Academic literature on the topic 'Sialylmimetics'

Cholera is an epidemic infectious diarrhoeal disease that for centuries has proven its frightening ability to cause rapid and widespread loss of human life. All symptoms associated with cholera are a result of rapid dehydration due to infection by pathogenic strains of the bacterium Vibrio cholerae. The damaging effects associated with cholera are mainly attributed to the toxin, which is secreted by the bacterium and infects cells lining the gastrointestinal tract. A sialidase, also secreted by the bacterium, is believed to facilitate toxin uptake by the gastrointestinal epithelium. V. cholerae sialidase is therefore a potential target for therapeutic intervention. A survey of the literature reveals that sialidases from different species share common features with respect to their structure, substrate specificity and catalytic mechanism. The unsaturated sialic acid, Neu5Ac2en, inhibits most exosialidases with a dissociation constant of inhibitor of -10-4 to-10-6 M and has frequently been used as a template in the design of more potent sialidase inhibitors. In the case of V. cholerae sialidase, there have been no inhibitors reported to date that are significantly more potent than Neu5Ac2en itself The present research aimed to develop a range of mimics of Neu5Ac2en, which contain various substituents to replace the C-6 glycerol side chain, as potential inhibitors of V cholerae sialidase. The x-ray crystal structure of V cholerae sialidase was used to explore potential interactions between active site residues and C-6 modified Neu5Ac2en mimetics of known inhibitory potency. Opportunities for interactions within the glycerol side chain pocket in the active site of V cholerae sialidase are discussed. A novel synthetic strategy was developed for the synthesis of a series of glucuronidebased Neu5Ac2en mimetics starting from readily available GIcNAc. This approach was employed for the preparation of Neu5Ac2en mimetics that contained an ether or thioether substituent as replacement of the glycerol side chain of Neu5Ac2en. Progress was also made towards the synthesis of a series of C-6 acylamino Neu5Ac2en mimetics. Analysis by 1H NMR spectroscopy showed that the acylamino derivatives adopted a half-chair conformation that was similar to the conformation of Neu5Ac2en but different to the conformation adopted by the ether and thioether derivatives prepared. The inhibitory activity of the C-6 ether and thioether Neu5Ac2en mimetics prepared was evaluated in vitro using an enzyme assay. It was found that most of the derivatives inhibited V. cholerae sialidase with a K1 of approximately 1O-4 M. The derivatives containing a hydrophobic side chain were found to be slightly more potent compared to derivatives with more hydrophilic side chains. A more detailed study of binding interactions between the C-6 thioether Neu5Ac2en mimetics and V cholerae sialdiase was carried out using STD 1H NMR spectroscopy and computational molecular modelling.

Cholera is an epidemic infectious diarrhoeal disease that for centuries has proven its frightening ability to cause rapid and widespread loss of human life. All symptoms associated with cholera are a result of rapid dehydration due to infection by pathogenic strains of the bacterium Vibrio cholerae. The damaging effects associated with cholera are mainly attributed to the toxin, which is secreted by the bacterium and infects cells lining the gastrointestinal tract. A sialidase, also secreted by the bacterium, is believed to facilitate toxin uptake by the gastrointestinal epithelium. V. cholerae sialidase is therefore a potential target for therapeutic intervention. A survey of the literature reveals that sialidases from different species share common features with respect to their structure, substrate specificity and catalytic mechanism. The unsaturated sialic acid, Neu5Ac2en, inhibits most exosialidases with a dissociation constant of inhibitor of -10-4 to-10-6 M and has frequently been used as a template in the design of more potent sialidase inhibitors. In the case of V. cholerae sialidase, there have been no inhibitors reported to date that are significantly more potent than Neu5Ac2en itself The present research aimed to develop a range of mimics of Neu5Ac2en, which contain various substituents to replace the C-6 glycerol side chain, as potential inhibitors of V cholerae sialidase. The x-ray crystal structure of V cholerae sialidase was used to explore potential interactions between active site residues and C-6 modified Neu5Ac2en mimetics of known inhibitory potency. Opportunities for interactions within the glycerol side chain pocket in the active site of V cholerae sialidase are discussed. A novel synthetic strategy was developed for the synthesis of a series of glucuronidebased Neu5Ac2en mimetics starting from readily available GIcNAc. This approach was employed for the preparation of Neu5Ac2en mimetics that contained an ether or thioether substituent as replacement of the glycerol side chain of Neu5Ac2en. Progress was also made towards the synthesis of a series of C-6 acylamino Neu5Ac2en mimetics. Analysis by 1H NMR spectroscopy showed that the acylamino derivatives adopted a half-chair conformation that was similar to the conformation of Neu5Ac2en but different to the conformation adopted by the ether and thioether derivatives prepared. The inhibitory activity of the C-6 ether and thioether Neu5Ac2en mimetics prepared was evaluated in vitro using an enzyme assay. It was found that most of the derivatives inhibited V. cholerae sialidase with a K1 of approximately 1O-4 M. The derivatives containing a hydrophobic side chain were found to be slightly more potent compared to derivatives with more hydrophilic side chains. A more detailed study of binding interactions between the C-6 thioether Neu5Ac2en mimetics and V cholerae sialdiase was carried out using STD 1H NMR spectroscopy and computational molecular modelling.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Biomolecular and Physical Sciences
Full Text

Typically found at the terminus of glycoconjugates, the sialic acids are ideally located to participate in carbohydrate-protein interactions that mediate important recognition phenomena. These include acting as receptors for viruses and bacteria, mediating cell-cell adhesion via lectins, and enabling cell-cell communication. N-Acetylneuraminic acid alpha(2,3)-linked to galactose is a commonly found epitope in several important cell-surface glycoconjugates, and is associated with diseases like cancer (through the upregulation of alpha(2,3)-sialyltransferases), Chagas' disease (and the involvement of Trypanosoma cruzi trans-sialidase), and rotaviral infection. The apparent strong specificity of some sialic acid-recognising proteins for substrates with N-acetylneuraminic acid alpha(2,3)-linked to galactose suggests that this interaction may be a potential target for therapeutic intervention. This thesis focuses on investigations towards the synthesis of biological probes for sialic acid-recognising proteins that naturally recognise N-acetylneuraminic acid alpha(2,3)-linked to galactose, as well as the development of potential inhibitors for rotavirus and Trypanosoma cruzi trans-sialidase. Chapter 1 provides a general introduction of sialic acids, and the proteins that recognise them, with an emphasis on their involvement in certain disease states. There is also a discussion of the involvement of sialic acids in relation to rotavirus infection, as well as the relationship of sialic acids to Chagas' disease, via the Trypanosoma cruzi trans-sialidase enzyme. Chapter 2 describes the development of an efficient synthetic route towards S-linked lactose-based mimics of N-acetylneuraminic acid alpha(2,3)-linked to galactose. The inclusion of varying functionality in the mimetic portion was designed to allow the exploration of the effects of hydrophobic, hydrophilic and sterically demanding groups upon interaction with a given biomolecule. En route to the successful synthesis of these sialylmimetics, several protecting group manipulations of lactoside were investigated, with the preparation of the key intermediate seemingly dependent on the protecting group strategy used. Furthermore, the synthesis of the metabolically stable substrate analogue of the natural substrate of Trypanosoma cruzi trans-sialidase has been described. Approaches towards the synthesis of the O-linked series of lactose-based sialylmimetics are described in Chapter 3. Several methodologies are reported, as well as protecting group manipulations, for the successful preparation of these sialylmimetics. It appears from these investigations that gaining access into the O -linked series is not as straightforward as for the S-linked series, with the reactivity of the coupling partner being a critical factor in the success of such reactions. In Chapter 4, the synthesis of galactose- and lactose-based sialosides as sialyl donors for Trypanosoma cruzi trans-sialidase has been described, as well as the preparation of an asialo acceptor molecule. These substrates were used to probe the mechanism of the enzyme using H NMR spectroscopy, aimed at investigating the hydrolysis and transfer rates of Trypanosoma cruzi trans-sialidase with a variety of sialyl donors and asialo acceptor molecules. These studies have shown that the hydrolysis of sialyl donors is slow compared to the transfer rates of sialic acid to asialo acceptor molecules, and have provided further insight into the preference of the enzyme for various substrates. The biological evaluation of some of the compounds reported in Chapters 2 and 3 is detailed in Chapter 5. The sialylmimetics were evaluated for their ability to inhibit rotavirus infection in vitro. In addition, a novel H NMR spectroscopic-based assay was developed for inhibition studies of Trypanosoma cruzi trans-sialidase with some of the compounds prepared through the course of this project, and some preliminary inhibition results have been obtained. The conclusions drawn in Chapters 2 to 5 are summarised in Chapter 6 and the future directions of this work are briefly outlined. Chapter 7 brings together all of the experimental procedures and details that support the results and observations presented in Chapters 2 to 5.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
Institute for Glycomics
Full Text

The combat against viral diseases has been, and still is, a major challenge in the field of drug development. Viruses are intracellular parasites that use the host cell ma-chinery for their replication and release. Therefore it is difficult to target and destroy the viral particle without disturbing the essential functions of the host cell. This thesis describes studies towards antiviral agents targeting adenovirus type 37 (Ad37), which causes the severe ocular infection epidemic keratoconjunctivitis (EKC). Cell surface oligosaccharides serve as cellular receptors for many pathogens, including viruses and bacteria. For EKC-causing adenoviruses, cell surface oligo-saccharides with terminal sialic acid have recently been shown to be critical for their attachment to and infection of host cells. The work in this thesis support these re-sults and identifies the minimal binding epitope for viral recognition. As carbo-hydrate–protein interactions in general, the sialic acid–Ad37 interaction is very weak. Nature overcomes this problem and vastly improves the binding affinity by presenting the carbohydrates in a multivalent fashion. Adenoviruses interact with their cellular receptors via multiple fiber proteins, whereby it is likely that the ideal inhibitor of adenoviral infections should be multivalent. This thesis includes design and synthesis of multivalent sialic acid glycoconjugates that mimic the structure of the cellular receptor in order to inhibit adenoviral attachment to and infection of human corneal epithelial (HCE) cells. Synthetic routes to three different classes of sialic acid conjugates, i.e. derivatives of sialic acid, 3’-sialyllactose and N-acyl modified sialic acids, and their multivalent counterparts on human serum albumine (HSA) have been developed. Evaluation of these conjugates in cell binding and cell infectivity assays revealed that they are effective as inhibitors. Moreover the results verify the hypothesis of the multivalency effect and clearly shows that the power of inhibition is significantly increased with higher orders of valency. Potential inhibi-tors could easily be transferred to the eye using a salve or eye drops, and thereby they would escape the metabolic processes of the body, a major drawback of using carbohydrates as drugs. The results herein could therefore be useful in efforts to develop an antiviral drug for treatment of EKC.