siaalhappe

Siaalhappo, or sialic acid, refers to a family of acidic monosaccharides with a nine-carbon backbone and a terminal carboxyl group that confers a negative charge at physiological pH. The most common member in humans is N-acetylneuraminic acid (Neu5Ac), while other forms such as N-glycolylneuraminic acid (Neu5Gc) occur in many mammals. Sialic acids are usually found at the outermost positions of glycan chains on glycoproteins and glycolipids, decorating cell surfaces and secreted molecules.

In biology, the position and linkage of sialic acids—such as α2-3, α2-6, or α2-8—shape interactions with other molecules and influence immune recognition, signaling, and pathogen binding. The diversity of sialic acid structures creates a complex code that modulates the stability and lifetime of glycoconjugates and their interactions with lectins and receptors.

Biosynthesis and turnover: Sialic acids are produced in the cytosol from N-acetylmannosamine and phosphoenolpyruvate, converted to CMP-sialic acid, and transferred to glycans by sialyltransferases. They can be removed by neuraminidases, a process known as desialylation, which can alter cell signaling and clearance.

Clinical and evolutionary notes: Humans lack the CMAH enzyme, so Neu5Gc is largely absent in normal human tissues, differing from many other mammals and affecting dietary uptake and immune recognition. Altered sialylation is associated with cancer and inflammatory states. Pathogens, including influenza viruses and several bacteria, exploit sialic acids as receptors; host range and tissue tropism are influenced by sialic acid linkage preferences.