*The complexity of sugars: regarding their structures, they are not linear instead they are highly branched. Moreover, glycans can be modified (modified sugars), this increases its complexity.
*Complex biosynthetic pathways for glycans.
*Usually glycans are found either bound to protein (glycoprotein) or conjugated with lipids (glycolipids).
*Unlike genomes, glycans are highly dynamic.
This area of research has to deal with an inherent level of complexity not seen in other areas of applied biology. 68 building blocks (molecules for DNA, RNA and proteins; categories for lipids; types of sugar linkages for saccharides) provide the structural basis for the molecular choreography that constitutes the entire life of a cell. DNA and RNA have four building blocks each (the nucleosides or nucleotides). Lipids are divided into eight categories based on ketoacyl and isoprene. Proteins have 20 (the amino acids). Saccharides have 32 types of sugar linkages. While these building blocks can be attached only linearly for proteins and genes, they can be arranged in a branched array for saccharides, further increasing the degree of complexity.
Add to this the complexity of the numerous proteins involved, not only as carriers of carbohydrate, the glycoproteins, but proteins specifically involved in binding and reacting with carbohydrate:
*Carbohydrate-specific enzymes for synthesis, modulation, and degradation
*Lectins, carbohydrate-binding proteins of all sorts
*Receptors, circulating or membrane-bound carbohydrate-binding receptors