An Introduction to Glycans

From unicellular organisms to mammals, all living things owe their functionality and integrity to 4 building blocks: nucleic acids (DNA and RNA), proteins, lipids, and polysaccharides, also known as glycans. Among these macromolecules, glycans are perhaps the most mysterious due to their dazzling structural diversity and various functions. In fact, the more we learn about glycans, the more we realize that we are still scratching the surface of glycobiology. To add to that, it is impossible to think of glycans in isolation. They are composed of glycosidically linked monosaccharides conjugated to other macromolecules. That’s why understanding the fundamentals of glycans can pave the way for elucidating their roles in essential biological processes. 

To understand the factors behind glycan structural diversity, we need to go back to their building blocks: monosaccharides. More specifically, the answer lies in how 2 monosaccharides join together by a glycosidic bond. Unlike amino acids and nucleotides, with a more standardized coupling to form proteins and nucleic acids, 2 monosaccharides can bind at several locations, resulting in the possibility of several isomeric structures. To add to that, 1 monosaccharide can form glycosidic linkages at multiple locations, giving rise to a structural phenomenon not encountered in other macromolecules: branching. As the size of a polysaccharide grows, the number of possible structures increases exponentially.

Glycans are conjugated with proteins and lipids to confer specific structures and functions, such as protein folding, recognition, and cell migration. Despite the staggering diversity of glycans, a high-level classification is possible depending on the conjugation type and the binding site.

The covalent bonding of a glycan to a protein, glycosylation, is one of the most common posttranslational modifications in eukaryotic proteins. Unlike the diverse nature of glycosidic linkages between monosaccharides, glycosylation is more constrained since glycans can bind peptides only at the binding sites composed of specific amino acid sequences. These glycans can be classified according to the atom on the amino acid residue side chain at the binding site. 

Besides proteins, glycans can also be covalently attached to lipids to form glycolipids crucial to cell membrane integrity, cell-cell interactions, and immune response. Glycolipids are classified according to their hydrocarbon backbone: glycerolipids with glycerol and sphingosine with sphingolipids.

It is nearly impossible to attribute a specific function to a glycan or glycan subclass. The same glycan structure can serve a range of tasks across different organisms as well as at different locations of the same organism. That’s why we present a broadened and simplified classification of glycan function.

Glycans are ubiquitous in cellular compartments, membranes, and extracellular matrix, where they alter the structure and function of the proteins and lipids that they are attached to. Therefore, glycans play a critical part in shaping primary macromolecule structures, contributing to the integrity of cells and cellular components. Glycosylated macromolecules are paramount to a long list of structural and modulatory functions, from protection and transport to site-specific functions, such as lubrication of the joints to prevent friction ⁽¹⁾.  

Perhaps the most well-known example is the mucin peptide, which forms protective layers on the epithelial cell surfaces to protect tissues against pathogens and free radicals (2). In fact, aberrations in cell surface glycosylation are often associated with inflammatory diseases and cancer. Another example of a protective function is that provided by cellulose (3) and chitin (4), glycan polymers made of glucose and GlcNAc, respectively, which are indispensable components of cell walls that provide strength and stability for plants and fungi.   

Glycan-induced structural modifications can also alter the physical properties of macromolecules to confer advantages. Blood plasma is the first example to come to mind since it contains a high concentration of glycoproteins, where hydrophilic glycans augment protein solubility (5). In addition, glycosylation alters the charge density of proteins, thereby preventing their degradation by proteases (6).

Many organisms tend to form different forms of symbiotic relationships with one another, and it has become evident that cell surface glycans are one of the main control points of these interactions.  

Terminal glycan residues are particularly critical to pathogen-host interactions. For instance, parasitic bacteria, including E. coli (7), and Helicobacter (8), adhere to the host cells by recognizing cell surface sialoglycans. Another example is malaria, where P. falciparum invades red blood cells by binding specific sialic acid residues on the surface (9). 

Extrinsic recognition also applies to viral infections, including the most recent pandemic caused by COVID-19. A growing body of evidence suggests that N- and O-glycosylation of the spike protein strengthens its binding affinity to the ACE2 receptor while helping it evade the host immune system (10).  

Fortunately, extrinsic recognition is bidirectional, meaning that host immune systems combat pathogens through glycan recognition. More specifically, glycan-binding proteins (which we will cover in the next blog post in more detail) can recognize the surface glycans of pathogens to entrap and destroy them.

Cell-cell communication and cellular trafficking are greatly influenced by glycan recognition. Cellular uptake mechanisms, such as endocytosis and phagocytosis, are often mediated by the cell surface receptors that recognize macrophage terminal glycans. This allows cells to pull in molecules from outside the cell, transport material across organelles, and discard unwanted material.  

Essential processes in organelles are also mediated by glycan receptors. For example, receptors in the ER carry out specific folding patterns for the incoming proteins based on their glycan epitopes. By the same logic, incorrectly folded proteins can be detected and discarded upon recognizing aberrant glycan structures.   

Cell-cell interactions also rely on glycan recognition by surface proteins and are crucial for the survival of individual organisms and the harmony of entire ecosystems. Well-characterized examples include glycosylated fibrinogen-induced blood clotting (11) and egg-sperm interactions (12). On a larger scale, glycan-mediated intercellular signaling between rhizobacteria is largely responsible for establishing their symbiotic relationship with plants for nitrogen fixation, which has a large impact on the food chain (13).

Pathogens that often evade innate immune reactions decorate their cell surfaces with glycan structures found on host cell surfaces (14). This means that pathogens can camouflage their antigenic epitopes behind these glycans. This can be achieved through various mechanisms, such as the appropriation of host glycans or convergent evolution of the pathogen, enabling the mimicry of the biosynthetic pathways of the target host.  

The study of glycans can provide us valuable insights about their roles in our biological systems. While there is an overwhelming diversity in glycan structure and function, we believe that this article recaps the fundamentals of structural diversity and the functional significance of glycans.  

That said, there are several resources for you to expand your knowledge of glycan biochemistry. In the article “Sweet systems: technologies for glycomic analysis and their integration into systems biology”, Chen, et al. explains glycan function with examples from the 4 classes of functions. The article also discusses the current setbacks and questions in glycobiology. For more specific carbohydrate related data, you can check out GlyGen: Computational and Informatics Resources for Glycoscience. Last but not least, our SpeakEasy Science Blog and Glycobiology eBook serve as a scientific hub where you can find practical information about glycan detection and quantification as well as reviews of peer-reviewed articles exploring the roles of glycans in cancer. ​