Sialidase specificity determined by chemoselective modification of complex sialylated glycans

Human Neuraminidases: Structures and Stereoselective Inhibitors

This Perspective describes the classification, structures, substrates, mechanisms of action, and implications of human neuraminidases (hNEUs) in various pathologies. Some inhibitors have been developed for each isoform, leading to more precise interactions with hNEUs. Although crystal structure data are available for NEU2, most of the findings are based on NEU1 inhibition, and limited information is available for other hNEUs. Therefore, the synthesis of new compounds would facilitate the enrichment of the arsenal of inhibitors to better understand the roles of hNEUs and their mechanisms of action. Nevertheless, due to the already known inhibitors of human neuraminidase enzymes, a structure-activity relationship is presented along with different approaches to inhibit these enzymes for the development of potent and selective inhibitors. Among the different emerging strategies, one is the inhibition of the dimerization of NEU1 or NEU3, and the second is the inhibition of certain receptors located close to hNEU.

Impacting Bacterial Sialidase Activity by Incorporating Bioorthogonal Chemical Reporters onto Mammalian Cell-Surface Sialosides

Bioorthogonal chemical reporters, in synergy with click chemistry, have emerged as a key technology for tagging complex glycans in living cells. This strategy relies on the fact that bioorthogonal chemical reporters are highly reactive species while being biologically noninvasive. Here, we report that chemical reporters and especially sydnones may have, on the contrary, enormous impact on biomolecule processing enzymes. More specifically, we show that editing cell-surface sialic acid-containing glycans (sialosides) with bioorthogonal chemical reporters can significantly affect the activity of bacterial sialidases, enzymes expressed by bacteria during pathogenesis for cleaving sialic acid sugars from mammalian cell-surface glycans. Upon screening various chemical reporters, as well as their position on the sialic acid residue, we identified that pathogenic bacterial sialidases were unable to cleave sialosides displaying a sydnone at the 5-position of sialic acids in vitro as well as in living cells. This study highlights the importance of investigating more systematically the metabolic fate of glycoconjugates modified with bioorthogonal reporters.

Hierarchical Fluorescence Imaging Strategy for Assessment of the Sialylation Level of Lipid Rafts on the Cell Membrane

Glycosylation is one of the most ubiquitous and complicated modifications of proteins and lipids. The revelation of glycosylation-mediated regulation mechanisms of biological processes relies critically on the tools that can reflect the spatial heterogeneity of cell surface glycans, for example, distinguishing glycans exhibited in lipid raft or nonraft domains. To achieve simultaneous visualization of raft and raft-harbored glycans on the cell surface, we combine specific raft recognition, glycan chemoselective labeling, and DNA dynamic hybridization techniques to develop a hierarchical fluorescence imaging strategy using N-acetyl-neuraminic acid (Sia) as the model sugar. We fabricate a raft probe and Sia probe for rafts and Sia, respectively. After specifically anchoring the two probes on the cell surface, the raft probe can be cyclically utilized to turn on the fluorescence of the Sia probe, only residing in rafts, via a proximity cascade DNA reaction. The duplex imaging capability for spatially relevant levels of biological structures enables the revelation of the reason for raft-confined Sia variation in different biological processes. Thus, this work provides an elegant and powerful tool for interrogation of the glycan regulation mechanisms on raft composition, organization, and functions and also contributes to the development of raft-carried glycoconjugate-based theranostic techniques.

Virus-Induced Changes of the Respiratory Tract Environment Promote Secondary Infections With Streptococcus pneumoniae

Secondary bacterial infections enhance the disease burden of influenza infections substantially. Streptococcus pneumoniae (the pneumococcus) plays a major role in the synergism between bacterial and viral pathogens, which is based on complex interactions between the pathogen and the host immune response. Here, we discuss mechanisms that drive the pathogenesis of a secondary pneumococcal infection after an influenza infection with a focus on how pneumococci senses and adapts to the influenza-modified environment. We briefly summarize what is known regarding secondary bacterial infection in relation to COVID-19 and highlight the need to improve our current strategies to prevent and treat viral bacterial coinfections.

Sialidase substrates for Sialdiase assays - activity, specificity, quantification and inhibition

Sialidases are glycosidases responsible for the removal of sialic acid (Sia) residues (desialylation) from glycan portions of either glycoproteins or glycolipids. By desialylation, sialidases are able to modulate the functionality and stability of the Sia-containing molecules and are involved in both physiological and pathological pathways. Therefore, evaluation of sialidase activity and specificity is important for understanding the biological significance of desialylation by sialidases and its function and the related molecular mechanisms of the physiological and pathological pathways. In addition, it is essential for developing novel mechanisms and approaches for disease treatment and diagnosis and pathogen detection as well. This review summarizes the most recent sialidase substrates for evaluating sialidase activity and specificity and screening sialidase inhibitors, including (i) general sialidase substrates, (ii) specific sialidase substrates, (iii) native sialidase substrates and (iv) cellular sialidase substrates. This review also provides a brief introduction of recent instrumental methods for quantifying the sialidase activity, such as UV, fluorescence, HPLC and LC-MS methods.

Anti-Neuraminidase Bioactives from Manggis Hutan (Garcinia celebica L.) Leaves: Partial Purification and Molecular Characterization

The neuraminidase enzyme (NA) from the influenza virus is responsible for the proliferation and infections of the virus progeny, prompting several efforts to discover and optimize effective neuraminidase inhibitors. The main aim of this study is to discover a new potential neuraminidase inhibitor that comes from Garcinia celebica leaves (GCL). The bioassay-guided isolation method was performed to obtain lead compounds. The binding interaction of the isolated compounds was predicted by using molecular docking studies. Friedeline (GC1, logP > 5.0), two lanastone derivatives (methyl-3α,23-dihydroxy-17,14-friedolanstan-8,14,24-trien-26-oat (GC2) and 24E-3a,9,23-trihydroxy-17,14-friedolanostan-14,24-dien-26-oate (GC3) with LogP > 5.0) and catechin (GC4, LogP = 1.4) were identified. The inhibitory potency of these four compounds on NA from C. perfringens and H₁N₁ was found to be as follows: GC4 > GC2 > GC3 > GC1. All compounds exhibited higher inhibitory activity towards C. perfringens NA compared to H₁N₁ NA. From the molecular docking results, GC4 favorably docked and interacted with Arg118, Arg371, Arg292, Glu276 and Trp178 residues, whilst GC2 interacted with Arg118, Arg371, Arg292, Ile222, Arg224 and Ser246. GC3 interacted with Tyr406 only. GC4 had potent NA inhibition with free energy of binding of −12 kcal/mol. In the enzyme inhibition study, GC4 showed the highest activity with an IC₅₀ of 60.3 µM and 91.0 µM for C. perfringens NA and H₁N₁ NA—respectively.

Separation of Sialylated Glycan Isomers by Differential Mobility Spectrometry

Mass spectrometry has proven itself to be an important technology for characterizing intact glycoproteins, glycopeptides, and released glycans. However, these molecules often present significant challenges during analysis. For example, glycans of identical molecular weights can be present in many isomeric forms, with one form having dramatically more biological activity than the others. Discriminating among these isomeric forms using mass spectrometry alone can be daunting, which is why orthogonal techniques, such as ion mobility spectrometry, have been explored. Here, we demonstrate the use of differential mobility spectrometry (DMS) to separate isomeric glycans differing only in the linkages of sialic acid groups (e.g., α 2,3 versus α 2,6). This ability extends from a small trisaccharide species to larger biantennary systems and is driven, in part, by the role of intramolecular solvation of the charge site(s) on these ions within the DMS environment.

Sialidase-catalyzed one-pot multienzyme (OPME) synthesis of sialidase transition-state analogue inhibitors

Sialidase transition state analog inhibitor 2,3-dehydro-2-deoxy-N-acetylneuraminic acid (Neu5Ac2en, DANA) has played a leading role in developing clinically used anti-influenza virus drugs. Taking advantage of the Neu5Ac2en-forming catalytic property of Streptococcus pneumoniae sialidase SpNanC, an effective one-pot multienzyme (OPME) strategy has been developed to directly access Neu5Ac2en and its C-5, C-9, and C-7-analogs from N-acetylmannosamine (ManNAc) and analogs. The obtained Neu5Ac2en analogs can be further derivatized at various positions to generate a larger inhibitor library. Inhibition studies demonstrated improved selectivity of several C-5- or C-9-modified Neu5Ac2en derivatives against several bacterial sialidases. The study provides an efficient enzymatic method to access sialidase inhibitors with improved selectivity.

Evaluation of ion mobility for the separation of glycoconjugate isomers due to different types of sialic acid linkage, at the intact glycoprotein, glycopeptide and glycan level

The study of protein glycosylation can be regarded as an intricate but very important task, making glycomics one of the most challenging and interesting, albeit under-researched, type of "omics" science. Complexity escalates remarkably when considering that carbohydrates can form severely branched structures with many different constituents, which often leads to the formation of multiple isomers. In this regard, ion mobility (IM) spectrometry has recently demonstrated its power for the separation of isomeric compounds. In the present work, the potential of traveling wave IM (TWIMS) for the separation of isomeric glycoconjugates was evaluated, using mouse transferrin (mTf) as model glycoprotein. Particularly, we aim to assess the performance of this platform for the separation of isomeric glycoconjugates due to the type of sialic acid linkage, at the intact glycoprotein, glycopeptide and glycan level. Straightforward separation of isomers was achieved with the analysis of released glycans, as opposed to the glycopeptides which showed a more complex pattern. Finally, the developed methodology was applied to serum samples of mice, to investigate its robustness when analyzing real complex samples.

BIOLOGICAL SIGNIFICANCE

Ion mobility mass spectrometry is a promising analytical technique for the separation of glycoconjugate isomers due to type of sialic acid linkage. The impact of such a small modification in the glycan structure is more evident in smaller analytes, reason why the analysis of free glycans was easier compared to the intact protein or the glycopeptides. The established methodology could be regarded as starting point in the separation of highly decorated glycoconjugates. This is an important topic nowadays, as differences in the abundance of some glycan isomers could be the key for the early diagnosis, control or differentiation of certain diseases, such as inflammation or cancer.

Streptococcus pneumoniae Senses a Human-like Sialic Acid Profile via the Response Regulator CiaR

Streptococcus pneumoniae is a human-adapted pathogen that encounters terminally sialylated glycoconjugates and free sialic acid (Sia) in the airways. Upon scavenging by the bacterial sialidase NanA, Sias serve as carbon sources for the bacteria. Unlike most animals in which cytidine-monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) converts Sia N-acetylneuraminic acid (Neu5Ac) into N-glycolylneuraminic acid (Neu5Gc), humans have an inactive CMAH, causing an absence of Neu5Gc and excess Neu5Ac. We find that pneumococcal challenge in Cmah(-/-) mice leads to heightened bacterial loads, virulence, and NanA expression. In vitro, NanA is upregulated in response to Neu5Ac compared with Neu5Gc, a process controlled by the two-component response regulator CiaR and requiring Sia uptake by the transporter SatABC. Additionally, compared with Neu5Gc, Neu5Ac increases pneumococcal resistance to antimicrobial reactive oxygen species in a CiaR-dependent manner. Thus, S. pneumoniae senses and responds to Neu5Ac, leading to CiaR activation and increased virulence and potentially explaining the greater susceptibility in humans.

Precision glycocalyx editing as a strategy for cancer immunotherapy

Cell surface sialosides constitute a central axis of immune modulation that is exploited by tumors to evade both innate and adaptive immune destruction. Therapeutic strategies that target tumor-associated sialosides may therefore potentiate antitumor immunity. Here, we report the development of antibody-sialidase conjugates that enhance tumor cell susceptibility to antibody-dependent cell-mediated cytotoxicity (ADCC) by selective desialylation of the tumor cell glycocalyx. We chemically fused a recombinant sialidase to the human epidermal growth factor receptor 2 (HER2)-specific antibody trastuzumab through a C-terminal aldehyde tag. The antibody-sialidase conjugate desialylated tumor cells in a HER2-dependent manner, reduced binding by natural killer (NK) cell inhibitory sialic acid-binding Ig-like lectin (Siglec) receptors, and enhanced binding to the NK-activating receptor natural killer group 2D (NKG2D). Sialidase conjugation to trastuzumab enhanced ADCC against tumor cells expressing moderate levels of HER2, suggesting a therapeutic strategy for cancer patients with lower HER2 levels or inherent trastuzumab resistance. Precision glycocalyx editing with antibody-enzyme conjugates is therefore a promising avenue for cancer immune therapy.

Glycan specificity of neuraminidases determined in microarray format

Neuraminidases hydrolytically remove sialic acids from glycoconjugates. Neuraminidases are produced by both humans and their pathogens, and function in normal physiology and in pathological events. Identification of neuraminidase substrates is needed to reveal their mechanism of action, but high-throughput methods to determine glycan specificity of neuraminidases are limited. Here we use two glycan labeling reactions to monitor neuraminidase activity toward glycan substrates. While both periodate oxidation and aniline-catalyzed oxime ligation (PAL) and galactose oxidase and aniline-catalyzed oxime ligation (GAL) can be used to monitor neuraminidase activity toward glycans in microtiter plates, only GAL accurately measured neuraminidase activity toward glycans displayed on a commercial glass slide microarray. Using GAL, we confirm known linkage specificities of three pneumococcal neuraminidases and obtain new information about underlying glycan specificity.

Pneumococcal Neuraminidase Substrates Identified through Comparative Proteomics Enabled by Chemoselective Labeling

Neuraminidases (sialidases) are enzymes that hydrolytically remove sialic acid from sialylated proteins and lipids. Neuraminidases are encoded by a range of human pathogens, including bacteria, viruses, fungi, and protozoa. Many pathogen neuraminidases are virulence factors, indicating that desialylation of host glycoconjugates can be a critical step in infection. Specifically, desialylation of host cell surface glycoproteins can enable these molecules to function as pathogen receptors or can alter signaling through the plasma membrane. Despite these critical effects, no unbiased approaches exist to identify glycoprotein substrates of neuraminidases. Here, we combine previously reported glycoproteomics methods with quantitative proteomics analysis to identify glycoproteins whose sialylation changes in response to neuraminidase treatment. The two glycoproteomics methods-periodate oxidation and aniline-catalyzed oxime ligation (PAL) and galactose oxidase and aniline-catalyzed oxime ligation (GAL)-rely on chemoselective labeling of sialylated and nonsialylated glycoproteins, respectively. We demonstrated the utility of the combined approaches by identifying substrates of two pneumococcal neuraminidases in a human cell line that models the blood-brain barrier. The methods deliver complementary lists of neuraminidase substrates, with GAL identifying a larger number of substrates than PAL (77 versus 17). Putative neuraminidase substrates were confirmed by other methods, establishing the validity of the approach. Among the identified substrates were host glycoproteins known to function in bacteria adherence and infection. Functional assays suggest that multiple desialylated cell surface glycoproteins may act together as pneumococcus receptors. Overall, this method will provide a powerful approach to identify glycoproteins that are desialylated by both purified neuraminidases and intact pathogens.

Enhanced Cross-Linking of Diazirine-Modified Sialylated Glycoproteins Enabled through Profiling of Sialidase Specificities

Sialic-acid-mediated interactions play critical roles on the cell surface, providing an impetus for the development of methods to study this important monosaccharide. In particular, photo-cross-linking sialic acids incorporated onto cell surfaces have allowed covalent capture of transient interactions between sialic acids and sialic-acid-recognizing proteins via cross-linking. However, natural sialic acids also present on the cell surface compete with photo-cross-linking sialic acids in binding events, limiting cross-linking yields. In order to improve the utility of one such photo-cross-linking sialic acid, SiaDAz, we examined a number of sialidases, enzymes that remove sialic acids from glycoconjugates, to find one that would cleave natural sialic acids but remain inactive toward SiaDAz. Using this sialidase, we improved SiaDAz-mediated cross-linking of an antisialyl Lewis X antibody and of endoglin. This protocol can be applied generally to sialic-acid-mediated interactions and will facilitate identification of sialic acid binding partners.

Streptococcus pneumoniae NanC: STRUCTURAL INSIGHTS INTO THE SPECIFICITY AND MECHANISM OF A SIALIDASE THAT PRODUCES A SIALIDASE INHIBITOR

Streptococcus pneumoniae is an important human pathogen that causes a range of disease states. Sialidases are important bacterial virulence factors. There are three pneumococcal sialidases: NanA, NanB, and NanC. NanC is an unusual sialidase in that its primary reaction product is 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (Neu5Ac2en, also known as DANA), a nonspecific hydrolytic sialidase inhibitor. The production of Neu5Ac2en from α2–3-linked sialosides by the catalytic domain is confirmed within a crystal structure. A covalent complex with 3-fluoro-β-N-acetylneuraminic acid is also presented, suggesting a common mechanism with other sialidases up to the final step of product formation. A conformation change in an active site hydrophobic loop on ligand binding constricts the entrance to the active site. In addition, the distance between the catalytic acid/base (Asp-315) and the ligand anomeric carbon is unusually short. These features facilitate a novel sialidase reaction in which the final step of product formation is direct abstraction of the C3 proton by the active site aspartic acid, forming Neu5Ac2en. NanC also possesses a carbohydrate-binding module, which is shown to bind α2–3- and α2–6-linked sialosides, as well as N-acetylneuraminic acid, which is captured in the crystal structure following hydration of Neu5Ac2en by NanC. Overall, the pneumococcal sialidases show remarkable mechanistic diversity while maintaining a common structural scaffold.

Discovery of intramolecular trans-sialidases in human gut microbiota suggests novel mechanisms of mucosal adaptation

The gastrointestinal mucus layer is colonized by a dense community of microbes catabolizing dietary and host carbohydrates during their expansion in the gut. Alterations in mucosal carbohydrate availability impact on the composition of microbial species. Ruminococcus gnavus is a commensal anaerobe present in the gastrointestinal tract of >90% of humans and overrepresented in inflammatory bowel diseases (IBD). Using a combination of genomics, enzymology and crystallography, we show that the mucin-degrader R. gnavus ATCC 29149 strain produces an intramolecular trans-sialidase (IT-sialidase) that cleaves off terminal α2-3-linked sialic acid from glycoproteins, releasing 2,7-anhydro-Neu5Ac instead of sialic acid. Evidence of IT-sialidases in human metagenomes indicates that this enzyme occurs in healthy subjects but is more prevalent in IBD metagenomes. Our results uncover a previously unrecognized enzymatic activity in the gut microbiota, which may contribute to the adaptation of intestinal bacteria to the mucosal environment in health and disease.

Mucosal sialoglycans contribute to host–microbe interactions at mucosal surfaces and impact bacterial colonization of the digestive system. Here the authors identify and characterize an intramolecular trans-sialidase produced by the gut bacterium R. gnavus ATCC 29149 that may contribute to adaptation to the mucosal environment.

Using common spatial distributions of atoms to relate functionally divergent influenza virus N10 and N11 protein structures to functionally characterized neuraminidase structures, toxin cell entry domains, and non-influenza virus cell entry domains

The ability to identify the functional correlates of structural and sequence variation in proteins is a critical capability. We related structures of influenza A N10 and N11 proteins that have no established function to structures of proteins with known function by identifying spatially conserved atoms. We identified atoms with common distributed spatial occupancy in PDB structures of N10 protein, N11 protein, an influenza A neuraminidase, an influenza B neuraminidase, and a bacterial neuraminidase. By superposing these spatially conserved atoms, we aligned the structures and associated molecules. We report spatially and sequence invariant residues in the aligned structures. Spatially invariant residues in the N6 and influenza B neuraminidase active sites were found in previously unidentified spatially equivalent sites in the N10 and N11 proteins. We found the corresponding secondary and tertiary structures of the aligned proteins to be largely identical despite significant sequence divergence. We found structural precedent in known non-neuraminidase structures for residues exhibiting structural and sequence divergence in the aligned structures. In N10 protein, we identified staphylococcal enterotoxin I-like domains. In N11 protein, we identified hepatitis E E2S-like domains, SARS spike protein-like domains, and toxin components shared by alpha-bungarotoxin, staphylococcal enterotoxin I, anthrax lethal factor, clostridium botulinum neurotoxin, and clostridium tetanus toxin. The presence of active site components common to the N6, influenza B, and S. pneumoniae neuraminidases in the N10 and N11 proteins, combined with the absence of apparent neuraminidase function, suggests that the role of neuraminidases in H17N10 and H18N11 emerging influenza A viruses may have changed. The presentation of E2S-like, SARS spike protein-like, or toxin-like domains by the N10 and N11 proteins in these emerging viruses may indicate that H17N10 and H18N11 sialidase-facilitated cell entry has been supplemented or replaced by sialidase-independent receptor binding to an expanded cell population that may include neurons and T-cells.

Unusual enzymatic glycoside cleavage mechanisms

Over the sixty years since Koshland initially formulated the classical mechanisms for retaining and inverting glycosidases, researchers have assembled a large body of supporting evidence and have documented variations of these mechanisms. Recently, however, researchers have uncovered a number of completely distinct mechanisms for enzymatic cleavage of glycosides involving elimination and/or hydration steps. In family GH4 and GH109 glycosidases, the reaction proceeds via transient NAD(+)-mediated oxidation at C3, thereby acidifying the proton at C2 and allowing for elimination across the C1-C2 bond. Subsequent Michael-type addition of water followed by reduction at C3 generates the hydrolyzed product. Enzymes employing this mechanism can hydrolyze thioglycosides as well as both anomers of activated substrates. Sialidases employ a conventional retaining mechanism in which a tyrosine functions as the nucleophile, but in some cases researchers have observed off-path elimination end products. These reactions occur via the normal covalent intermediate, but instead of an attack by water on the anomeric center, the catalytic acid/base residue abstracts an adjacent proton. These enzymes can also catalyze hydration of the enol ether via the reverse pathway. Reactions of α-(1,4)-glucan lyases also proceed through a covalent intermediate with subsequent abstraction of an adjacent proton to give elimination. However, in this case, the departing carboxylate "nucleophile" serves as the base in a concerted but asynchronous syn-elimination process. These enzymes perform only elimination reactions. Polysaccharide lyases, which act on uronic acid-containing substrates, also catalyze only elimination reactions. Substrate binding neutralizes the charge on the carboxylate, which allows for abstraction of the proton on C5 and leads to an elimination reaction via an E1cb mechanism. These enzymes can also cleave thioglycosides, albeit slowly. The unsaturated product of polysaccharide lyases can then serve as a substrate for a hydration reaction carried out by unsaturated glucuronyl hydrolases. This hydration is initiated by protonation at C4 and proceeds in a Markovnikov fashion rather than undergoing a Michael-type addition, giving a hemiketal at C5. This hemiketal then undergoes a rearrangement that results in cleavage of the anomeric bond. These enzymes can also hydrolyze thioglycosides efficiently and slowly turn over substrates with inverted anomeric configuration. The mechanisms discussed in this Account proceed through transition states that involve either positive or negative charges, unlike the exclusively cationic transition states of the classical Koshland retaining and inverting glycosidases. In addition, the distribution of this charge throughout the substrate can vary substantially. The nature of these mechanisms and their transition states means that any inhibitors or inactivators of these unusual enzymes probably differ from those presently used for Koshland retaining or inverting glycosidases.

Exploration of sialic acid diversity and biology using sialoglycan microarrays

Sialic acids (Sias) are a group of α-keto acids with a nine-carbon backbone, which display many types of modifications in nature. The diversity of natural Sia presentations is magnified by a variety of glycosidic linkages to underlying glycans, the sequences and classes of such glycans, as well as the spatial organization of Sias with their surroundings. This diversity is closely linked to the numerous and varied biological functions of Sias. Relatively large libraries of natural and unnatural Sias have recently been chemically/chemoenzymatically synthesized and/or isolated from natural sources. The resulting sialoglycan microarrays have proved to be valuable tools for the exploration of diversity and biology of Sias. Here we provide an overview of Sia diversity in nature, the approaches used to generate sialoglycan microarrays, and the achievements and challenges arising.

Catalytic preference of Salmonella typhimurium LT2 sialidase for N-acetylneuraminic acid residues over N-glycolylneuraminic acid residues

In a comparison of sialidase activities toward N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc), we found that Salmonella typhimurium LT2 sialidase (STSA) hardly cleaved 4-methylumbelliferyl Neu5Gc (4MU-Neu5Gc). The k cat/K m value of STSA for 4MU-Neu5Gc was found to be 110 times lower than that for 4-methylumbelliferyl Neu5Ac (4MU-Neu5Ac). Additionally, STSA had remarkably weak ability to cleave α2-3-linked-Neu5Gc contained in gangliosides and equine erythrocytes. In silico analysis based on first-principle calculations with transition-state analogues suggested that the binding affinity of Neu5Gc2en is 14.3 kcal/mol more unstable than that of Neu5Ac2en. The results indicated that STSA preferentially cleaves Neu5Ac residues rather than Neu5Gc residues, which is important for anyone using this enzyme to cleave α2-3-linked sialic acids.

Unified theory of bacterial sialometabolism: how and why bacteria metabolize host sialic acids

Sialic acids are structurally diverse nine-carbon ketosugars found mostly in humans and other animals as the terminal units on carbohydrate chains linked to proteins or lipids. The sialic acids function in cell-cell and cell-molecule interactions necessary for organismic development and homeostasis. They not only pose a barrier to microorganisms inhabiting or invading an animal mucosal surface, but also present a source of potential carbon, nitrogen, and cell wall metabolites necessary for bacterial colonization, persistence, growth, and, occasionally, disease. The explosion of microbial genomic sequencing projects reveals remarkable diversity in bacterial sialic acid metabolic potential. How bacteria exploit host sialic acids includes a surprisingly complex array of metabolic and regulatory capabilities that is just now entering a mature research stage. This paper attempts to describe the variety of bacterial sialometabolic systems by focusing on recent advances at the molecular and host-microbe-interaction levels. The hope is that this focus will provide a framework for further research that holds promise for better understanding of the metabolic interplay between bacterial growth and the host environment. An ability to modify or block this interplay has already yielded important new insights into potentially new therapeutic approaches for modifying or blocking bacterial colonization or infection.