N-acetyl-D-neuraminic acid lyase generates the sialic acid for colominic acid biosynthesis in Escherichia coli K1

Comparison of the modulatory effects of three structurally similar potential prebiotic substrates on an in vitro multi-species oral biofilm

Previous research identified potential prebiotic substrates for oral health like the structural analogues N-acetyl-D-mannosamine (NADM) and N-acetyl-D-glucosamine (NADG). The main hypothesis of the current study was twofold. Firstly, it was hypothesized that the modulatory effects of NADM are not limited to changes in multi-species oral biofilm composition, but also include effects on metabolism, virulence, and inflammatory potential. Secondly, the presence and orientation of their N-acetyl group could play a role. Therefore, a comparison was made between the effects of NADM, NADG and D-(+)-mannose on multi-species oral biofilms. Besides a beneficial compositional shift, NADM-treated biofilms also showed an altered metabolism, a reduced virulence and a decreased inflammatory potential. At a substrate concentration of 1 M, these effects were pronounced for all biofilm aspects, whereas at ~ 0.05 M (1%_(w/v)) only the effects on virulence were pronounced. When comparing between substrates, both the presence and orientation of the N-acetyl group played a role. However, this was generally only at 1 M and dependent on the biofilm aspect. Overall, NADM was found to have different effects at two concentrations that beneficially modulate in vitro multi-species oral biofilm composition, metabolism, virulence and inflammatory potential. The presence and orientation of the N-acetyl group influenced these effects.

An overview and future prospects of sialic acids

Sialic acids (Sias) are negatively charged functional monosaccharides present in a wide variety of natural sources (plants, animals and microorganisms). Sias play an important role in many life processes, which are widely applied in the medical and food industries as intestinal antibacterials, antivirals, anti-oxidative agents, food ingredients, and detoxification agents. Most Sias are composed of N-acetylneuraminic acid (Neu5Ac, >99%), and Sia is its most commonly used name. In this article, we review Sias in terms of their structures, applications, determination methods, metabolism, and production strategies. In particular, we summarise and compare different production strategies, including extraction from natural sources, chemical synthesis, polymer decomposition, enzymatic synthesis, whole-cell catalysis, and de novo biosynthesis via microorganism fermentation. We also discuss research on their physiological functions and applications, barriers to efficient production, and strategies for overcoming these challenges. We focus on efficient de novo biosynthesis strategies for Neu5Ac via microbial fermentation using novel synthetic biology tools and methods that may be applied in future. This work provides a comprehensive overview of recent advances on Sias, and addresses future challenges regarding their functions, applications, and production.

The role of RcsA in the adaptation and survival of Escherichia coli K92

The Rcs phosphorelay is a two-component signal transduction system that senses stressful environmental signals such as desiccation or low temperatures, which serve as natural inducers in bacteria. RcsA is an important coregulator in this system involved in some functions regulated by the Rcs system, including biofilm formation and capsule synthesis. In this sense, we previously showed that RcsA is necessary for colanic acid synthesis in Escherichia coli K92. Here, using an E. coli K92ΔrcsA mutant lacking rcsA gene we further characterize the implications of RcsA on E. coli K92 survival under osmotic and oxidative stressful conditions, and bacterial attachment and biofilm formation on both biotic and abiotic surfaces. Our results show that RcsA protects E. coli K92 against osmotic and, especially, oxidative stress at low temperatures. In addition, RcsA did not interfere in biofilm formation in any surface tested, including polystyrene, stainless steel, silicone, Teflon, aluminum and glass. By contrast, deletion of rcsA increased bacterial attachment to the caco-2 cells monolayer used as biotic surface.

Features and structure of a cold active N-acetylneuraminate lyase

N-acetylneuraminate lyases (NALs) are enzymes that catalyze the reversible cleavage and synthesis of sialic acids. They are therefore commonly used for the production of these high-value sugars. This study presents the recombinant production, together with biochemical and structural data, of the NAL from the psychrophilic bacterium Aliivibrio salmonicida LFI1238 (AsNAL). Our characterization shows that AsNAL possesses high activity and stability at alkaline pH. We confirm that these properties allow for the use in a one-pot reaction at alkaline pH for the synthesis of N-acetylneuraminic acid (Neu5Ac, the most common sialic acid) from the inexpensive precursor N-acetylglucosamine. We also show that the enzyme has a cold active nature with an optimum temperature for Neu5Ac synthesis at 20°C. The equilibrium constant for the reaction was calculated at different temperatures, and the formation of Neu5Ac acid is favored at low temperatures, making the cold active enzyme a well-suited candidate for use in such exothermic reactions. The specific activity is high compared to the homologue from Escherichia coli at three tested temperatures, and the enzyme shows a higher catalytic efficiency and turnover number for cleavage at 37°C. Mutational studies reveal that amino acid residue Asn 168 is important for the high k_cat. The crystal structure of AsNAL was solved to 1.65 Å resolution and reveals a compact, tetrameric protein similar to other NAL structures. The data presented provides a framework to guide further optimization of its application in sialic acid production and opens the possibility for further design of the enzyme.

Production of N-acetyl-D-neuraminic Acid by Recombinant Single Whole Cells Co-expressing N-acetyl-D-glucosamine-2-epimerase and N-acetyl-D-neuraminic Acid Aldolase

N-acetyl-D-neuraminic acid (Neu5Ac) is a costly precursor for many drugs such as anti-influenza antivirals. In a previous study, a whole-cell process for Neu5Ac production was developed using a combination of two Escherichia coli cells expressing Anabaena sp. CH1 N-acetyl-D-glucosamine-2-epimerase (bage) and E. coli N-acetyl-D-neuraminic acid aldolase (nanA), respectively. In this study, we constructed a bAGE and NanA co-expression system to improve Neu5Ac production. Two recombinant E. coli strains, E. coli BL21 (DE3) pET-bage-nanA (HA) and E. coli BL21 (DE3) pET-bage-2nanA (HAA), synchronously expressing bAGE and NanA were used as biocatalysts to generate Neu5Ac from N-acetyl-D-glucosamine (GlcNAc) and pyruvate. The HA biocatalysts produced 187.5 mM Neu5Ac within 8 h. The yield of GlcNAc was 15.6%, and the Neu5Ac production rate was 7.25 g/L/h. The most active HAA biocatalysts generated 412.6 mM Neu5Ac and a GlcNAc yield of 34.4%. HAA achieved a Neu5Ac production rate of 15.9 g/L/h, which surpassed those for all reported Neu5Ac production processes so far. The present study demonstrates that using recombinant E. coli cells synchronously expressing bAGE and NanA as biocatalysts could potentially be used in the industrial mass production of Neu5Ac.

Extending enzyme molecular recognition with an expanded amino acid alphabet

Natural enzymes are constructed from the 20 proteogenic amino acids, which may then require posttranslational modification or the recruitment of coenzymes or metal ions to achieve catalytic function. Here, we demonstrate that expansion of the alphabet of amino acids can also enable the properties of enzymes to be extended. A chemical mutagenesis strategy allowed a wide range of noncanonical amino acids to be systematically incorporated throughout an active site to alter enzymic substrate specificity. Specifically, 13 different noncanonical side chains were incorporated at 12 different positions within the active site of N-acetylneuraminic acid lyase (NAL), and the resulting chemically modified enzymes were screened for activity with a range of aldehyde substrates. A modified enzyme containing a 2,3-dihydroxypropyl cysteine at position 190 was identified that had significantly increased activity for the aldol reaction of erythrose with pyruvate compared with the wild-type enzyme. Kinetic investigation of a saturation library of the canonical amino acids at the same position showed that this increased activity was not achievable with any of the 20 proteogenic amino acids. Structural and modeling studies revealed that the unique shape and functionality of the noncanonical side chain enabled the active site to be remodeled to enable more efficient stabilization of the transition state of the reaction. The ability to exploit an expanded amino acid alphabet can thus heighten the ambitions of protein engineers wishing to develop enzymes with new catalytic properties.

Structural basis for substrate specificity and mechanism of N-acetyl-D-neuraminic acid lyase from Pasteurella multocida

N-Acetylneuraminate lyases (NALs) or sialic acid aldolases catalyze the reversible aldol cleavage of N-acetylneuraminic acid (Neu5Ac, the most common form of sialic acid) to form pyruvate and N-acetyl-d-mannosamine. Although equilibrium favors sialic acid cleavage, these enzymes can be used for high-yield chemoenzymatic synthesis of structurally diverse sialic acids in the presence of excess pyruvate. Engineering these enzymes to synthesize structurally modified natural sialic acids and their non-natural derivatives holds promise in creating novel therapeutic agents. Atomic-resolution structures of these enzymes will greatly assist in guiding mutagenic and modeling studies to engineer enzymes with altered substrate specificity. We report here the crystal structures of wild-type Pasteurella multocida N-acetylneuraminate lyase and its K164A mutant. Like other bacterial lyases, it assembles into a homotetramer with each monomer folding into a classic (β/α)₈ TIM barrel. Two wild-type structures were determined, in the absence of substrates, and trapped in a Schiff base intermediate between Lys164 and pyruvate, respectively. Three structures of the K164A variant were determined: one in the absence of substrates and two binary complexes with N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc). Both sialic acids bind to the active site in the open-chain ketone form of the monosaccharide. The structures reveal that every hydroxyl group of the linear sugars makes hydrogen bond interactions with the enzyme, and the residues that determine specificity were identified. Additionally, the structures provide some clues for explaining the natural discrimination of sialic acid substrates between the P. multocida and Escherichia coli NALs.

Characterization of a novel N-acetylneuraminate lyase from Staphylococcus carnosus TM300 and its application to N-acetylneuraminic acid production

The possibility of incorporating N-acetylneuraminic acid (Neu5Ac) in infant formulas and other functional foods has opened up the need to synthesize N-acetylneuraminic acid using N-acetylneuraminate lyases (NALs) by reversible aldol condensation of pyruvate and N-acetyl-d-mannosamine. Until now, NALs have been cloned from pathogenic microorganisms; however, this Article describes the expression and characterization of an N-acetylneuraminate lyase from the Staphylococcus carnosus TM300, a GRAS microorganism used in fermented meat. ScNAL showed a high level of expression in E. coli (403 mg L(-1) culture). This, combined with its simple two-step purification procedure, the highest recovery described to date (86%), its kinetic parameters, which are in the same order of magnitude as best reported NALs, and its optimum pH and temperature, make ScNAL a promising and cheap biocatalyst. To confirm its biotechnological potential, the Neu5Ac was synthesized in 3 h in simple industrial working conditions with a high degree of conversion (94%).

Growth temperature regulation of some genes that define the superficial capsular carbohydrate composition of Escherichia coli K92

We studied growth temperature as a factor controlling the expression of genes involved in capsular polymers of Escherichia coli K92. These genes are shown to be regulated by growth temperature. Expression levels of genes belonging to the kps cluster, responsible for polysialic acid (PA) biosynthesis, were significantly increased at 37 °C compared with at 19 °C, being up to 500-fold increased for neuE and neuS genes. Similarly, the genes for the nan operon, responsible for PA catabolism, also reached higher expression levels at 37 °C, although with slightly lower values (39-141-fold). In contrast, genes of the cps operon, which are implicated in colanic acid (CA) metabolism, were upregulated when the bacteria were grown at 19 °C, albeit to a much lesser extent (around twofold). This different regulation of genes involved in the biosynthesis of polysialic and CAs correlates with the reported maximal production temperatures for the two polymers. The results suggest that the metabolism of PA is predominantly regulated by changes in gene expression, while CA production may be regulated mainly by post-transcriptional processes such as phosphorylation-dephosphorylation reactions.

Molecular characterization of a novel N-acetylneuraminate lyase from Lactobacillus plantarum WCFS1

N-Acetylneuraminate lyases (NALs) or sialic acid aldolases catalyze the reversible aldol cleavage of N-acetylneuraminic acid (Neu5Ac) to form pyruvate and N-acetyl-d-mannosamine (ManNAc). In nature, N-acetylneuraminate lyase occurs mainly in pathogens. However, this paper describes how an N-acetylneuraminate lyase was cloned from the human gut commensal Lactobacillus plantarum WCFS1 (LpNAL), overexpressed, purified, and characterized for the first time. This novel enzyme, which reaches a high expression level (215 mg liter(-1) culture), shows similar catalytic efficiency to the best NALs previously described. This homotetrameric enzyme (132 kDa) also shows high stability and activity at alkaline pH (pH > 9) and good temperature stability (60 to 70°C), this last feature being further improved by the presence of stabilizing additives. These characteristics make LpNAL a promising biocatalyst. When its sequence was compared with that of other, related (real and putative) NALs described in the databases, it was seen that NAL enzymes could be divided into four structural groups and three subgroups. The relation of these subgroups with human and other mammalian NALs is also discussed.

Biosynthesis and production of polysialic acids in bacteria

Polysialic acids (PA) are protective capsular sialohomopolymers present in some bacteria which can invade the mammalian host and cause lethal bacteremia and meningitis. Biosynthesis and translocation of PA to the cell surface are equivalent in different species and bacterial strains which are produced. The diversity in PA structure is derived from the PA linkages and is a consequence of the specific sialyltransferase activities. The monomer acetylation and the polymer length could be important factors in the potential virulence. In vivo PA production is affected by different physical and chemical factors. The temperature of cellular growth strictly regulates PA genesis through a molecular complex and multifactorial mechanism that operate to transcription level.

Temperature has reciprocal effects on colanic acid and polysialic acid biosynthesis in E. coli K92

Escherichia coli K92 is an opportunistic pathogen bacterium able to produce polysialic acid (PA) capsules when grows at 37 degrees C. PA polysaccharides are cell-associated homopolymers tailored from acid sialic monomers that function as virulence factors in different neuroinvasive diseases caused by certain Enterobacteriaceae. Conversely, when grows at 19 degrees C (restrictive conditions), PA synthesis was negligible, whereas in such condition, a slimy substance started to be accumulated in the culture broths. Analysis by uronic acids colorimetric determinations, gas chromatography-mass spectrometry, and Fourier transform infrared spectroscopy allowed the isolation and identification of mucoid substance as colanic acid (CA). CA is a heteropolymer containing glucose, galactose, fucose, and glucuronic acid as monomers which seems to be involved in the protection of this bacterium against environment assaults. The study of physicochemical conditions required for CA synthesis revealed that in E. coli K92, nutrient (carbon and nitrogen sources) modulates CA production, reaching the maximal values when glucose and proline were as carbon and nitrogen sources, respectively. Furthermore, we have found that E. coli K92 is able to produce CA at all temperatures tested (from 42 degrees C to 15 degrees C), whereas PA synthesis only occurred when bacteria were cultured at temperatures higher than 25 degrees C. Additionally, genetic engineering approaches revealed that the CA cluster including several genes required for synthesis was placed into a DNA fragment of 100 kb using polymerase chain reaction methodology.

Pasteurella multocida sialic acid aldolase: a promising biocatalyst

Sialic acid aldolases or N-acetylneuraminate lyases (NanAs) catalyze the reversible aldol cleavage of N-acetylneuraminic acid (Neu5Ac) to form pyruvate and N-acetyl-D: -mannosamine (ManNAc). A capillary electrophoresis assay was developed to directly characterize the activities of NanAs in both Neu5Ac cleavage and Neu5Ac synthesis directions. The assay was used to obtain the pH profile and the kinetic data of a NanA cloned from Pasteurella multocida P-1059 (PmNanA) and a previously reported recombinant Escherichia coli K12 NanA (EcNanA). Both enzymes are active in a broad pH range of 6.0-9.0 in both reaction directions and have similar kinetic parameters. Substrates specificity studies showed that 5-O-methyl-ManNAc, a ManNAc derivative, can be used efficiently as a substrate by PmNanA, but not efficiently by EcNanA, for the synthesis of 8-O-methyl Neu5Ac. In addition, PmNanA (250 mg l(-1) culture) has a higher expression level (2.5-fold) than EcNanA (94 mg l(-1) culture). The higher expression level and a broader substrate tolerance make PmNanA a better catalyst than EcNanA for the chemoenzymatic synthesis of sialic acids and their derivatives.

Hexose/Pentose and Hexitol/Pentitol Metabolism

Escherichia coli and Salmonella enterica serovar Typhimurium exhibit a remarkable versatility in the usage of different sugars as the sole source of carbon and energy, reflecting their ability to make use of the digested meals of mammalia and of the ample offerings in the wild. Degradation of sugars starts with their energy-dependent uptake through the cytoplasmic membrane and is carried on further by specific enzymes in the cytoplasm, destined finally for degradation in central metabolic pathways. As variant as the different sugars are, the biochemical strategies to act on them are few. They include phosphorylation, keto-enol isomerization, oxido/reductions, and aldol cleavage. The catabolic repertoire for using carbohydrate sources is largely the same in E. coli and in serovar Typhimurium. Nonetheless, significant differences are found, even among the strains and substrains of each species. We have grouped the sugars to be discussed according to their first step in metabolism, which is their active transport, and follow their path to glycolysis, catalyzed by the sugar-specific enzymes. We will first discuss the phosphotransferase system (PTS) sugars, then the sugars transported by ATP-binding cassette (ABC) transporters, followed by those that are taken up via proton motive force (PMF)-dependent transporters. We have focused on the catabolism and pathway regulation of hexose and pentose monosaccharides as well as the corresponding sugar alcohols but have also included disaccharides and simple glycosides while excluding polysaccharide catabolism, except for maltodextrins.

Transport of N-acetyl-D-galactosamine in Escherichia coli K92: effect on acetyl-amino sugar metabolism and polysialic acid production

The N-acetyl-D-galactosamine (GalNAc) transport system of Escherichia coli K92 was studied when the bacterium was grown in a chemically defined medium containing GalNAc as a carbon source. Kinetic measurements were carried out in vivo at 37 degrees C in 25 mM phosphate buffer, pH 7.0. Under these conditions, the uptake rate was linear for at least 3 min and the calculated Km for GalNAc was 3 microM. The transport system was strongly inhibited by sodium arsenate (70%), potassium cyanide (62%) and 2,4-dinitrophenol (75%). Analysis of bacterial GalNAc phosphotransferase activity revealed in vitro GalNAc phosphorylation activity only when phosphoenolpyruvate was present. These results strongly support the notion that GalNAc uptake depends on a specific phosphotransferase system. Study of activity regulation showed that N-acetylglucosamine and mannosamine specifically inhibit the transport of GalNAc in this bacterium. Analysis of expression revealed that the GalNAc transport system is specifically induced by GalNAc but not by N-acetylglucosamine (GlcNAc) or N-acetylmannosamine (ManNAc), two intimately related sugars. Moreover, full induction of GalNAc transport required the presence of both cAMP and GalNAc. Comparative studies revealed that E. coli K92 has developed a regulation mechanism that specifically induces the appropriate permease based on the presence of each respective phospho-amino sugar (GlcNAc, ManNAc and GalNAc). In this regulation system, GlcNAc is the preferred amino sugar as the carbon source. Finally, when E. coli K92 was grown using GalNAc, capsular polysialic acid production was strongly affected. The presence of intracellular phosphoderivative acetylamino sugars, generated by the action of the phosphotransferase transport system, can be responsible for this effect.

Diversity of microbial sialic acid metabolism

Sialic acids are structurally unique nine-carbon keto sugars occupying the interface between the host and commensal or pathogenic microorganisms. An important function of host sialic acid is to regulate innate immunity, and microbes have evolved various strategies for subverting this process by decorating their surfaces with sialylated oligosaccharides that mimic those of the host. These subversive strategies include a de novo synthetic pathway and at least two truncated pathways that depend on scavenging host-derived intermediates. A fourth strategy involves modification of sialidases so that instead of transferring sialic acid to water (hydrolysis), a second active site is created for binding alternative acceptors. Sialic acids also are excellent sources of carbon, nitrogen, energy, and precursors of cell wall biosynthesis. The catabolic strategies for exploiting host sialic acids as nutritional sources are as diverse as the biosynthetic mechanisms, including examples of horizontal gene transfer and multiple transport systems. Finally, as compounds coating the surfaces of virtually every vertebrate cell, sialic acids provide information about the host environment that, at least in Escherichia coli, is interpreted by the global regulator encoded by nanR. In addition to regulating the catabolism of sialic acids through the nan operon, NanR controls at least two other operons of unknown function and appears to participate in the regulation of type 1 fimbrial phase variation. Sialic acid is, therefore, a host molecule to be copied (molecular mimicry), eaten (nutrition), and interpreted (cell signaling) by diverse metabolic machinery in all major groups of mammalian pathogens and commensals.

Transport of N-acetyl-D-mannosamine and N-acetyl-D-glucosamine in Escherichia coli K1: effect on capsular polysialic acid production

N-Acetyl-D-mannosamine (ManNAc) and N-acetyl-D-glucosamine (GlcNAc) are the essential precursors of N-acetylneuraminic acid (NeuAc), the specific monomer of polysialic acid (PA), a bacterial pathogenic determinant. Escherichia coli K1 uses both amino sugars as carbon sources and uptake takes place through the mannose phosphotransferase system transporter, a phosphoenolpyruvate-dependent phosphotransferase system that shows a broad range of specificity. Glucose, mannose, fructose, and glucosamine strongly inhibited the transport of these amino-acetylated sugars and GlcNAc and ManNAc strongly affected ManNAc and GlcNAc uptake, respectively. The ManNAc and the GlcNAc phosphorylation that occurs during uptake affected NeuAc synthesis in vitro. These findings account for the low in vivo PA production observed when E. coli K1 uses ManNAc or GlcNAc as a carbon source for growth.

N-Acetylneuraminic acid uptake in Pasteurella (Mannheimia) haemolytica A2 occurs by an inducible and specific transport system

The N-acetylneuraminic acid (NeuAc) transport system of Pasteurella (Mannheimia) haemolytica A2 was studied when this bacterium was grown in both complex and chemically defined media. Kinetic measurements were carried out at 37 degrees C in 50 mM Tris-HCl buffer, pH 8.0, containing 50 microg/ml bovine serum albumin. Under these conditions, the uptake rate was linear for at least 3 min and the calculated K(m) for NeuAc was 0.1 microM. The transport rate was increased by the addition of several cations and was inhibited by sodium arsenite (95%), N,N'-dicyclohexyl-carbodiimide (50%), and 2,4-dinitrophenol (40%) at final concentration of 1 mM (each). These results support the notion that NeuAc uptake is an active sugar cation symporter. Study of specificities showed that glucosamine, mannose and mannosamine inhibited the transport of NeuAc in this bacterium. Analysis of expression revealed that the NeuAc transport system was induced by NeuAc and by the simultaneous presence of glucose and galactose in the growth medium.

Kinetic properties of the acylneuraminate cytidylyltransferase from Pasteurella haemolytica A2

Neuroinvasive and septicaemia-causing pathogens often display a polysialic acid capsule that is involved in invasive behaviour. N-Acetylneuraminic acid (NeuAc) is the basic monomer of polysialic acid. The activated form, CMP-Neu5Ac, is synthesized by the acylneuraminate cytidylyltransferase (ACT; EC 2.7.7.43). We have purified this enzyme from Pasteurella haemolytica A2 to apparent homogeneity (522-fold). The protein behaved homogeneously on SDS/PAGE as a 43 kDa band, a size similar to that of Escherichia coli, calf, mouse and rat. Specific activity in crude lysate displayed one of the highest values cited in the literature (153 m-units/mg). We have studied the steady-state kinetic mechanism of the enzyme by using normalized plot premises. The catalysis proceeds through a Ping Pong Bi Bi mechanism, with CTP as the first substrate and CMP-NeuAc as the last product. The true Km values were 1.77 mM for CTP and 1.82 mM for NeuAc. The nucleotides CDP, UTP, UDP and TTP, and the modified sialic acid N-glycolylneuraminic acid were also substrates of the ACT activity. The enzyme is inhibited by cytidine nucleotides through binding to a second cytidyl-binding site. This inhibition is greater with nucleotides that display a long phosphate tail, and the genuine inhibitor is the substrate CTP. At physiological concentrations, ATP is an activator, and AMP an inhibitor, of the ACT activity. The activated sugar UDP-N-acetylglucosamine acts as an inhibitor, thus suggesting cross-regulation of the peptidoglycan and polysialic acid pathways. Our findings provide new mechanistic insights into the nature of sialic acid activation and suggest new targets for the approach to the pathogenesis of encapsulated bacteria.

Characterization and mutagenesis of the recombinant N-acetylneuraminate lyase from Clostridium perfringens: insights into the reaction mechanism

The N-acetylneuraminate lyase from Clostridium perfringens was expressed in Escherichia coli as a fusion protein with a His-tag and purified to homogeneity using metal chelate affinity and anion exchange chromatography. The purified enzyme has a pH optimum of 7.6 and a temperature optimum of 65-70 degrees C. In kinetic studies the lyase exhibits a Km of 3.2 mM for Neu5Ac and a Vmax of 27.5 U x mg(-1). To clarify the functional role of some putative active site residues, site-directed mutagenesis was performed. Lysine 161 was identified as the residue forming the Schiff base intermediate with the substrate. Tyrosine 133 was shown to be also a catalytically important residue; it seems to function as an acceptor for the proton of the C4 hydroxyl group, as already suggested by other groups. Furthermore, it is involved in stabilizing the Schiff base intermediate. Mutations of aspartate 187 and glutamate 188 indicate that both residues are involved in substrate binding. In this respect the carboxy group of aspartate 187 seems to be particularly important. Based on the results of these studies, a model of the reaction mechanism is discussed.

Sialic acid metabolism's dual function in Haemophilus influenzae

Many bacterial commensals and pathogens use the sialic acids as carbon and nitrogen sources. In Escherichia coli, the breakdown of these sugars is catalysed by gene products of the nan (Nacylneuraminate) operon; other microorganisms may use a similar catabolic strategy. Despite the known ligand and antirecognition functions of the sialic acids, the contribution of their catabolism to infection or host colonization has never been directly investigated. We addressed these questions with Haemophilus influenzae type b, which metabolizes relatively few carbohydrates, using the infant-rat infection model. The predicted H. influenzae homologue (HI0142) of the E. coli sialic acid aldolase structural gene, nanA, was subcloned and mutagenized by insertion of a kanamycin resistance cassette. Phenotypic investigation of the resulting H. influenzae aldolase mutants showed that: (i) HI0142 is essential for sialic acid degradation; (ii) the products of the open reading frames (ORFs) flanking HI0142 (HI0140, 41, 44 and 45) are likely to have the same functions as those of their counterparts in E. coli; (iii) sialylation of the lipooligosaccharide (LOS) epitope recognized by monoclonal antibody 3F11 is dependent on an environmental source of sialic acid; (iv) a nanA mutant hypersialylates its LOS sialyl acceptor, corresponding to an apparent increased fitness of the mutant in the infant-rat model; and (v) expression of the LOS sialyl acceptor is altered in cells grown without exogenous sialic acid, indicating the direct or indirect effect of sialic acid metabolism on LOS antigenicity. Taken together the data show the dual role of sialic acid catabolism in nutrition and cell surface modulation.

Determination of different amino sugar 2'-epimerase activities by coupling to N-acetylneuraminate synthesis

A new procedure for quantitating the amount of N-acetyl-D-mannosamine (ManNAc) or ManNAc-6-phosphate produced by 2'-epimerase activities involved in sialic acid metabolism has been developed. The ManNAc generated by the action of N-acetyl-D-glucosamine (GlcNAc) and UDP-GlcNAc 2'-epimerases is condensed with pyruvate through the action of N-acetylneuraminate lyase and the sialic acid released is measured by the thiobarbituric acid assay. For the analysis of prokaryotic GlcNAc-6-phosphate 2'-epimerase, ManNAc-6-phosphate can also be evaluated by this coupled assay after dephosphorylation of the sugar phosphate. This system provides a sensitive, rapid, reproducible, specific and simple procedure (feasible with commercial reagents) for measuring amino sugar 2'-epimerases from eukaryotic and prokaryotic sources. The technique reported here permitted us to detect UDP-GlcNAc 2'-epimerase and GlcNAc 2'-epimerase in mammalian cell extracts and GlcNAc-6-phosphate 2'-epimerase in bacterial extracts.

Investigation of the kinetic mechanism of cytidine 5'-monophosphate N-acetylneuraminic acid synthetase from Haemophilus ducreyi with new insights on rate-limiting steps from product inhibition analysis

The presence of sialic acid as a component of cell surface lipooligosaccharides or capsular polysaccharides has been shown to be correlated with the virulence of a number of Gram-negative mucosal pathogens, including several Haemophilus and Neisseria spp. As part of our efforts to evaluate the role of sialic acid in the pathobiology of these organisms, we have initiated studies of the enzymes from Haemophilus ducreyi (the infectious agent of chancroid) responsible for the activation and attachment of sialic acid to the lipooligosaccharide. In this report, we describe results of an investigation of the steady-state kinetic mechanism of the activating enzyme, cytidine 5'-monophosphate N-acetylneuraminic acid (CMP-NeuAc) synthetase. Using a combination of initial velocity, product inhibition, and dead-end inhibition studies, the reaction is shown to be freely reversible and to proceed through an ordered bi-bi kinetic mechanism in which CTP binds first and CMP-NeuAc dissociates last. In addition, a detailed analysis of the kinetic expressions for the observable constants is presented showing how the variation in apparent product inhibition constants (Kii) can be used to predict the rate-limiting step in kcat, which appears to be dissociation of CMP-NeuAc in this enzyme. To our knowledge, this relationship has not been previously recognized.

Uptake of N-acetyl-D-mannosamine: an essential intermediate in polysialic acid biosynthesis by Escherichia coli K92

The N-acetyl-D-mannosamine (ManNAc) transport system of Escherichia coli K92 was studied when this bacterium was grown in a chemically defined medium containing ManNAc as carbon source. Kinetic measurements were carried out in vivo at 37 degrees C in 25 mM phosphate buffer, pH 7.5. Under these conditions, the uptake rate was linear for at least 15 min and the calculated Km for ManNAc was 280 microM. The transport system was strongly inhibited by sodium arsenate (97%), potassium cyanide (84%) and 2,4-dinitrophenol (88%) added at final concentrations of 1 mM (each). Analysis of bacterial ManNAc phosphotransferase activity revealed in vitro ManNAc phosphorylation activity only when phosphoenolpyruvate was present. These results strongly support the notion that ManNAc uptake depends on a specific phosphotransferase system. Study of specificities showed that N-acetylglucosamine and mannosamine specifically inhibited the transport of ManNAc in this bacterium. Analysis of expression revealed that the ManNAc transport system was induced by ManNAc, glucosamine, galactosamine, mannosamine and mannose but not by N-acetylglucosamine or N-acetylgalactosamine. Moreover, ManNAc permease was subject to glucose repression and cAMP stimulation. Full induction of the ManNAc transport system required the simultaneous presence of both cAMP and ManNAc.

Regulation of capsular polysialic acid biosynthesis by temperature in Pasteurella haemolytica A2

The capsular polysaccharide of Pasteurella haemolytica A2 consists of a linear polymer of N-acetylneuraminic acid (Neu5Ac) with alpha(2-8) linkages. The production of this polymer is strictly regulated by the growth temperature and above 40 degrees C no production is detected. Analysis of the enzymatic activities directly involved in its biosynthesis reveals that Neu5Ac lyase, CMP-Neu5Ac synthetase and polysialyltransferase are involved in this regulation. Very low activities were found in P. haemolytica grown at 43 degrees C (at least 25 times lower than those observed when the growth temperature was 37 degrees C). The synthesis of these enzymes increased rapidly when bacteria grown at 43 degrees C were transferred to 37 degrees C and decreased dramatically when cells grown at 37 degrees C were transferred to 43 degrees C. These findings indicate that the cellular growth temperature regulates the synthesis of these enzymes and hence the concentration of the intermediates necessary for capsular polysaccharide genesis in P. haemolytica A2.

Biochemical conditions for the production of polysialic acid by Pasteurella haemolytica A2

The capsular polysaccharide of Pasteurella haemolytica A2 consists of a linear polymer of N-acetylneuraminic acid (Neu5Ac) with alpha(2-8) linkages. When the bacterium was grown at 37 degrees C for 90 h in 250 ml shake flasks at 200 rpm in Brain heart infusion broth (BHIB), it accumulated, attaining a level of 60 microg/ml. Release of this polymer was strictly regulated by the growth temperature, and above 40 degrees no production was detected. The pathway for the biosynthesis of this sialic acid capsular polymer was also examined in P. haemolytica A2 and was seen to involve the sequential presence of three enzymatic activities: Neu5Ac lyase activity, which synthesizes Neu5Ac by condensation of Nacetyl-D-mannosamine and pyruvate with apparent Km values of 91 mM and 73 mM, respectively; a CMP-Neu5Ac synthetase, which catalyzes the production of CMP-Neu5Ac from Neu5Ac and CTP with apparent Km values of 2 mM and 0.5 mM, respectively, and finally a membrane-associated polysialyltransferase, which catalyzes the incorporation of sialic acid from CMP-Neu5Ac into polymeric products with an apparent CMP-Neu5Ac Km of 250 microM.

Regulation of capsular polysialic acid biosynthesis by N-acetyl-D-mannosamine, an intermediate of sialic acid metabolism

N-Acetyl-D-mannosamine (ManNAc) is a specific substrate for the synthesis of N-acetylneuraminic acid, the essential precursor of bacterial capsular polysialic acid (PA). When Escherichia coli K92 used ManNAc as a carbon source, we observed a dramatic reduction (up to 90%) in in vivo PA production. Experiments in which the carbon source was changed revealed that the maximal inhibitory effect occurred when this sugar was present in the medium before the logarithmic phase of bacterial growth had started. Enzymatic analysis revealed that high concentrations of ManNAc-6-phosphate inhibit NeuAc lyase, the enzyme that synthesizes NeuAc for PA biosynthesis in E. coli. These results indicate that ManNAc-6-phosphate is able to regulate NeuAc lyase activity and modulate the PA synthesis.

Identification, expression and tissue distribution of cytidine 5'-monophosphate N-acetylneuraminic acid synthetase activity in the rat

We report the postnatal developmental profiles of N-acetylneuraminic acid cytidylyltransferase (EC 2.7.7.43) (CMP-Neu5Ac synthetase) in different rat tissues. This enzyme, which catalyses the activation of NeuAc to CMP-Neu5Ac, was detected in brain, kidney, heart, spleen, liver, stomach, intestine, lung, thymus, prostate and urinary bladder but not in skeletal muscle. Comparative analysis of the different specific activity profiles obtained shows that the expression of CMP Neu5Ac synthetase is tissue-dependent and does not seem to be embryologically determined. Changes in the level of sialylation during development were also found to be intimately related to variations in the expression of this enzyme, at least in brain, heart, kidney, stomach, intestine and lung.

Cloning, sequencing and expression of the acylneuraminate lyase gene from Clostridium perfringens A99

The acylneuraminate lyase gene from Clostridium perfringens A99 was cloned on a 3.3 kb HindIII DNA fragment identified by screening the chromosomal DNA of this species by hybridization with an oligonucleotide probe that had been deduced from the N-terminal amino acid sequence of the purified protein, and another probe directed against a region that is conserved in the acylneuraminate lyase gene of Escherichia coli and in the putative gene of Clostridium tertium. After cloning, three of the recombinant clones expressed lyase activity above the background of the endogenous enzyme of the E. coli host. The sequenced part of the cloned fragment contains the complete acylneuraminate lyase gene (ORF2) of 864 bp that encodes 288 amino acids with a calculated molecular weight of 32.3 kDa. The lyase structural gene follows a noncoding region with an inverted repeat and a ribosome binding site. Upstream from this regulatory region another open reading frame (ORF1) was detected. The 3'-terminus of the lyase structural gene is followed by a further ORF (ORF3). A high homology was found between the amino acid sequences of the sialate lyases from Clostridium perfringens and Haemophilus influenzae (75% identical amino acids) or Trichomonas vaginalis (69% identical amino acids), respectively, whereas the similarity to the gene from E. coli is low (38% identical amino acids). Based on our new sequence data, the 'large' sialidase gene and the lyase gene of C. perfringens are not arranged next to each other on the chromosome of this species.