Sialihappo

Sialihappo refers to a family of acidic sugars, commonly known in English as sialic acids, including neuraminic acid and its derivatives. They typically have a nine-carbon backbone and bear a carboxyl group that confers a negative charge at physiological pH. In biology they most often occur as terminal residues on glycoproteins and glycolipids, influencing cell–cell and cell–matrix interactions.

The most abundant sialic acid in humans is N-acetylneuraminic acid (Neu5Ac). Other forms, such as N-glycolylneuraminic acid (Neu5Gc), are found in most mammals but are largely absent in humans due to CMAH gene inactivation; Neu5Gc can be absorbed from diet and incorporated into tissues in trace amounts.

Biosynthesis occurs in cells via the CMP-Neu5Ac pathway: fructose-6-phosphate and N-acetylmannosamine are converted to Neu5Ac, activated to CMP-Neu5Ac, and transferred by sialyltransferases onto nascent glycans. Sialic acids can be removed by neuraminidases (sialidases), enabling remodeling of glycoconjugates.

Functionally, sialihappo residues modulate protein half-life, receptor binding, and immune interactions. They participate in embryonic development, neural communication, and pathogen recognition. Many microbes exploit host sialic acids for adhesion or camouflage; conversely host receptors such as Siglecs bind sialic acids to regulate immune responses.

Clinically, altered sialylation is associated with aging and cancer; the pattern of sialic acids on serum glycoproteins affects serum half-life and stability of therapeutic proteins. In biotechnology, controlling sialylation is important for efficacy and pharmacokinetics of biopharmaceuticals.