Viral glycoprotein neuraminidase

All the nearest-neighbor interactions between sialic acid or Neu5Ac2en and the protein are with totally conserved amino acids. Thus an inhibitor designed to bind only to the conserved active-site residues of neuraminidase would inhibit neuraminidase activity across all strains of influenza. This would enable the development of an antiviral drug that would affect the spread of viral replication potentially in three ways, i.e., transport through the protective mucosal layer, desialyation of freshly synthesized viral glycoproteins, and elution of progeny virions from infected cells. 

Neuraminidase is an essential viral glycoprotein for virus replication and release. The neuraminidase inhibitors zanamivir and oseltamivir have recently been approved for the treatment of acute uncomplicated influenza infection. When a 5-day course of therapy is initiated within 36-48 hours after the onset of symptoms, use of either agent shortens the severity and duration of illness and may decrease the incidence of respiratory complications in children and adults. Unlike amantadine and rimantidine, zanamivir and oseltamivir have activity against both influenza A and influenza B. Zanamivir is administered via oral inhaler. The compound displays poor oral bioavailability, limited plasma protein binding, rapid renal clearance, and absence of significant metabolism. Nasal and throat discomfort may occur—as well as bronchospasm in patients with reactive airway disease. 

Studies with simian viral hemagglutinin neuraminidase and yeast acid phosphatase suggest that AT-glycans are needed for proper folding of glycoproteins... 

Influenza A viruses cause severe infections in the respiratory system and are responsible for seasonal epidemics and sporadic pandemics. The primary method for prevention is through vaccination but vaccine production by current methods cannot be carried out in time to stop the progress of a new strain of the influenza virus. Therefore, effective antiviral agents are used for prophylactic and therapeutic treatments. Among several viral molecular targets for anti-influenza drugs, the surface glycoprotein neuraminidase appears particularly attractive. Selective inhibitors of this enzyme have been developed and two of which, oseltamivir phosphate 18 (Tamiflu ) " and zanamivir 19 (Relenza ), have been approved for human use (Scheme 6). 

Varghese IN, Colman PM (1991) Three-dimensional structure of the neuraminidase of influenza virus A/Tokyo/3/67 at 2.2 A resolution. 1 Mol Biol 221 473 86 Varghese IN, Laver WG, Colman PM (1983) Structure of the influenza virus glycoprotein antigen neuraminidase at 2.9 A resolution. Nature 303 35 0 Varghese IN, McKimm-Breschkin IL, Caldwell IB, Kortt AA, Colman PM (1992) The structure of the complex between influenza virus neuraminidase and sialic acid, the viral receptor. Proteins 14 327-332... 

Zanamivir (2) is a potent competitive inhibitor of viral neuraminidase glycoprotein, which is essential in the infective cycle of both influenza A and B viruses. It inhibits a wide range of influenza A and B types in vitro as well as in vivo. The concentrations of inhibiting in vitro plaque formation of influenza A and B virus by 50% in Madin-Darby canine kidney (MDCK) cells were 0.004-0.014 p.mol/L in laboratory-passaged strains, and 0.002-16 p.mol/L in assays of clinical isolates. Due to its low bioavailability, it is delivered by inhalation via the Diskhaler , 10 mg twice daily, or intranasally 2-4 times daily for 5 days. After an intravenous dose of 1 -16 mg, the median elimination half-life was ti/2 = 7 h, the volume of distribution at steady state was Vdss = 16 L, and 90% of the dose was excreted unchanged in the urine. After intranasal and inhaled (dry powder) administration, maximum serum concentrations occurred within 2h and the terminal phase half-lives were 3.4 and 2.9 h, respectively. The bioavailabilities were 10 and 25%, respectively, and 20% after inhalation of zanamivir (2) by nebulizer. 

Oseltamivir (1) is a prodrug of GS-4071 (7, Fig. 7.2). It is also a potent competitive inhibitor of viral neuraminidase glycoprotein for both influenza A and B types. Similar to zanamivir (2), oseltamivir (1) is also the fmit of SBDD in the sense that the protein structure directed the design of the ligand (sialic acid) and the protein-bound ligand conformation is close to the designed stmcture. In particular, the neuraminidase active site contains several... 

In addition to the lipid bilayer, enveloped viruses generally have two or more distinct layers of protein that are organized across the membrane. Thus, most viruses have an outer layer of proteins, usually glycoproteins, which are anchored in the membrane as integral membrane proteins. These proteins function to attach the virion to target host cell receptors and facilitate the entry or fusion of the viral membrane with that of the host cell. In addition, some viruses also contain enzymatic activities associated with this outer layer of protein. For example, influenza virus carries with it a neuraminidase that is responsible for cleaving sialic acid residues on host cells. 

Influenza virus neuraminidase Influenza Viral envelope glycoprotein involved in viral release, cleavage of sialic add residues of new virus particles and host membranes Inhibition at active site... 

Figure 11.29 Viral receptors. Influenza virus targets cells by binding to sialic acid residues (purple diamonds) located at the termini of oligosaccharides present on cell-surface glycoproteins and glycolipids. These carbohydrates are bound by hemagglutinin (interaction circles), one of the major proteins expressed on the surface of the virus. The other major viral surface protein, neuraminidase, iis an enzyme that cleaves oligosaccharide chains to release the viral particle at a later stage of the viral life cycle.

Castanospermine has been screened for efficacy against simian immunodeficiency virus (265), and has been shown to prevent syncytium formation in feline astrocyte cultures infected with the feline immimodeficiency virus by modifying the viral cell envelope (266). It suppressed syncytium formation and hemolytic activity in baby hamster kidney cells infected with Newcastle disease virus however, synthesis and cell surface expression of the hemagglutinin-neuraminidase glycoprotein in the viral envelope were not affected, which strengthens the hypothesis that poor transport of the parent alkaloid across membrane barriers may limit its therapeutic use (267). Both 239 and its 6-0-butanoyl ester had comparable relative toxicities and antiviral effects on Rauscher murine leukemia virus (268), but the ester was more potent than the parent alkaloid in inhibiting replication of Moloney murine leukemia virus (258). The ester was also active against herpes simplex viruses types 1 and 2 (269,270). In the latter case, conclusive evidence was provided for intracellular hydrolysis to 239. 

The enzyme catalyzes the cleavage of terminal sialic acid from adjacent sugar residues. Neuraminidase and hemagglutinin are the two glycoproteins attach to the influenza viral membrane. They make up the antigenic surface of the virus, and changes in these proteins form the basis for evasion of immune recognition from previous infections. Data extracted from Davies et ai (1990). Individual references are a) Sheriff et al. (1987) b) Padlan et al. (1989) c) Amit et al. (1986) d) Tulip et oL (1989). 

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