Free oligosaccharides in serum

Glycans are sugars/sugar chains that are usually linked to proteins or lipids. The attachment of glycans often results in alterations of physicochemical/physiological properties of the carrier molecules, e.g., glycosylation of proteins can modulate their fate, intracellular localization, or interaction with cells/other proteins. On the other hand, unconjugated N-glycans (free N-glycans; FNGs) have been identified in the cytosol of eukaryotic cells. The processing pathway of intracellular FNGs has been clarified in recent years, but their biological functions remain unclear. Free oligosaccharides have also been identified in the sera of various animals. Structurally, these extracellular free glycans can be classified into three types: sialyl FNGs, oligomannose-type FNGs, and sialyl lactose/N-acetyllactosamine-type glycans. The extracellular FNGs show different structural features from intracellular FNGs, implying that their mechanism of formation is distinct. This mini-review summarizes current knowledge about the structures and formation mechanisms of free oligosaccharides in serum, and suggests their possible biological functions.