sialic acid
Summary
Sialic acid is a family of nine-carbon acidic sugars, chiefly N-acetylneuraminic acid, that cap the ends of glycoprotein and glycolipid chains on cell surfaces. It serves as the influenza receptor and, on host cells, as a self-marker that restrains complement activation.
Detail
Sialic acid's negative charge and terminal position give it several roles. Influenza hemagglutinin binds it to enter cells and neuraminidase cleaves it to release progeny, and the linkage preference (alpha-2,6 in human upper airway versus alpha-2,3 in avian gut and human lower airway) is a key determinant of host range for avian strains such as H5N1. On host cells sialic acid recruits complement factor H, which accelerates decay of C3 convertase and thereby protects self surfaces from alternative pathway attack; loss of this regulation underlies atypical haemolytic uraemic syndrome. Some bacteria exploit the same trick by coating themselves in sialic acid, most importantly Neisseria meningitidis group B and Escherichia coli K1, whose sialylated capsules mimic host tissue, evade complement, and are poorly immunogenic — which is why there is no conventional polysaccharide vaccine against meningococcus B. Sialic acid also caps desialylated glycoproteins whose removal signals hepatic clearance, and it terminates the ABO blood group and Lewis antigen structures.
Sources
- Lippincott Illustrated Reviews: Biochemistry
- Levinson Medical Microbiology and Immunology
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