Sialic acid occupies the terminal position within glycan molecules on the surfaces of many vertebrate cells, where it functions in diverse cellular processes such as intercellular adhesion and cell signalling. Pathogenic bacteria have evolved to use this molecule beneficially in at least two different ways: they can coat themselves in sialic acid, providing resistance to components of the host's innate immune response, or they can use it as a nutrient. Sialic acid itself is either synthesized de novo by these bacteria or scavenged directly from the host.
The sialic acids are a family of sugars with a shared nine-carbon backbone, a carboxylic acid at the C1 position, and various alpha-glycosidic linkages to the underlying sugar chain (R) from the C2 position. Various substitutions at the C4, C5, C7, C8 and C9 positions combine with linkage variation to generate the diversity of sialic acids in nature. Sialic acids are typically found at the terminal position of N- and O-linked glycans attached to the cell surface and to secreted glycoproteins, as well as on glycosphingolipids expressed at the cell surface.

==========Variant codes:======================

1.0 - Organisms that synthesize sialic acid de novo but do not catabolize it (like all Campylobacter);

2.0 - Organisms that catabolize but do not synthesize (use exogenous sialic acids);

3.0 - Organisms that both catabolize sialic acid and synthesize it de novo from simple metabolites;

4.0 - Organisms that both catabolize exogenous sialic acid and use for surface decortion (precursor scavenging -Haemophilus influenzae );

5.0 - Organisms that just use exogenous sialic acids for surface decoration;

x - stands for a missing gene in the pathway

The sialic acids are a structurally complex family of nine-carbon monosaccharides whose unique physiochemical properties confer biologically diverse activities to mammalian glycoproteins and glycolipids (glycoconjugates). Neither N-acetylneuraminic acid (Neu5Ac), the most common sialic acid, nor any of its >40 structural derivatives is synthesized by most plants, lower metazoans, protists, Archaea or Bacteria.

Sialic acids are 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.
Many pathogens secrete a sialidase that releases sialic acid from a diverse range of host sialoglycoconjugates; however, other sialic acid-utilizing bacteria, such as the respiratory pathogen H. influenzae, lack genes for a sialidase yet are reliant on host-derived sialic acid. Presumably free sialic acid is made available to such pathogens by other, sialidase-expressing bacteria living in the same niche, or by host sialidases that are activated in the course of inflammation. The latter process is part of the normal recycling of sialic acid and there has been a recent suggestion that the host cells might use free sialic acid to help them cope with oxidative stress.
Many commensal and pathogenic bacteria also use environmental (host) sialic acids as sources of carbon, nitrogen, energy, and amino sugars for cell wall synthesis. Microbial sialic acid metabolism has now been firmly established as a virulence determinant in a range of infectious diseases.

********De novo sialic acid synthesis:*************

De novo sialic acid synthesis is thought to begin with the conversion of the common cell wall precursor UDP-GlcNAc to ManNAc by NeuC. The elimination reaction catalyzed by NeuC is thought to involve the formation of a 2-acetamidoglucal intermediate followed by the irreversible epimerization of this intermediate to ManNAc, as happens in mammalian systems.
It was demonstrated that in mammalian systems UDP-GlcNAc 2-epimerase and ManNAc kinase are parts of one bifunctional enzyme( Ref.5). Free ManNAc is produced as the obligate substrate for the condensing (ManNAc plus phosphoenolpyruvate) enzyme, NeuB.

*******Sialic Acid Catabolism - Nan-operon:*******

All Escherichia coli possess a chromosomally encoded nanATEK-yhcH operon for the catabolism of sialic acids.
The nun operon (for N-acylneuraminate) is catabolite repressed and is inducible by free sialic acid.
A complete nan system was defined as one that minimally includes orthologues of genes encoding NanA, NanE, and NanK.
A complete nan system also should include a permease of some type for sialic acid uptake. The only sialic acid transporter in E. coli is sialate transporter NanT.
But it was also shown that different types of bacterial transporters can be used for sialic acid uptake: ABC transporters, secondary (ion- or proton-coupled) symporters or antiporters, and tripartite ATP-independent periplasmic (TRAP) transporters (ref.1).
It was indicated that TRAP transporters are either closely linked or part of the nan operons of H. influenzae, Pasteurella multocida, V. cholerae , and Fusobacterium nucleatum.

===Bacteroides species possesses a new pathway of NANA utilization (ref.16):====

We characterized the nanLET operon in Bacteroides fragilis, whose products are required for the utilization of the sialic acid N-acetyl neuraminic acid (NANA) as a carbon and energy source. The first gene of the operon is nanL, which codes for an aldolase that cleaves NANA into N-acetyl mannosamine (manNAc) and pyruvate. The next gene, nanE, codes for a manNAc/N-acetylglucosamine (NAG) epimerase, which, intriguingly, possesses more similarity to eukaryotic renin binding proteins than to other bacterial NanE epimerase proteins. Unphosphorylated manNAc is the substrate of NanE, while ATP is a cofactor in the epimerase reaction. The third gene of the operon is nanT, which shows similarity to the major transporter facilitator superfamily and is most likely to be a NANA transporter.
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Exogenous sialic acid is transported by a secondary transporter (NanT) of the major facilitator superfamily and degraded intracellularly by NanA to yield pyruvate and the amino sugar ManNAc. The products of nanK and nanE were suggested to function by first phosphorylating ManNAc and then epimerizing the ManNAc-6-P generated by the kinase reaction to GlcNAc-6-P. Recent biochemical analyses confirmed that NanK is an ATP-dependent kinase specific for ManNAc and that NanE is a reversible 2-epimerase. The upstream regulatory open reading frame yhcK was renamed nanR and shown to repress the nan operon in the absence of sialic acid. The function of yhcH is unknown.

The GlcNAc-6-P produced by NanE was shown to enter the amino sugar degradative pathway encoded by nagBA , converting GlcNAc-6-P ultimately to fructose-6-P by GlcNAc-6-P deacetylase (NagA) and glucosamine-6-P deaminase (NagB). Sialic acid thus can serve as the sole carbon or nitrogen source in E. coli and as a source of amino sugars (GlcNAc and glucosamine) for cell wall biosynthesis.

NanC:

The Escherichia coli yjhA (renamed nanC) gene encodes a protein of the KdgM family of outer membrane-specific channels. NanC is an outer membrane channel protein allowing the entry of Neu5Ac into E. coli.

YhcH:

-Putative sugar isomerase involved in processing of exogenous sialic acid (ref. 9 )



A variety of pathogens use at least one of four distinct mechanisms for decorating their surfaces with sialic acid:

(a) De novo synthesis:
E. coli K1 and Neisseria meningitidis were the first microorganisms shown to synthesize Neu5Ac from simple metabolites. After synthesis, Neu5Ac must be activated by conversion to the nucleotide sugar donor cytidine monophosphate (CMP)-Neu5Ac before it can be added to appropriate acceptors by linkage-specific sialyltransferases.


(b) Donor scavenging:
The gonococcal surface sialyltransferase uses the exogenous donor to sialylate its surface. Exogenous CMP-Neu5Ac was the substrate for an N. gonorrhoeae enzyme that sialylates the bacterial cell surface. N. gonorrhoeae thus appears to encode a dramatically truncated sialylation pathway comprising just the terminal sialyltransferase.

(c) Trans-sialidase mechanism:
Sialidases are a superfamily of N-acylneuraminate-releasing (sialic-acid-releasing) exoglycosidases found mainly in higher eukaryotes and in some, mostly pathogenic, viruses, bacteria and protozoans.
The trypanosome trans-sialidase removes sialic acid from a red blood cell membrane and transfers it to the parasite galactosyl acceptor (transglycosylation) instead of to water (hydrolysis).
While sialidases are obviously encoded by the nan systems of V. cholerae and S. pneumoniae, all strains of some organisms such as E. coli and H. influenzae appear to lack these enzymes, while other organisms such as P. multocida encode sialidases that are unlinked to nan.

Precursor scavenging:
Haemophilus influenzae transports host-derived sialic acid and either degrades it to N-acetylmannosamine and pyruvate, or activates it to CMP-sialic acid for subsequent sialyltransfer to an LOS acceptor.
H. influenzae lacks NeuBC but encodes a NeuA orthologue as well as multiple sialyltransferases. It was hypothesized that H. influenzae scavenges free sialic acids from its host, presumably by using a TRAP transporter system, and then activates the internalized sialic acid with its NeuA orthologue and transfers sialic acid to LOS acceptors.
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NanK is missing in many Staphylococcus.

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REFERENCES:

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Microbiology. 2007 Sep;153(Pt 9):2817-22. Review. PMID: 17768226;

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3. Ringenberg, Michael A., Steenbergen, Susan M. & Vimr, Eric R. The first committed step in the biosynthesis of sialic acid by Escherichia coli K1 does not involve a phosphorylated N-acetylmannosamine intermediate. Molecular Microbiology 50 (3), 961-975.

4. Condemine, G., Berrier, C., Plumbridge, J., Ghazi, A. (2005). Function and Expression of an N-Acetylneuraminic Acid-Inducible Outer Membrane Channel in Escherichia coli. J. Bacteriol. 187: 1959-1965


5. Hinderlich, S., Stäsche, R., Zeitler, R., and Reutter, W. (1997) A bifunctional enzyme catalyzes the first two steps in N-acetylneuraminic acid biosynthesis of rat liver. Purification and characterization of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase. J Biol Chem 272: 24313-24318.

6. S. M. Steenbergen, C. A. Lichtensteiger, R. Caughlan, J. Garfinkle, T. E. Fuller, and E. R. Vimr. Sialic Acid Metabolism and Systemic Pasteurellosis.Infect. Immun., March 1, 2005; 73(3): 1284 - 1294.

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Human renin-binding protein is the enzyme N-acetyl-D-glucosamine 2-epimerase.
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