Polysialic acid (polySia) is polymer of linearly repeating monomer units of α2,8- and α2,9-glycosidic linked sialic acid residues. Sialic acid refers to carboxylated 9-carbon sugars, 2-keto-3-dexoxy-D-glycero-nononic acids. An unusual property of this sugar is that it often polymerizes into polySia. This is accomplished by attaching the monomers to the nonreducing end of the glycan. This mostly consists of Neu5Ac subunits. It is polyanionic and bulky, meaning there is little ability to reach its central molecules. polySia is useful in signaling in vertebrates and on the cell surface of few glycoproteins and glycolipids causing modifications, and it has been recently found that the function of polySia relates almost directly to its degree of polymerization. The number of units can range from 8 to greater than 400. This vast range causes differences in the polySias ability to adhere different cells, assist in cellular migration, synapse formation, and regulate adhesion in nerve cells by modeling and formating them. polySias most prominent role is in post-translational modifications in a few proteins, with the main one being NCAM. polySia links to adhesion molecules causing their adhesive properties to be subdued allowing for the detailed control of cell migration and cell to cell relations. This is caused by polySia's bulky and polyanionic properties.
The human body produces polySia naturally and attaches it to a various number of proteins. This is done by linking polySia on the α2,3- or α2,6- terminal of the glycoprotein. O-linked glycosylation through threonine or N-linked glycosylation through asparagine is employed. This polySia linkage is found in proteins such as NCAM, E-selectin ligand 1 (ESL-1), C–C chemokine receptor type 7 (CCR7), synaptic cell adhesion molecule-1 (SynCAM-1), neuropilin-2 (NRP-2), the CD36 scavenger receptor found in the milk of humans, and the α-subunit of the voltage-sensitive sodium channel. The synthesis of polySia is enzymatically formed by α2,8-sialyltransferase (ST8Sia) in a Type II transmembrane protein located on the Golgi Apparatus membrane. ST8Sia does this by adding sialic acids to the terminal end of the glycan through the CMP-sialic acid donor at various lengths depending on necessity. The length is controlled extensively by the expression of polysialyltransferase enzymes, once again controlling the function of polySia.