Exo-α-sialidase (EC 3.2.1.18, sialidase, neuraminidase; systematic name acetylneuraminyl hydrolase) is a
glycoside hydrolase that cleaves the
glycosidic linkages of
neuraminic acids:
: Hydrolysis of α-(2→3)-, α-(2→6)-, α-(2→8)- glycosidic linkages of terminal sialic acid residues in oligosaccharides, glycoproteins, glycolipids, colominic acid and synthetic substrates
Neuraminidase enzymes are a large family, found in a range of organisms. The best-known neuraminidase is the
viral neuraminidase, a drug target for the prevention of the spread of
influenza
Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptom ...
infection. The viral neuraminidases are frequently used as
antigenic determinants found on the surface of the influenza virus. Some variants of the influenza neuraminidase confer more
virulence
Virulence is a pathogen's or microorganism's ability to cause damage to a host.
In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ...
to the virus than others. Other homologues are found in mammalian cells, which have a range of functions. At least four mammalian
sialidase homologues have been described in the
human genome
The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. These are usually treated separately as the ...
(see
NEU1,
NEU2,
NEU3,
NEU4).
Sialidases may act as pathogenic factors in microbial infections.
Reaction
There are two major classes of Neuraminidase that cleave exo or endo poly-sialic acids:
* Exo hydrolysis of α-(2→3)-, α-(2→6)-, α-(2→8)-glycosidic linkages of terminal sialic acid residues
* Endo hydrolysis of (2→8)-α-sialosyl linkages in oligo- or poly(sialic) acids
(see endo-α-sialidase.)
Function
Sialidases, also called neuraminidases, catalyze the hydrolysis of terminal
sialic acid Sialic acids are a class of alpha-keto acid sugars with a nine-carbon backbone.
The term "sialic acid" (from the Greek for saliva, - ''síalon'') was first introduced by Swedish biochemist Gunnar Blix in 1952. The most common member of this ...
residues from the newly formed
virions and from the host cell receptors.
Sialidase activities include assistance in the mobility of virus particles through the respiratory tract mucus and in the elution of virion progeny from the infected cell.
Subtypes
Swiss-Prot
UniProt is a freely accessible database of protein sequence and functional information, many entries being derived from genome sequencing projects. It contains a large amount of information about the biological function of proteins derived fro ...
lists 137 types of neuraminidase from various species as of October 18, 2006. Nine subtypes of influenza neuraminidase are known; many occur only in various species of duck and chicken. Subtypes N1 and N2 have been positively linked to epidemics in humans, and strains with N3 or N7 subtypes have been identified in a number of isolated deaths.
CAZy defines a total of 85 glycosyl hydrolase families, of which families
GH34 (viral),
GH33 (cellular organisms),
GH58 (viral and bacterial),
GH83 (viral) are major families that contain this enzyme. GH58 is the only endo-acting family.
The following is a list of major classes of neuraminidase enzymes:
*
Viral neuraminidase
*
Bacterial neuraminidase
Bacterial neuraminidase is type of neuraminidase and a virulence factor for many bacteria including '' Bacteroides fragilis'' and '' Pseudomonas aeruginosa''.
Its function is to cleave a sialic acid residue off ganglioside- GM1 (a modulator of ce ...
* Mammalian neuraminidases:
Structure
Influenza
Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptom ...
neuraminidase is a mushroom-shaped projection on the surface of the influenza virus. It has a head consisting of four co-planar and roughly spherical subunits, and a hydrophobic region that is embedded within the interior of the virus' membrane. It comprises a single polypeptide chain that is oriented in the opposite direction to the
hemagglutinin antigen. The composition of the polypeptide is a single chain of six conserved polar amino acids, followed by hydrophilic, variable amino acids. β-Sheets predominate as the secondary level of protein conformation.
The structure of trans-sialidase includes a catalytic
β-propeller domain, a ''N''-terminal
lectin
Lectins are carbohydrate-binding proteins that are highly specific for sugar groups that are part of other molecules, so cause agglutination of particular cells or precipitation of glycoconjugates and polysaccharides. Lectins have a role in rec ...
-like domain and an irregular beta-stranded domain inserted into the catalytic domain.
Recent emergence of
oseltamivir
Oseltamivir, sold under the brand name Tamiflu, is an antiviral medication used to treat and prevent influenza A and influenza B, viruses that cause the flu. Many medical organizations recommend it in people who have complications or are at h ...
and
zanamivir resistant
human influenza A(
H1N1
In virology, influenza A virus subtype H1N1 (A/H1N1) is a subtype of influenza A virus. Major outbreaks of H1N1 strains in humans include the Spanish flu, the 1977 Russian flu pandemic and the 2009 swine flu pandemic. It is an orthomyxoviru ...
) H274Y has emphasized the need for suitable expression systems to obtain large quantities of highly pure and stable,
recombinant neuraminidase through two separate artificial tetramerization domains that facilitate the formation of catalytically active neuraminidase homotetramers from
yeast
Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to consti ...
and ''
Staphylothermus marinus
In taxonomy, ''Staphylothermus'' is a genus of the Desulfurococcaceae. Taxonomy
Desulfurococcaceae are anaerobic, sulfur respiring, extreme thermophiles. Desulfurococcaceae share the same family as Desulfurococcus. Two species of ''Staphylotherm ...
'', which allow for secretion of
FLAG-tagged
proteins and further purification.
Mechanism
The enzymatic mechanism of influenza virus sialidase has been studied by Taylor et al., shown in Figure 1. The enzyme catalysis process has four steps. The first step involves the distortion of the α-sialoside from a
2C
5 chair conformation (the lowest-energy form in solution) to a pseudoboat conformation when the sialoside binds to the sialidase. The second step leads to an oxocarbocation intermediate, the sialosyl cation. The third step is the formation of Neu5Ac initially as the α-anomer, and then mutarotation and release as the more thermodynamically stable β-Neu5Ac.
Inhibitors
Neuraminidase inhibitors are useful for combating
influenza
Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptom ...
infection:
zanamivir, administered by inhalation;
oseltamivir
Oseltamivir, sold under the brand name Tamiflu, is an antiviral medication used to treat and prevent influenza A and influenza B, viruses that cause the flu. Many medical organizations recommend it in people who have complications or are at h ...
, administered orally;
peramivir administered
parenterally, that is through intravenous or intramuscular injection; and
laninamivir which is in phase III clinical trials.
There are two major proteins on the surface of influenza virus particles. One is the lectin haemagglutinin protein with three relatively shallow sialic acid-binding sites and the other is enzyme sialidase with the active site in a pocket. Because of the relative deep active site in which low-molecular-weight inhibitors can make multiple favorable interactions and approachable methods of designing transition-state analogues in the hydrolysis of sialosides, the sialidase becomes more attractive anti-influenza drug target than the haemagglutinin.
After the X-ray crystal structures of several influenza virus sialidases were available, the structure-based inhibitor design was applied to discover potent inhibitors of this enzyme.
The unsaturated sialic acid (''N''-acetylneuraminic acid
eu5ac derivative 2-deoxy-2, 3-didehydro-
D-''N''-acetylneuraminic acid (Neu5Ac2en), a sialosyl cation transition-state (Figure 2) analogue, is believed the most potent inhibitor core template. Structurally modified Neu5Ac2en derivatives may give more effective inhibitors.
Many Neu5Ac2en-based compounds have been synthesized and tested for their influenza virus sialidase inhibitory potential. For example:
The 4-substituted Neu5Ac2en derivatives (Figure 3), 4-amino-Neu5Ac2en (Compound 1), which showed two orders of magnitude better inhibition of influenza virus sialidase than Neu5Ac2en5 and 4-guanidino-Neu5Ac2en (Compound 2), known as Zanamivir, which is now marketed for treatment of influenza virus as a drug, have been designed by von Itzstein and coworkers.
A series of amide-linked C9 modified Neu5Ac2en have been reported by Megesh and colleagues as NEU1 inhibitors.
See also
*
Glycoside hydrolase family 33
*
Neuraminidase inhibitors
*
Hemagglutinin (influenza)
Influenza hemagglutinin (HA) or haemagglutinin ">/sup> (British English) is a homotrimeric glycoprotein found on the surface of influenza viruses and is integral to its infectivity.
Hemagglutinin is a Class I Fusion Protein, having multifunc ...
References
External links
*
Orthomyxoviruses Robert B. Couch, UTMB. Article includes a good clear line drawing of a neuraminidase on an influenza virus.
{{Authority control
Carbohydrate chemistry
EC 3.2.1
Glycobiology
Neuraminidase inhibitors