Synthesis and inhibitory properties of a thiomethylmercuric sialic acid with application to the X-ray structure determination of 9-O-acetylsialic acid esterase from influenza C virus

Sialic acid O-acetylation: From biosynthesis to roles in health and disease

Sialic acids are nine-carbon sugars that frequently cap glycans at the cell surface in cells of vertebrates as well as cells of certain types of invertebrates and bacteria. The nine-carbon backbone of sialic acids can undergo extensive enzymatic modification in nature and O-acetylation at the C-4/7/8/9 position in particular is widely observed. In recent years, the detection and analysis of O-acetylated sialic acids have advanced, and sialic acid-specific O-acetyltransferases (SOATs) and O-acetylesterases (SIAEs) that add and remove O-acetyl groups, respectively, have been identified and characterized in mammalian cells, invertebrates, bacteria, and viruses. These advances now allow us to draw a more complete picture of the biosynthetic pathway of the diverse O-acetylated sialic acids to drive the generation of genetically and biochemically engineered model cell lines and organisms with altered expression of O-acetylated sialic acids for dissection of their roles in glycoprotein stability, development, and immune recognition, as well as discovery of novel functions. Furthermore, a growing number of studies associate sialic acid O-acetylation with cancer, autoimmunity, and infection, providing rationale for the development of selective probes and inhibitors of SOATs and SIAEs. Here, we discuss the current insights into the biosynthesis and biological functions of O-acetylated sialic acids and review the evidence linking this modification to disease. Furthermore, we discuss emerging strategies for the design, synthesis, and potential application of unnatural O-acetylated sialic acids and inhibitors of SOATs and SIAEs that may enable therapeutic targeting of this versatile sialic acid modification.

Synthesis and inhibitory activity of sialic acid derivatives targeted at viral sialate-O-acetylesterases

A series of sialosides modified at the 4- and 9-hydroxy group were synthesised and tested for inhibition of the viral haemagglutinin-esterase activity from various Orthomyxoviruses and Coronaviruses. While no inhibition of the sialate-4-O-acetylesterases from mouse hepatitis virus strain S or sialodacryoadenitis virus was found, a 9-O-methyl derivative displayed inhibitory activity against recombinant sialate-9-O-acetylesterase from influenza C virus.

Synthesis of sialic acid derivatives as ligands for the myelin-associated glycoprotein (MAG)

The trisaccharide substructure 13 of the ganglioside GQ1balpha shows a remarkable affinity for the myelin-associated glycoprotein (MAG). In the search for structurally simplified and pharmacokinetically improved mimics of 13, sialosides with modifications at the reducing and non-reducing end were synthesized. The biological evaluation of mimics 12a-o was performed in a competitive target-based assay. It was found that the relative inhibitory potency (rIP) of antagonist 12h was enhanced by more than 1000-fold in comparison to the reference trisaccharide 13, despite the former having a much simpler structure. In addition, the sialic acid derivatives, for example, 12h, have clearly improved pharmacokinetic properties due to the presence of aromatic moieties, a lower molecular weight, and a reduced number of polar hydroxy functions compared to the reference compound 13.

Synthesis of soluble multivalent glycoconjugates that target the Hc region of botulinum neurotoxin A

The design, synthesis, and initial inhibitory studies of di- and tetravalent glycoconjugates that target the heavy chain of botulinum neurotoxin A are reported.

Carbohydrate chips for studying high-throughput carbohydrate-protein interactions

Carbohydrate-protein interactions play important biological roles in living organisms. For the most part, biophysical and biochemical methods have been used for studying these biomolecular interactions. Less attention has been given to the development of high-throughput methods to elucidate recognition events between carbohydrates and proteins. In the current effort to develop a novel high-throughput tool for monitoring carbohydrate-protein interactions, we prepared carbohydrate microarrays by immobilizing maleimide-linked carbohydrates on thiol-derivatized glass slides and carried out lectin binding experiments by using these microarrays. The results showed that carbohydrates with different structural features selectively bound to the corresponding lectins with relative binding affinities that correlated with those obtained from solution-based assays. In addition, binding affinities of lectins to carbohydrates were also quantitatively analyzed by determining IC(50) values of soluble carbohydrates with the carbohydrate microarrays. To fabricate carbohydrate chips that contained more diverse carbohydrate probes, solution-phase parallel and enzymatic glycosylations were performed. Three model disaccharides were in parallel synthesized in solution-phase and used as carbohydrate probes for the fabrication of carbohydrate chips. Three enzymatic glycosylations on glass slides were consecutively performed to generate carbohydrate microarrays that contained the complex oligosaccharide, sialyl Le(x). Overall, these works demonstrated that carbohydrate chips could be efficiently prepared by covalent immobilization of maleimide-linked carbohydrates on the thiol-coated glass slides and applied for the high-throughput analyses of carbohydrate-protein interactions.

Inhibition of neuraminidase with neuraminic acid C-glycosides

Neuraminic (sialic) acid based alpha-C-glycosides have been synthesized and their inhibitory activity towards bacterial neuraminidase (sialidase) was examined. While some C-glycosides were found to be potent inhibitors (Ki 15-30 microM) of this neuraminidase, others afforded no measurable activity. The structure-activity relationship of these C-glycosides is discussed in the context of other previously reported sialidase inhibitors.

Structure of the haemagglutinin-esterase-fusion glycoprotein of influenza C virus

The spike glycoproteins of the lipid-enveloped orthomyxoviruses and paramyxoviruses have three functions: to recognize the receptor on the cell surface, to mediate viral fusion with the cell membrane, and to destroy the receptor. In influenza C virus, a single glycoprotein, the haemagglutinin-esterase-fusion (HEF) protein, possesses all three functions. In influenza A and B, the first two activities are mediated by haemagglutinin and the third by a second glycoprotein, neuraminidase. Here we report the crystal structure of the HEF envelope glycoprotein of influenza C virus. We have identified the receptor-binding site and the receptor-destroying enzyme (9-O-acetylesterase) sites, by using receptor analogues. The receptor-binding domain is structurally similar to the sialic acid-binding domain of influenza A haemagglutinin, but binds 9-O-acetylsialic acid. The esterase domain has a structure similar to the esterase from Streptomyces scabies and a brain acetylhydrolase. The receptor domain is inserted into a surface loop of the esterase domain and the esterase domain is inserted into a surface loop of the stem. The stem domain is similar to that of influenza A haemagglutinin, except that the triple-stranded, alpha-helical bundle diverges at both of its ends, and the amino terminus of HEF2, the fusion peptide, is partially exposed. The segregation of HEF's three functions into structurally distinct domains suggests that the entire stem region, including sequences at the amino and carboxy termini of HEF1 which precede the post-translational cleavage site between HEF1 and HEF2, forms an independent fusion domain which is probably derived from an ancestral membrane fusion protein.

The synthesis and enzymatic incorporation of sialic acid derivatives for use as tools to study the structure, activity, and inhibition of glycoproteins and other glycoconjugates

Methods have been developed for the enzymatic synthesis of complex carbohydrates and glycoproteins containing in the sialic acid moiety the heavy metal mercury or the transition-state analog phosphonate of the influenza C 9-O-acetyl-neuraminic acid esterase-catalyzed reaction. 5-Acetamido-3, 5-dideoxy-9-methylphosphono-beta-D-glycero-D-galacto-nonulopyra nosidonic acid (1), 5-acetamido-3,5-dideoxy-9-methylphosphono-2-propyl-alpha-D- glycero-D-galacto-nonulopyranosidonic acid triethylammonium salt (2), and 5-acetamido-9-thiomethylmercuric-3, 5,9-trideoxy-beta-D-glycero-D-galacto-nonulopyranosidonic acid (3) were synthesized. Compounds 1 and 2 are proposed transition state inhibitors of an esterase vital for the binding and infection of influenza C. Compound 3 was enzymatically incorporated into an oligosaccharide and a non-natural glycoprotein for use as an aid in the structure determination of these compounds by X-ray crystallography.