Glycobiology

Glycosylation, the addition of N- or O- linked carbohydrates to proteins, either as a co-translational or post translational modification can be key for the proteins’ interaction with other proteins and it’s own function if folding is affected.Glycosylation is known to be the most common post-translational modification, and more than 50% of all proteins are glycosylated. Not surprisingly, changes in glycosylation have therefore been suggested to be implicated in a number of diseases. (1)

Glycans introduce complexity to the proteins to which they are attached. These modifications vary during the progression of many diseases; thus, they serve as potential biomarkers for disease diagnosis and prognosis. (2) In the field of therapeutic antibodies, the glycosylation of the antibody can fine tune the efficacy of the antibody with changes in glycosylation altering the antibody-dependent cellular cytotoxicity (ADCC) effector function by up to 40 fold, while decreasing Complement-dependent cytotoxicity (CDC) activity. (3)

Micro arrays are frequently used to assess which glycans are bound by a protein, or to discover which carbohydrates maybe playing a part in a biological system. They help elucidate the fine specificities of glycan binding proteins. Although knowing this information is useful, there is still much that can be learnt from the starting point of knowing the terminal glycan motifs of the carbohydrate, since these are the parts of the structure free for interaction.

Lectins are a broad class of naturally occurring glycan binding proteins, initially discovered from plant extracts, that had the ability to agglutinate red blood cells. They are a simple and relatively inexpensive way to check for a variety of terminal glycan motifs and can be used in a wide range of assays in much the same way primary antibodies can be, some can also be  used for neuronal tracing, and others have mitogenic properties.

Raising primary antibodies against the carbohydrates themselves has historically been problematic with so many carbohydrate structures being involved in immune signalling pathways when antibody technologies were being initially developed, since glycosylation influences the function of all immune cells. Glycans play a crucial role in intercellular contacts and leukocytes migration. These interactions are important in activation and proliferation of leukocytes and during immune response.(4)

With the advent of antibody engineering techniques, the first anti-glycan antibodies are reaching the market. Although there are limited numbers currently commercially available, they offer the benefit of carbohydrate specific binding with the familiarity of IgG antibodies, for which much more is known as therapeutic entities than lectins. Currently available antibodies include those against Stage-specific embryonic antigen-4 (SSEA-4), whose expression changes both qualitatively and quantitatively during development, differentiation and in tumorigenesis, and 3’-Sulfo-Lewis X (3-SLeX), a glycan epitope that has been reported to act as a ligand for the adhesion molecules E- and L-selectin which may help identify tumours with higher likelihood of progression and metastasis.  Glykogen has further antibodies in development including antibodies specific for SSEA3 and GloboH. 

SigLecs are sialic acid binding ImmunoGlobulin-type Lectins – a whole family of proteins expressed by cells of the immune system - that are sensitive to variations in the presentation of sialic acid and it is suggested that different immune responses can be generated based on the Sialic acid linkage detected.  

Lectins can have a number of sugar recognition sites, with the binding affinity increasing with the number of correct matches between the lectin and the carbohydrate, which also relies on the spatial presentation of the sugars. The strength of the binding of a lectin to a sugar can therefore also help to distinguish between different isotypes of carbohydrate that would appear the same on a mass spectrometer. 

Of themselves, lectins are a study area of interest. Some of the well-characterized roles of lectins are in cell-cell communication, cancer metastasis, embryogenesis, host-pathogen interactions, and tissue development (5).

As lectins are purified from natural sources, there can be some variation in the exact affinity of the fine carbohydrate structure; there can be hypervariability in the binding site loops which  generates carbohydrate recognition diversity (6) although the primary carbohydrate preference is secure. Thus, lectin specificity is usually generalised to a broad specificity determined from a sugar monomer that can inhibit lectin binding, or elute the carbohydrate if used with an agarose bound lectin. Further characterisation of lectin binding is determined with micro arrays, sometimes provided by commercial entities and published as at The National Centre for Functional Glycomics. As with antibodies, the use of recombinant technology can help ensure consistency over the long term and recombinant lectins are available in the market.

Other approaches to detecting carbohydrates comes in the form of LectEnz – modified carbohydrate recognising enzymes that retain their binding function but not enzymatic activity.

There are various offerings on the market now that enable a high level overview of glycosylation in an affordable way that can help screen possibilities and direct further studies if more in depth knowledge is required.