Sialylation

Sialylation is the enzymatic process that attaches sialic acid residues to the terminal positions of glycan chains on glycoproteins and glycolipids. In animals, the most common sialic acid is N-acetylneuraminic acid (Neu5Ac), though other forms exist. The addition is mediated by sialyltransferases in the Golgi apparatus, which transfer activated CMP-sialic acid donors to galactose or other sugars on N- or O-linked glycans and glycosphingolipids. Sialylation can also occur in polysialylation, where long chains of sialic acid are added by specific enzymes.

Sialylation types include N-linked and O-linked glycoprotein sialylation, and various linkages such as alpha-2,3 and alpha-2,6 to galactose, as well as alpha-2,8 in polysialic acid chains. The pattern and extent of sialylation influence the stability and half-life of glycoproteins in circulation, modulate receptor binding and cell–cell interactions, and affect immune recognition. Desialylation by neuraminidases (sialidases) removes these residues and can alter function or mark molecules for clearance.

Biological roles are diverse, contributing to development, inflammation, and pathogen interactions. In health, appropriate sialylation helps regulate self-recognition and signaling; in disease, altered sialylation is observed in cancer and inflammatory conditions and can affect metastasis, immune evasion, and vaccine or therapeutic targeting. Congenital disorders of glycosylation and other metabolic diseases can involve defective sialylation.

Analytical approaches include lectin binding assays using SNA (alpha-2,6), MAA or MAL-II (alpha-2,3), as well as mass spectrometry and chromatography to characterize sialylation patterns.