Studies on the Formation Mechanism for Free Glycans

Glycans are sugars/sugar chains that are commonly attached to proteins or lipids where they can influence the physiological and biological properties of the carrier molecules. However, free, unconjugated glycans have also been identified in the cytosol of mammalian cells. Structural characterization of cytosolic free glycans demonstrates that they have structural similarities with N-linked glycans on proteins/lipids and their biosynthetic pathway has been gradually clarified. Recently, free glycans have also been identified in sera of various animal species. These extracellular free glycans also include free N-glycans, which have distinct structural features compared to intracellular free N-glycans, implying that they are formed through a different pathway than intracellular free glycans. In addition to free N-linked glycans, sialyl lactose/N-acetyllactosamine-type free glycans have been identified in the serum. Here, this mini-review briefly summarizes recent progress on understanding the mechanisms by which free glycans in mammalian cells as well as in animal sera are formed.