctivitis (Sereny test) and intraperitoneal challenge with virulent S. flexneri 2a and 3a, respectively. Our results support further development of this platform conjugation technology in the development of shigella conjugate vaccines for use in resource-limited settings.

Vaccination of Rabbits with a Cholera Conjugate Vaccine Comprising O-Specific Polysaccharide and a Recombinant Fragment of Tetanus Toxin Heavy Chain Induces Protective Immune Responses against Vibrio cholerae O1.

There is a need for next-generation cholera vaccines that provide high-level and durable protection in young children in cholera-endemic areas. A cholera conjugate vaccine (CCV) is in development to address this need. This vaccine contains the O-specific polysaccharide (OSP) of Vibrio cholerae O1 conjugated via squaric acid chemistry to a recombinant fragment of the tetanus toxin heavy chain (OSP:rTTHc). This vaccine has been shown previously to be immunogenic and protective in mice and found to be safe in a recent preclinical toxicological analysis in rabbits. We took advantage of excess serum samples collected as part of the toxicological study and assessed the immunogenicity of CCV OSP:rTTHc in rabbits. We found that vaccination with CCV induced OSP-, lipopolysaccharide (LPS)-, and rTTHc-specific immune responses in rabbits, that immune responses were functional as assessed by vibriocidal activity, and that immune responses were protective against death in an established virulent challenge assay. CCV OSP:rTTHc immunogenicity in two animal model systems (mice and rabbits) is encouraging and supports further development of this vaccine for evaluation in humans.

Characterization and acceptor preference of a soluble meningococcal group C polysialyltransferase.

Vaccines against Neisseria meningitidis group C are based on its α-2,9-linked polysialic acid capsular polysaccharide. This polysialic acid expressed on the surface of N. meningitidis and in the absence of specific antibody serves to evade host defense mechanisms. The polysialyltransferase (PST) that forms the group C polysialic acid (NmC PST) is located in the cytoplasmic membrane. Until recently, detailed characterization of bacterial polysialyltransferases has been hampered by a lack of availability of soluble enzyme preparations. We have constructed chimeras of the group C polysialyltransferase that catalyzes the formation α-2,9-polysialic acid as a soluble enzyme. We used site-directed mutagenesis to determine the region of the enzyme necessary for synthesis of the α-2,9 linkage. A chimera of NmB and NmC PSTs containing only amino acids 1 to 107 of the NmB polysialyltransferase catalyzed the synthesis of α-2,8-polysialic acid. The NmC polysialyltransferase requires an exogenous acceptor for catalytic activity. While it requires a minimum of a disialylated oligosaccharide to catalyze transfer, it can form high-molecular-weight α-2,9-polysialic acid in a nonprocessive fashion when initiated with an α-2,8-polysialic acid acceptor. De novo synthesis in vivo requires an endogenous acceptor. We attempted to reconstitute de novo activity of the soluble group C polysialyltransferase with membrane components. We found that an acapsular mutant with a defect in the polysialyltransferase produces outer membrane vesicles containing an acceptor for the α-2,9-polysialyltransferase. This acceptor is an amphipathic molecule and can be elongated to produce polysialic acid that is reactive with group C-specific antibody.

Scalable production and immunogenicity of a cholera conjugate vaccine.

There is a need to develop cholera vaccines that are protective in young children under 5 years of age, which induce long-term immunity, and which can be incorporated into the Expanded Programme of Immunization (EPI) in cholera-endemic countries. The degree of protection afforded by currently available oral cholera vaccines (OCV) to young children is significantly lower than that induced by vaccination of older vaccine recipients. Immune responses that protect against cholera target the O-specific polysaccharide (OSP) of Vibrio cholerae, and young children have poor immunological responses to bacterial polysaccharides, which are T cell independent antigens. To overcome this, we have developed a cholera conjugate vaccine (CCV) containing the OSP of V. cholerae O1, the main cause of endemic and epidemic cholera. Here, we describe production of CCV through a scalable manufacturing process and preclinical evaluation of immunogenicity in the presence and absence of aluminum phosphate (alum) as an adjuvant. The vaccine displays V. cholerae O1 Inaba OSP in sun-burst display via single point attachment of core oligosaccharide to a recombinant tetanus toxoid heavy chain fragment (rTTHc). Two different pilot-scale production batches of non-GMP CCV were manufactured and characterized in terms of physico-chemical properties and immunogenicity. In preclinical testing, the vaccine induced OSP- and lipopolysaccharide (LPS)-specific IgG and IgM responses, vibriocidal responses, memory B cell responses, and protection in a V. cholerae O1 challenge model. The addition of alum to the administered vaccine increased OSP-specific immune responses. These results support evaluation of CCV in humans.

Multimeric bivalent immunogens from recombinant tetanus toxin HC fragment, synthetic hexasaccharides, and a glycopeptide adjuvant.

Using recombinant tetanus toxin H(C) fragment (rTT-H(C)) as carrier, we prepared multimeric bivalent immunogens featuring the synthetic hexasaccharide fragment of O-PS of Vibrio cholerae O:1, serotype Ogawa, in combination with either the synthetic hexasaccharide fragment of O-PS of Vibrio cholerae O:1, serotype Inaba, or a synthetic disaccharide tetrapeptide peptidoglycan fragment as adjuvant. The conjugation reaction was effected by squaric acid chemistry and monitored in virtually real time by SELDI-TOF MS. In this way, we could prepare well-defined immunogens with predictable carbohydrate-carrier ratio, whose molecular mass and the amount of each saccharide attached could be independently determined. The ability to prepare such neoglycoconjugates opens unprecedented possibilities for preparation of conjugate vaccines for bacterial diseases from synthetic carbohydrates.

Chemical structure and genetic organization of the E. coli O6:K15 capsular polysaccharide.

Capsular polysaccharides are important virulence factors in pathogenic bacteria. Characterizing the structural components and biosynthetic pathways for these polysaccharides is key to our ability to design vaccines and other preventative therapies that target encapsulated pathogens. Many gram-negative pathogens such as Neisseria meningitidis and Escherichia coli express acidic capsules. The E. coli K15 serotype has been identified as both an enterotoxigenic and uropathogenic pathogen. Despite its relevance as a disease-causing serotype, the associated capsular polysaccharide remains poorly characterized. We describe in this report the chemical structure of the K15 polysaccharide, based on chemical analysis and nuclear magnetic resonance (NMR) data. The repeating structure of the K15 polysaccharide consists of 4)-α-GlcpNAc-(1 → 5)-α-KDOp-(2 → partially O-acetylated at 3-hydroxyl of GlcNAc. We also report, the organization of the gene cluster responsible for capsule biosynthesis. We identify genes in this cluster that potentially encode an O-acetyltransferase, an N-acetylglucosamine transferase, and a KDO transferase consistent with the structure we report.

Preclinical Development of a Fusion Peptide Conjugate as an HIV Vaccine Immunogen.

The vaccine elicitation of broadly neutralizing antibodies against HIV-1 is a long-sought goal. We previously reported the amino-terminal eight residues of the HIV-1-fusion peptide (FP8) - when conjugated to the carrier protein, keyhole limpet hemocyanin (KLH) - to be capable of inducing broadly neutralizing responses against HIV-1 in animal models. However, KLH is a multi-subunit particle derived from a natural source, and its manufacture as a clinical product remains a challenge. Here we report the preclinical development of recombinant tetanus toxoid heavy chain fragment (rTTHC) linked to FP8 (FP8-rTTHC) as a suitable FP-conjugate vaccine immunogen. We assessed 16 conjugates, made by coupling the 4 most prevalent FP8 sequences with 4 carrier proteins: the aforementioned KLH and rTTHC; the H. influenzae protein D (HiD); and the cross-reactive material from diphtheria toxin (CRM197). While each of the 16 FP8-carrier conjugates could elicit HIV-1-neutralizing responses, rTTHC conjugates induced higher FP-directed responses overall. A Sulfo-SIAB linker yielded superior results over an SM(PEG)2 linker but combinations of carriers, conjugation ratio of peptide to carrier, or choice of adjuvant (Adjuplex or Alum) did not significantly impact elicited FP-directed neutralizing responses in mice. Overall, SIAB-linked FP8-rTTHC appears to be a promising vaccine candidate for advancing to clinical assessment.

Elimination of 2-keto-3-deoxy-D-glycero-D-galacto-nonulosonic acid 9-phosphate synthase activity from human N-acetylneuraminic acid 9-phosphate synthase by a single mutation.

The most commonly occurring sialic acid Neu5Ac (N-acetylneuraminic acid) and its deaminated form, KDN (2-keto-3-deoxy-D-glycero-D-galacto-nonulosonic acid), participate in many biological functions. The human Neu5Ac-9-P (Neu5Ac 9-phosphate) synthase has the unique ability to catalyse the synthesis of not only Neu5Ac-9-P but also KDN-9-P (KDN 9-phosphate). Both reactions are catalysed by the mechanism of aldol condensation of PEP (phosphoenolpyruvate) with sugar substrates, ManNAc-6-P (N-acetylmannosamine 6-phosphate) or Man-6-P (mannose 6-phosphate). Mouse and putative rat Neu5Ac-9-P synthases, however, do not show KDN-9-P synthase activity, despite sharing high sequence identity (>95%) with the human enzyme. Here, we demonstrate that a single mutation, M42T, in human Neu5Ac-9-P synthase can abolish the KDN-9-P synthase activity completely without compromising the Neu5Ac-9-P synthase activity. Saturation mutagenesis of Met42 of the human Neu5Ac-9-P synthase showed that the substitution with all amino acids except leucine retains only the Neu5Ac-9-P synthase activity at levels comparable with the wild-type enzyme. The M42L mutant, like the wild-type enzyme, showed the additional KDN-9-P synthase activity. In the homology model of human Neu5Ac-9-P synthase, Met42 is located 22 A (1 A=0.1 nm) away from the substrate-binding site and the impact of this distant residue on the enzyme functions is discussed.

In vitro generation of polysialylated cervical mucins by bacterial polysialyltransferases to counteract cytotoxicity of extracellular histones.

Neutrophil extracellular traps (NET) are formed against pathogens. However, various diseases are directly linked to this meshwork of DNA. The cytotoxic properties of extracellular histones especially seem to be an important trigger during these diseases. Furthermore, NET accumulation on implants is discussed to result in an impaired efficiency or failure, depending on the category of implant. Interestingly, mucins have been investigated as surface coatings potentially capable of reducing neutrophil adhesion. Similarly, polysialic acid was shown to inactivate the cytotoxic properties of extracellular histones. We wanted to combine the probability to decrease the adhesion of neutrophils using mucins with the capability of sialic acid polymers to counteract histone-mediated cytotoxicity. To this end, we elongate cervical mucins using bacterial polysialyltransferases. Subsequent cell-based experiments demonstrated the activity of elongated mucins against histone-mediated cytotoxicity. Thus, polysialylated mucins may represent a novel component to coat implants or to combat diseases with exaggerated NET formation.

Escherichia coli BL21(DE3) chromosome contains a group II capsular gene cluster.

During our study of de novo synthesis of Escherichia coli K1 capsular polysaccharides, we found that E. coli BL21(DE3) has a capsular gene cluster, similar to those of group II capsular E. coli strains. Analysis of the nucleotide sequence of the E. coli BL21(DE3) gene cluster showed homologues to all group II regions 1 and 3 genes and the presence of an IS1 element in one of the region 2 ORFs, which likely prevents capsule expression. Complementation analysis showed that region 1 and 3 genes encode functional proteins that are sufficient for the export of newly synthesized polysaccharide. The gene products of Bl21(DE3) kpsC and kpsS supported in vitro de novo synthesis of K1 polysaccharide when co-expressed with K1 NeuE and NeuS. Sequence homology between BL21(DE3) region 2 open reading frames and capsule-related genes in other bacteria such as Haemophilus influenzae serotype b, suggests that the encapsulated ancestor of BL21(DE3) may have produced a ribose/ribitol-phosphate containing polysaccharide.

Characterization of N-acetylneuraminic acid synthase isoenzyme 1 from Campylobacter jejuni.

Escherichia coli NeuNAc (N-acetylneuraminic acid) synthase catalyses the condensation of PEP (phosphoenolpyruvate) and ManNAc (N-acetylmannosamine) to form NeuNAc and is encoded by the neuB gene. Campylobacter jejuni has three neuB genes, one of which is very similar to the E. coli neuB gene. We have characterized the C. jejuni neuraminic acid synthase with respect to acylamino sugar specificity and stereochemistry of the PEP condensation. We determined the specificity of C. jejuni NeuNAc synthase for N-acetylmannosamine, N-butanoylmannosamine, N-propionoylmannosamine and N-pentanoylmannosamine. We find that, although this enzyme exhibits similar K(m) values for N-acylmannosamine molecules with different N-acyl groups, the kcat/K(m) values decreased with increasing chain length. NeuNAc synthase is a member of a PEP-utilizing family of enzymes that form oxo acids from PEP and a monosaccharide. This family includes KDO 8-P (2-keto-3-deoxy-D-manno-octulosonate 8-phosphate) synthase and DAH 7-P (2-keto-3-deoxy-D-arabino-heptulosonate 7-phosphate) synthase. Both enzymes catalyse the condensation of the re face of the aldehyde group of the monosaccharide with the si face of the PEP molecule. The C. jejuni NeuNAc synthase catalysed the condensation of Z- and E-[3-2H]PEP with ManNAc, yielding (3S)-3-deutero-NeuNAc and (3R)-3-deutero-NeuNAc respectively. The condensation of Z-[3-F]PEP and ManNAc yielded (3S)-3-fluoro-NeuNAc. Results of our studies suggest that the C. jejuni NeuNAc synthase, similar to KDO 8-P synthase and DAH 7-P synthase, catalyses the condensation of the si face of PEP with the aldehyde sugar. The present study is the first stereochemical analysis of the reaction catalysed by a bacterial NeuNAc synthase.

Mapping the glycation sites in the neoglycoconjugate from hexasaccharide antigen of Vibrio cholerae, serotype Ogawa and the recombinant tetanus toxin C-fragment carrier.

We report herein the glycation sites in a vaccine candidate for cholera formed by conjugation of the synthetic hexasaccharide fragment of the O-specific polysaccharide of Vibrio cholerae, serotype Ogawa, to the recombinant tetanus toxin C-fragment (rTT-Hc) carrier. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis of the vaccine revealed that it is composed of a mixture of neoglycoconjugates with carbohydrate : protein ratios of 1.9 : 1, 3.0 : 1, 4.0 : 1, 4.9 : 1, 5.9 : 1, 6.9 : 1, 7.9 : 1 and 9.1 : 1. Liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis of the tryptic and GluC V8 digests allowed identification of 12 glycation sites in the carbohydrate-protein neoglycoconjugate vaccine. The glycation sites are located exclusively on lysine (Lys) residues and are listed as follows: Lys 22, Lys 61, Lys 145, Lys 239, Lys 278, Lys 318, Lys 331, Lys 353, Lys 378, Lys 389, Lys 396 and Lys 437. Based on the 3-D representation of the rTT-Hc protein, all the glycation sites correspond to lysines located at the outer surface of the protein.

Successive glycosyltransfer of sialic acid by Escherichia coli K92 polysialyltransferase in elongation of oligosialic acceptors.

Escherichia coli K92 produces a capsular polysialic acid with alternating alpha2,8 alpha2,9 NeuNAc linkages. This polysaccharide is cross-reactive with the neuroinvasive pathogen Neisseria meningitidis Group C. The K92 polysialyltransferase (PST) catalyzes the synthesis of the polysialic acid with alternating linkages by the transfer of NeuNAc from CMP-NeuNAc to the nonreducing end of the growing polymer. We used a fluorescent-based high-performance liquid chromatography assay to characterize the process of chain extension. The PST elongates the acceptor GT3-FCHASE in a biphasic fashion. The initial phase polymers are characterized by accumulation of product containing 1-8 additional sialic acid residues. This phase is followed by a very rapid formation of high-molecular weight (MW) polymer as the accumulated oligosaccharides containing 8-10 sialic acids are consumed. The high-MW polymer contains 90-100 sialic acids and is sensitive to degradation by periodate and K1-5 endoneuraminidase, suggesting that the polymer contains the alternating structure. The polymerization reaction does not appear to be strictly processive, since oligosaccharides of each intermediate size were detected before accumulation of high-molecular weight polymer. Synthesis can be blocked by CMP-9-azido-NeuNAc. These results suggest that the K92 PST forms both alpha2,8 and alpha2,9 linkages in a successive and nonprocessive fashion.