Chemo-enzymatic Synthesis of N-glycans of Schistosoma mansoni and their Molecular Interactions with Glycan Binding Proteins

Abstract

The glycome comprises of glyco-conjugates like N-glycans, that are the most diverse and abundant types of glycoproteins expressed across all domains of life. N-glycans of lower species like parasites and bacteria have still not been fully explored, owing to a poor understanding of their enzyme expression. As such, there is a tremendous lag in the amount of information available with respect to their interaction and invasion of humans. Research suggests that seemingly unimportant changes in glycosylation patterns in such pathogens can have unpredictable and immense effects on biological functions. However, biological extraction of these glycans yields heterogeneous and impure mixtures in minute quantities. Until we gain extensive advancement in technologies like gene expression and recombinant protein expression, chemical approach aided with enzymatic extensions seems to be the only way to get access to comprehensive libraries of desired glycans.

One such parasitic organism, S. mansoni utilizes glycan expression on its cell surface to invade its host and manipulate their immune system. They express a number of glycan motifs in a stage-dependent manner, which contain highly unusual fucoside linkages recognized as immunogenic epitopes, that are reported to bind to DC-SIGN to modulate the human immune response and evade detection. The synthesis of these minimal epitopes and their interactions with DC-SIGN using multi-disciplinary approaches such as STD-NMR, molecular modeling and micro-array was accomplished. The studies established the importance of multivalency to increase avidity and selectivity of binding from a biological perspective.

Further, a library of characteristic N-glycans of S. mansoni was synthesized via a chemo-enzymatic methodology. The N-glycans were attributed with characteristic features such as a core xyloside, core fucoside and terminal epitopes like Lewis-x, LDN-F and di-Lewis-x. These glycans were studied by STD-NMR, computation, and electron microscopy and it was concluded that multi-antennary glycans bind with higher affinity to DC-SIGN compared to mono-valent minimal epitopes. It was presented that the multi-antennary glycans could cross-link DC-SIGN tetramers into a dense network which could contribute to clustering of DC-SIGN, thus enhancing antigen recognition.

The synthesis of characteristic N-glycans of S. mansoni and their in-depth binding studies by multidisciplinary approaches offer unique insight into the molecular aspects of protein-glycan interactions. Access to such well-defined glycan libraries will enable future studies to identify the ideal candidate that can generate an appropriate antibody response in infections, thus activating the immune system towards the death and expulsion of the parasite.