Siaalihappo

Siaalihappo, or sialic acid, is a family of acidic monosaccharides with a nine-carbon backbone that typically appears at the terminal positions of glycan chains on cell surface glycoproteins and glycolipids. The most common forms in vertebrates are N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc).

In humans, Neu5Gc is not synthesized due to inactivation of the CMAH gene, though trace amounts can be acquired dietarily and incorporated into tissues, where it may elicit an immune response. Sialic acids influence a range of biological processes, including cell–cell interactions, receptor recognition, and immune modulation. They can shield underlying glycans from degradation, alter the half-life of circulating glycoproteins, and affect complement activation and leukocyte trafficking. Pathogens and toxins often recognize sialic acid residues, while some organisms also employ strategies to mimic or remove sialic acids to invade or evade host defenses.

Biosynthesis and attachment occur through the sialylation pathway, which uses CMP-Neu5Ac as the activated donor of sialic acid. In vertebrates, CMP-Neu5Ac is produced from UDP-N-acetylglucosamine via a series of enzymes centered on GNE (UDP-GlcNAc 2-epimerase/ManNAc kinase). Sialyltransferases then transfer sialic acid to glycans, forming sialylated structures. Removal of sialic acids is carried out by neuraminidases (sialidases), highlighting the dynamic regulation of sialylation on cell surfaces.

Clinical relevance includes associations between altered sialylation patterns and cancer progression, inflammatory diseases, and autoimmunity. Sialic acids also play a role in viral infections and vaccine research, notably in influenza, where receptor preference for different sialic acid linkages can influence host range.