Targeting human respiratory syncytial virus transcription anti-termination factor M2-1 to inhibit in vivo viral replication

Human respiratory syncytial virus (hRSV) is a leading cause of acute lower respiratory tract infection in infants, elderly and immunocompromised individuals. To date, no specific antiviral drug is available to treat or prevent this disease. Here, we report that the Smoothened receptor (Smo) antagonist cyclopamine acts as a potent and selective inhibitor of in vitro and in vivo hRSV replication. Cyclopamine inhibits hRSV through a novel, Smo-independent mechanism. It specifically impairs the function of the hRSV RNA-dependent RNA polymerase complex notably by reducing expression levels of the viral anti-termination factor M2-1. The relevance of these findings is corroborated by the demonstration that a single R151K mutation in M2-1 is sufficient to confer virus resistance to cyclopamine in vitro and that cyclopamine is able to reduce virus titers in a mouse model of hRSV infection. The results of our study open a novel avenue for the development of future therapies against hRSV infection.

Recent strategies in the search for new anti-influenza therapies

Influenza is a highly contagious, acute upper respiratory tract disease caused by influenza virus, a member of the Orthomyxoviridae family. The viral particles have two surface antigens, haemagglutinin and sialidase (neuraminidase) that extensively decorate the surface of the virus and have been implicated in viral attachment and fusion, and the release of virion progeny, respectively. The receptor for haemagglutinin is the terminal sialic acid residue of host cell surface sialyloligosaccharides, while sialidase catalyses the hydrolysis of terminal sialic acid residues from sialyloligosaccharides. Extensive crystallographic studies of both these proteins have revealed that the residues that interact with the sialic acid are strictly conserved. Therefore, these proteins make attractive targets for the design of drugs to halt the progression of the virus. Recent successful efforts in the search for new cures for influenza have led to the development of three clinically-useful anti-influenza drugs. All three are potent, selective inhibitors of influenza virus A and B sialidase. Strategies for the development of haemagglutinin inhibitors have also been devised.

Synthesis and biological evaluation of sialylmimetics as rotavirus inhibitors

Rotaviruses cause severe gastroenteritis in infants and are estimated to be responsible for over 600 000 deaths annually, primarily in developing countries. The development of potential inhibitors of this virus is therefore of great interest, particularly since the safety and efficacy of rotaviral vaccines has recently been questioned. This study describes the synthesis of a variety of compounds that can be considered as mimetics of N-acetylneuraminic acid thioglycosides and the subsequent in vitro biological evaluation of these sialylmimetics as inhibitors of rotaviral infection. Our results show that readily accessible carbohydrate-based compounds have the potential to act as inhibitors of rotaviral replication in vitro, presumably through inhibition of the rotaviral adhesion process.

Synthesis of novel sialylmimetics as biological probes

Glycomimetics are increasingly being recognised as powerful tools in the search for novel compounds that possess useful biological properties. This paper describes our preliminary efforts towards the development of novel mimetics of sialic acid thioglycosides. These sialylmimetics are readily prepared and have been shown, in some instances, to have biological properties similar to sialic acid thioglycosides.

Synthesis of C-3 nitrogen-containing derivatives of N-acetyl-alpha,beta-D-mannosamine as substrates for N-acetylneuraminic acid aldolase

The synthesis of 3-azido-3-deoxy, 3-amino-3-deoxy and 3-N-tert-butyloxycarbonyl-3-deoxy derivatives of 2-acetamido-2-deoxy-alpha,beta-D-mannose (N-acetyl-alpha,beta-D-mannosamine, ManNAc), is presented. The 3-azido-3-deoxy- and 3-N-tert-butyloxycarbonyl compounds were further characterised as their peracetates. A preliminary study has found that these C-3 nitrogen-substituted derivatives of ManNAc not to be substrates for Neu5Ac aldolase.

A unique sialidase that cleaves the Neu5Gcalpha2-->5-O(glycolyl)Neu5Gc linkage: comparison of its specificity with that of three microbial sialidases toward four sialic acid dimers

We found that the hepatopancreas of oyster, Crassostrea virginica, contained a sialidase capable of releasing Neu5Gc from the novel polysialic acid chain (-->5-O(glycolyl)Neu5Gcalpha2-->)n more efficiently than from the conventional type of polysialic acid chains, (-->8Neu5Acalpha2-->)n, or (-->8Neu5Gcalpha2-->)n. We have partially purified this novel sialidase and compared its reactivity with that of microbial sialidases using four different sialic acid dimers, Neu5Gcalpha2-->5-O(glycolyl)Neu5Gc (Gg2), Neu5Acalpha2-->8Neu5Ac (A2), Neu5Gcalpha2-->8Neu5Gc (G2), and KDNalpha2-->8KDN (K2) as substrates. Hydrolysis was monitored by high performance anion-exchange chromatography with a CarboPac PA-100 column and pulsed amperometric detection, the method by which we can accurately quantitate both the substrate (sialiac acid dimers) and the product (sialic acid monomers). The oyster sialidase effectively hydrolyzed Gg2 and K2, whereas A2 and G2 were poor substrates. Neu5Ac2en but not KDN2en effectively inhibited the hydrolysis of Gg2 by the oyster sialidase. Likewise, the hydrolysis of K2 by the oyster sialidase was inhibited by a cognate inhibitor, KDN2en, but not by Neu5Ac2en. Using the new analytical method we found that Gg2 was hydrolyzed less efficiently than A2 but much more readily than G2 by Arthrobacter ureafaciens sialidase. This result was at variance with the previous report using the thiobarbituric acid method to detect the released free sialic acid [Kitazume, S., et al. (1994) Biochem. Biophys. Res. Commun. 205, 893-898]. In agreement with previous results, Gg2 was a poor substrate for Clostridium perfringens sialidase, while K2 was refractory to all microbial sialidases tested. Thus, the oyster sialidase is novel and distinct from microbial sialidases with regards to glycon- and linkage-specificity. This finding adds an example of the presence of diverse sialidases, in line with the diverse sialic acids and sialic acid linkages that exist in nature. The new sialidase should become useful for both structural and functional studies of sialoglycoconjugates.

Preliminary 1H NMR investigation of sialic acid transfer by the trans-sialidase from Trypanosoma cruzi

1H NMR spectroscopy has been used to investigate the transfer of sialic acid from sialic acid donor molecules to acceptor molecules using the trans-sialidase from Typanosoma cruzi. It is clearly demonstrated that NMR spectroscopy is an efficient and powerful means of monitoring the trans-sialidase promoted transfer of sialic acid from donor to acceptor.

The synthesis of biotinylated carbohydrates as probes for carbohydrate-recognizing proteins

The intimate involvement of carbohydrate-protein interactions in a number of important biological processes has prompted several research efforts towards developing new methods of investigating these glycobiological interactions. Biotinylated oligosaccharides are emerging as a new and powerful tool in this area of research, primarily due to their high affinity towards streptavidin and their ease of immobilization on matrices. Here we describe a novel synthetic approach towards biotinylated saccharides which incorporate a UV absorbing group into the final compounds. The synthetic strategy described is applicable to a variety of saccharides, with examples of biotinylated mono-, di-, and trisaccharides being prepared with overall high efficiency.

Rapid access to uronic acid-based mimetics of Kdn2en from D-glucurono-6,3-lactone

A concise route to novel mimetics of Kdn2en, based on delta4-uronic acids, from D-glucurono-6,3-lactone is presented. Uronic acid-based mimetics in which an aliphatic ether (O-glycoside), a thioether (S-glycoside), or acetamide takes the place of the natural C-6 glycerol sidechain of the sialic acid were synthesized from the key intermediate, methyl 2,3,4-tri-O-acetyl-alpha-D-glucopyranosyluronate bromide.

A high-throughput assay for rat liver golgi and Saccharomyces cerevisiae-expressed murine CMP-N-acetylneuraminic acid transport proteins

Rat liver Golgi and Saccharomyces cerevisiae-expressed CMP-Neu5Ac transport protein were reconstituted in phosphatidylcholine liposomes and transport of CMP-Neu5Ac into these proteoliposomes was determined. The separation of transported substrate from free substrate was performed using Multiscreen minicolumns loaded with Sephadex G-50 resin (fine). The CMP-Neu5Ac transport characteristics of the rat liver Golgi and S. cerevisiae-expressed transporters, determined using this separation system, were very similar to those previously reported. Inhibition studies, utilizing the above procedure, revealed that the main structural features required for recognition of glycosyl nucleosides by the rat liver Golgi CMP-Neu5Ac transport protein were the nature of the nucleoside base and the anomeric configuration of the associated carbohydrate. In general, pyrimidine-based glycosyl nucleosides were found to inhibit transport to a far greater extent than purine-based glycosyl nucleosides, an observation that is in good agreement with previous reports. These results indicate that the reconstitution procedure, in conjunction with Multiscreen minicolumns, is an effective high-throughput method for the determination of CMP-Neu5Ac transport.

A one-pot synthesis of novel N,N-dialkyl-S-glycosylsulfenamides

An efficient new entry into N,N-dialkyl-S-glycosylsulfenamides is reported. The reaction of bis-activated alkyl halides in the presence of a secondary amine base with glycosylic S-acetyl derivatives (1-S-acetyl-1-thioaldoses or 2-S-acetyl-2-thioketoses) results in the formation of novel carbohydrate sulfonamides. These new carbohydrate-based sulfonamides may provide useful derivatives with biological activity, as well as provide reactive carbohydrate sulfonylating agents.

Synthesis and evaluation of C-9 modified N-acetylneuraminic acid derivatives as substrates for N-acetylneuraminic acid aldolase

Several C-9 modified N-acetylneuraminic acid derivatives have been synthesised and evaluated as substrates of N-acetylneuraminic acid aldolase. Simple C-9 acyl or ether modified derivatives of N-acetylneuraminic acid were found to be accepted as substrates by the enzyme, albeit being transformed more slowly than Neu5Ac itself. 1H NMR spectroscopy was used to evaluate the extent of the enzyme catalysed transformation of these compounds. Interestingly, the chain-extended Neu5Ac derivative 16 is not a substrate for N-acetylneuraminate lyase and behaves as an inhibitor of the enzyme.

The synthesis and evaluation of novel sialic acid analogues bound to matrices for the purification of sialic acid-recognising proteins

A novel N-acetylneuraminic acid analogue, 2-S-(5'-aminopentyl) 5-acetamido-3,5-dideoxy-2-thio-D-glycero-alpha-D-galacto-2- nonulopyranosidonic acid, as well as the thiosialoside 2-S-(2'-aminoethyl) 5-acetamido-3,5-dideoxy-2-thio-D-glycero-alpha-D-galacto-2- nonulopyranosidonic acid, have been synthesised and successfully coupled to CNBr-activated Sepharose 4B through the terminal amino group. The resultant affinity resins have proved efficient in purifying a number of sialic acid-recognising proteins such as Vibrio cholerae sialidase, sialidase-L from leech, trans-sialidase from Trypanosoma cruzi, and sialyltransferases from rat liver, all in high yield.

Investigation of the stability of thiosialosides toward hydrolysis by sialidases using NMR spectroscopy

[formula: see text] 1H NMR spectroscopy has been used to investigate whether the alpha(2-->6)-linked thiosialoside 3 and the alpha(2-->3)-linked thiosialoside 9 are hydrolyzed in the presence of Vibrio cholerae sialidase. Similarly, the hydrolysis of the O-ketosides Neu5Ac-2-O-alpha-(2-->3)-Gal beta Me (4) and the alpha-(2-->6)-sialyllactoside 7, representing natural alpha(2-->3)- and alpha(2-->6)-linked sialosides, respectively, was investigated. The results of the 1H NMR experiments clearly demonstrate that the thiosialosides are not hydrolyzed by Vibrio cholerae sialidase. As expected, the O-sialosides are hydrolyzed to give N-acetyl-alpha-D-neuraminic acid as the first product of substrate cleavage.

Synthesis of delta4-beta-D-glucopyranosiduronic acids as mimetics of 2,3-unsaturated sialic acids for sialidase inhibition

Mimetics of Neu5Ac2en and KDN2en, based on delta4-beta-delta-glucopyranosiduronic acids, have been synthesised. The Neu5Ac2en mimetic 5 showed inhibition of both bacterial and viral sialidases, with inhibition of the viral sialidase being comparable to that of Neu5Ac2en itself.

How pure is your thiosialoside? A reinvestigation into the HPLC purification of thioglycosides of N-acetylneuraminic acid

The synthesis of thiosialosides as potential biological probes for investigations involving the use of sialic acid-recognising proteins has been reinvestigated. It has been found that the most efficient method for the preparation of thiosialosides free from any 2,3-didehydro sialic acid contaminants involves an intermediate HPLC purification of thiosialosides as their methyl esters. Subsequent methyl ester hydrolysis provides thiosialosides (eg. 6 and 14) which are suitable for studies involving the use of sialic acid-recognising proteins.

Synthesis and evaluation of N-acetylneuraminic acid-based affinity matrices for the purification of sialic acid-recognizing proteins

The synthesis of 2-S-(2-aminoethyl) 5-acetamido-3,5-dideoxy-2-thio-D-galacto-2-nonulopyranosidonic acid (1) has been successfully achieved from the precursors methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-2-S-acetyl-3,5-dideoxy-2-thio-D-glyce ro-alpha-D-galacto-2-nonulopyranosonate (2) and 2-bromo-N-(tert-butoxycarbonyl)-ethylamine (5). Compounds 1 and 2 were coupled, via amino and thioglycosidic linkages, respectively, to epoxy-activated Sepharose 6B. The resultant affinity adsorbents have proved efficient in purifying the sialic acid-recognizing enzyme Vibrio cholerae sialidase, in a one-step process with yields in the order of 60%.

Novel bismuth compounds have in vitro activity against Helicobacter pylori

The increasing antimicrobial resistance in Helicobacter pylori has led to the search for new therapeutic agents. In this in vitro study, novel compounds combining bismuth with sialic acid inhibitors were investigated for bactericidal activity using time-kill methodology. The activity of these compounds was compared to bismuth subcitrate against a type strain and a clinical isolate of H. pylori. The compounds tested showed cidal activity which was related to the bismuth component of each drug. These compounds may offer a potential advantage over current bismuth preparations with the sialic acid inhibitor moiety interfering with adhesion of H. pylori to gastric epithelium.

A conformational study of the human and rat encephalitogenic myelin oligodendrocyte glycoprotein peptides 35-55

Myelin oligodendrocyte glycoprotein (MOG), is considered an important central-nervous system-specific target autoantigen for primary demyelination in autoimmune diseases like multiple sclerosis. We have recently demonstrated that MOG or its derived peptide, MOG-(35-55)-peptide, are able to produce in animals, clinicopathologic signs that mimic multiple sclerosis. The rat MOG sequence spanning amino acids 35-55 [rMOG-(35-55)-peptide] differs from the human sequence [hMOG-(35-55)-peptide] by a single amino acid substitution, i.e. Pro42-->Ser. Mice injected with rMOG-(35-55)-peptide showed severe inflammation and demyelination throughout the central nervous system but, interestingly, mice injected with hMOG-(35 -55)-peptide showed only a few foci of mild inflammation with no demyelination. Circular dichroism and nuclear magnetic resonance spectroscopy have been used to structurally characterise the bioactive peptides hMOG-(35-55)-peptide and rMOG-(35-55)-peptide. In 0.1 M K2HPO4/KOH, 90% H2O/D2O solutions, these derived peptides have been shown, by NMR spectroscopy, to adopt detectable levels of short-range structure in equilibrium with unfolded conformers. On addition of 2,2,2-trifluoro-(2H3)ethanol, rMOG-(35-55)-peptide and hMOG-(35-55)-peptide adopt folded structures which have nuclear Overhauser enhancements characteristic of a poorly defined alpha-helix over residues 44-51. There are some indications of secondary structure also evident in the N-terminal region of rMOG-(35-55)-peptide. CD spectroscopy has revealed that in aqueous solution both peptides are unfolded but in 2.2.2-trifluoroethanol and, at micellar concentrations of sodium dodecyl sulfate, rMOG-(35-55)-peptide and, to a lesser extent, hMOG-(35-55)-peptide adopt helical conformations. In contrast, at non-micellar concentrations of SDS rMOG-(35-55)-peptide and hMOG-(35-55)-peptide adopt, according to CD spectroscopy, a beta-structure indicating that the peptides change conformation depending on the microenvironment of the amino acids.

Catalysis by a new sialidase, deaminoneuraminic acid residue-cleaving enzyme (KDNase Sm), initially forms a less stable alpha-anomer of 3-deoxy-D-glycero-D-galacto-nonulosonic acid and is strongly inhibited by the transition state analogue, 2-deoxy-2, 3-didehydro-D-glycero-D-galacto-2-nonulopyranosonic acid, but not by 2-deoxy-2,3-didehydro-N-acetylneuraminic acid

Deaminoneuraminic acid residue-cleaving enzyme (KDNase Sm) is a new sialidase that has been induced and purified from Sphingobacterium multivorum. Catalysis by this new sialidase has been studied by enzyme kinetics and 1H NMR spectroscopy. Vmax/Km values determined for synthetic and natural substrates of KDNase Sm reveal that 4-methylumbelliferyl-KDN (KDNalpha2MeUmb, Vmax/Km = 0.033 min-1) is the best substrate for this sialidase, presumably because of its good leaving group properties. The transition state analogue, 2, 3-didehydro-2,3-dideoxy-D-galacto-D-glycero-nonulosonic acid, is a strong competitive inhibitor of KDNase Sm (Ki = 7.7 microM versus Km = 42 microM for KDNalpha2MeUmb). 2-Deoxy-2, 3-didehydro-N-acetylneuraminic acid and 2-deoxy-2, 3-didehydro-N-glycolylneuraminic acid are known to be strong competitive inhibitors for bacterial sialidases such as Arthrobacter ureafaciens sialidase; however, KDNase Sm activity is not significantly inhibited by these compounds. This observation suggests that the hydroxyl group at C-5 is important for recognition of the inhibitor by the enzyme. Reversible addition of water molecule (or hydroxide ion) to the reactive sialosyl cation, presumably formed at the catalytic site of KDNase Sm, eventually gives rise to two different adducts, the alpha- and beta-anomers of free 3-deoxy-D-glycero-D-galacto-nonulosonic acid. 1H NMR spectroscopic studies clearly demonstrate that the thermodynamically less stable alpha-form is preferentially formed as the first product of the cleavage reaction and that isomerization rapidly follows, leading to an equilibrium mixture of the two isomers, the beta-isomer being the major species at equilibrium. Therefore, we propose that KDNase Sm catalysis proceeds via a mechanism common to the known exosialidases, but the recognition of the substituent at C-5 by the enzyme differs.

A 1H NMR investigation of the hydrolysis of a synthetic substrate by KDN-sialidase from Crassostrea virginica

The mechanism of hydrolysis of 4-methylumbelliferyl 3-deoxy-D-glycero-alpha-D-galacto-2-nonulopyranosidonic acid (KDN alpha 2MeUmb, 4) by KDN-sialidase isolated from the hepatopancreas of the oyster Crassostrea virginica has been monitored by 1H NMR spectroscopy. The results of these experiments reveal that KDN-sialidase catalyses the hydrolysis of the synthetic substrate KDN alpha 2MeUmb, with initial release of alpha-D-KDN. This is consistent with an overall mechanism for the hydrolysis which proceeds with retention of anomeric configuration. These results agree with earlier NMR studies of other N-acetylneuraminic acid-recognising sialidases from both viral and bacterial sources.

Design and synthesis of carbohydrate-based inhibitors of protein-carbohydrate interactions

Our understanding of carbohydrate-protein interactions has significantly advanced over the past two years. In particular, a healthy amount of literature has appeared on selectins and their relevant ligands. A significant number of carbohydrate-metabolizing enzyme crystal structures have been solved which provide useful starting points for computer-assisted drug design. Some of these proteins have been implicated either directly or indirectly in playing roles in human-disease states, for example, in inflammation, in diabetes and its complications, and in microorganism-induced diseases such as influenza and cholera.

A structural and energetics analysis of the binding of a series of N-acetylneuraminic-acid-based inhibitors to influenza virus sialidase

A molecular dynamics/energy-minimisation protocol has been used to analyse the structural and energetic effects of functional group substitution on the binding of a series of C4-modified 2-deoxy-2,3-didehydro-N-acetylneuraminic acid inhibitors to influenza virus sialidase. Based on the crystal structure of sialidase, a conformational searching protocol, incorporating multiple randomisation steps in a molecular dynamics simulation was used to generate a range of minimum-energy structures. The calculations were useful for predicting the number, location, and orientation of structural water molecules within protein-ligand complexes. Relative binding energies were calculated for the series of complexes using several empirical molecular modelling approaches. Energies were computed using molecular-mechanics-derived interactions as the sum of pairwise atomic nonbonded energies, and in a more rigorous manner including solvation effects as the change in total electrostatic energy of complexation, using a continuum-electrostatics (CE) approach. The CE approach exhibited the superior correlation with observed affinities. Both methods showed definite trends in observed and calculated binding affinities; in both cases inhibitors with a positively charged C4 substituent formed the tightest binding to the enzyme, as observed experimentally.

Synthesis and biological evaluation of N-acetylneuraminic acid-based rotavirus inhibitors

Rotavirus can cause severe gastrointestinal disease, especially in infants and young children, and is particularly prevalent in Third-World countries. Therefore, the development of potential inhibitors of this virus is of great interest. The present study describes the synthesis and in vitro biological evaluation of a number of N-acetylneuraminic acid-based compounds as potential rotavirus inhibitors. Our data suggests that it is indeed possible to inhibit adhesion of the virus, and hence in vitro replication, with carbohydrate-based molecules, although this inhibition does appear to be strain dependent.

A study of the active site of influenza virus sialidase: an approach to the rational design of novel anti-influenza drugs

The development of sialidase inhibitor-based potential anti-influenza drugs using rational drug design techniques has been of recent interest. The present study details as investigation of the active site of influenza virus sialidase by using the program GRID in an attempt to design more potent inhibitors in the hope they will eventually lead to anti-influenza drugs. A number of different probes (amino, carboxy, hydroxy, methyl, etc) have been used in an effort to determine the functional groups most likely to improve the binding of the starting template 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (Neu5Ac2en). The data have correctly predicted the binding regions for the carboxylate, acetamido (NH and methyl), and glycerol (OH) groups of N-acetylneuraminic acid. Moreover, the data suggest that the addition of certain functionalities (amino group) at the C-4 position should enhance the overall binding.

Effect of substrate aglycon on enzyme mechanism in the reaction of sialidase from influenza virus

The effect of substrate aglycon on enzyme mechanism of sialidase from influenza virus was investigated by kinetic isotope effects using the substrates 4-methylumbelliferyl-N-acetyl-alpha-D-neuraminic acid (Neu5Ac alpha 2MU) and p-nitrophenyl-N-acetyl-alpha-D-neuraminic acid (Neu5Ac alpha 2PNP). The kinetic isotope effect on Vmax (beta DV), at pH 6.0, as revealed by direct comparison of rates obtained with Neu5Ac alpha 2MU and the [3,3-2H]-substituted substrate analogue, was shown to be inverse. This indicates that sialidase-catalysed hydrolysis of Neu5Ac alpha 2MU proceeds with substantial positive charge development at the reaction centre in the transition state for the formation of the glycosyl cation-enzyme intermediate. However, no such inverse effect on Vmax at pH 6.0 was observed when using Neu5Ac alpha 2PNP and the [3,3-2H]-substituted substrate. A mechanism by which hydrolysis proceeds through an alpha-lactone intermediate has been proposed by Guo et al. [8]. We propose that the differences in beta DV for the substrates investigated are due primarily to the differing properties of the aglycon leaving groups, which may result in influenza virus sialidase catalysing substrate hydrolysis by a similar mechanism with alternative stabilisation of transition state.

Aldose reductase as a target for drug design: molecular modeling calculations on the binding of acyclic sugar substrates to the enzyme

In an attempt to obtain a picture of the binding conformation of aldehyde substrates to human aldose reductase (hAR), modeling calculations have been performed on the binding of three substrates, D-xylose, L-xylose, and D-lyxose, to wild-type human aldose reductase and two of its site-directed mutants. It was found that the average geometry of D-xylose in the active site of wild-type aldose reductase is characterized by strong hydrogen bonds involving the reactive carbonyl oxygen of the substrate and both Tyr48 and His110. The calculations also suggest the importance of Trp111 in the binding of 2'-hydroxyl-containing aldehyde substrates. A good correlation between calculated interaction enthalpies and experimental log(Km) or log(kcat/Km) values was obtained when His110 was modeled with its N epsilon 2 atom protonated and N delta 1 unprotonated. No correlation was found for the other two configurations of His110. On the basis of comparisons of the calculated substrate binding conformations for the three possible His110 configurations, and on the correlations between measured log(Km) or log(kcat/Km) and calculated parameters, it is proposed that His110 is neutral and protonated at N epsilon 2 when an aldehyde substrate is bound to the hAR/NADPH complex. A chain of three hydrogen-bonded water molecules has been identified in all available crystal structures and is located in an enzyme channel which links the N delta 1 atom of His110 to the solvent-accessible surface of the enzyme. A possible role of this channel in the mechanism of catalysis of aldose reductase is suggested.

Slow-binding inhibition of sialidase from influenza virus

Sialidase from influenza virus A (Tokyo/3/67, N2) is inhibited in slow-binding fashion by 2,3-didehydro-2,4-dideoxy-4-guanidino-N-acetyl-D-neuraminic acid. The Ki observed for the tightly-bound form at steady-state is 3 x 10(-11) M. Slow-binding, which is a consequence of the guanidinyl moiety of the inhibitor, is observed only for influenza virus A sialidase and not for influenza virus B or any other viral, bacterial, or mammalian sialidase investigated. The different results obtained for sialidases from influenza virus A and B, whose active sites are conserved, point to the involvement of the expulsion of a structural water molecule in the slow-binding mechanism.

Molecular modeling studies on ligand binding to sialidase from influenza virus and the mechanism of catalysis

A molecular modeling study has been used to investigate the structural and energetic aspects of substrate and inhibitor binding and the mechanism of catalysis of influenza virus sialidase. A detailed analysis of the interactions of both N-acetylneuraminic acid (Neu5Ac,1) and a number of transition-state analogues with the active site of influenza A sialidase at an atomic level is reported. In each case the calculated structures favorably agreed with the results from X-ray studies. A qualitative agreement between the calculated binding energies for inhibitors with positive substituents at the C4 position on the sugar ring and experimental Ki values was observed. We propose that the hydrolysis of sialosides occurs via an SN1 type mechanism that is facilitated through an activated solvent water molecule which can be expelled upon inhibitor binding. A reaction scheme is presented that is consistent with previously observed crystallographic structures, anomeric products, and isotope effects.

Rational design of potent sialidase-based inhibitors of influenza virus replication

Two potent inhibitors based on the crystal structure of influenza virus sialidase have been designed. These compounds are effective inhibitors not only of the enzyme, but also of the virus in cell culture and in animal models. The results provide an example of the power of rational, computer-assisted drug design, as well as indicating significant progress in the development of a new therapeutic or prophylactic treatment for influenza infection.

Inhibition of sialidases from viral, bacterial and mammalian sources by analogues of 2-deoxy-2,3-didehydro-N-acetylneuraminic acid modified at the C-4 position

The inhibition of sialidase activity from influenza viruses A and B, parainfluenza 2 virus, Vibrio cholerae, Arthrobacter ureafaciens, Clostridium perfringens, and sheep liver by a range of 2-deoxy-2,3-didehydro-N-acetylneuraminic acid analogues modified at the C-4 position has been studied. All substitutions tested resulted in a decrease in the degree of inhibition of the bacterial and mammalian sialidases. For sialidases from influenza viruses A and B, on the other hand, most of the substitutions tested either had no significant effect on binding or, in the case of the basic amino and guanidino substituents, resulted in significantly stronger inhibition. The results for parainfluenza 2 virus sialidase were mostly intermediate, in that inhibition was neither significantly increased nor decreased by most of the modifications. We conclude that only the influenza A and B sialidase active sites possess acid groups correctly positioned to participate in charge-charge interactions in the region of C-4 of bound substrate, and that the C-4 binding pockets of the bacterial and mammalian sialidases examined are considerably smaller than is observed for either the influenza virus or parainfluenza virus sialidases.

High-level production and purification of Escherichia coli N-acetylneuraminic acid aldolase (EC 4.1.3.3)

The Escherichia coli gene which encodes N-acetylneuraminic acid aldolase was isolated by the polymerase chain reaction, cloned into the inducible expression vector pTTQ18, and overexpressed in E. coli. The high yield of aldolase was achieved through both optimum growth of cells and efficient expression of the aldolase gene (20-30% soluble cellular protein). The recombinant enzyme was purified to homogeneity with an activity of 1.2-2.2 U/mg, which compared favorably with that of commercial preparations of E. coli aldolase (1.1 U/mg) and Clostridium perfringens aldolase (0.4 U/mg). The cloning strategy, fermentation conditions, purification protocol, and activity assay are described.

Evidence for a sialosyl cation transition-state complex in the reaction of sialidase from influenza virus

The enzyme mechanism of sialidase from influenza virus has been investigated by kinetic isotope methods, NMR, and a molecular dynamics simulation of the enzyme-substrate complex. Comparison of the reaction rates obtained with the synthetic substrate 4-methylumbelliferyl-N-acetyl-alpha-D-neuraminic acid and the [3,3-2H]-substituted substrate revealed beta-deuterium isotope effects for V/Km ranging over 1.09-1.15 in the pH range 6.0-9.5, whereas the effects observed for V in this pH range increased from 0.979 to 1.07. In D2O, beta DV/Km was slightly increased by 2% and 5% at pD 6.0 and 9.5 respectively, while beta DV was unchanged. Solvent isotope effects of 1.74 were obtained for both beta DV/Km and beta DV at pD 9.5, with beta DV/Km decreasing and beta DV remaining constant at acidic pD. 1H-NMR experiments confirmed that the initial product of the reaction is the alpha-anomer of N-acetyl-D-neuraminic acid. Molecular dynamics studies identified a water molecule in the crystal structure of the sialidase-N-acetyl-D-neuraminic acid complex which is hydrogen-bonded to Asp151 and is available to act as a proton donor source in the enzyme reaction. The results of this study lead us to propose a mechanism for the solvent-mediated hydrolysis of substrate by sialidase that requires the formation of an endocyclic sialosyl cation transition-state intermediate.

Characterisation of an ionisable group involved in binding and catalysis by sialidase from influenza virus

The effect of pH on the kinetics of sialidase purified from influenza virus (A/Tokyo/3/67, H2N2) was investigated. A pK of 9.0 for inhibition of the enzyme by three competitive inhibitors, due to an ionisable group in the active site, was observed. A similar pK was observed for V/Km for the fluorogenic substrate 2-(4-methylumbelliferyl)-N-acetyl-alpha-D-neuraminic acid. However, the shape of the V/Km profile indicates that this substrate is sticky. Solvent perturbation experiments indicated that the observed ionisable active site group is likely to be a cationic amino acid. The results provide evidence against the hypothesis that Glu 276 acts as a proton donor in the enzyme reaction and supports the proposal of a role for one of the active site cationic amino acids in binding and catalysis.

Influenza virus sialidase: effect of calcium on steady-state kinetic parameters

Ca2+ increases the initial rate of activity of sialidase from influenza virus (A/Tokyo/3/67). Increasing ionic strength also activates influenza virus sialidase. When ionic strength is controlled, smaller but still significant Ca2+ effects are observed, with Vmax/Km increased from 0.8.10(5) to 1.4.10(5) M-1 s-1 and Vmax increased from 6.3 to 9.5 s-1 by saturating Ca2+. The Ki of the competitive inhibitor 2,3-dehydro-2-deoxy-N-acetylneuraminic acid was decreased from 2.7.10(-6) to 1.15.10(-6) M after the addition of saturating Ca2+. The data show that Ca2+ exerts a specific effect on Vmax/Km, leading to an increased rate of interaction of substrate with the enzyme. The Kd-app for the Ca2(+)-sialidase complex is 2 mM. Except for Mg2+ which behaves similarly to Ca2+, other mono- and divalent cations have little specific effect on sialidase kinetics. Sequence analysis of a range of subtypes of sialidases from influenza virus supports the proposal that Ca2+ binds at the subunit interface transmitting a conformational change to the enzyme active site. Ca2+ activation may have a physiological role in switching on sialidase activity during the release of newly synthesised virions from the host cell surface.