Sialic acid

Sialic acid is a generic term for the N- or O-substituted derivatives of neuraminic acid, a nine-carbon monosaccharide. It is also the name for the most common member of this group, N-acetylneuraminic acid (Neu5Ac or NANA). Sialic acids are found widely distributed in animal tissues and in bacteria, especially in glycoproteins and gangliosides. The amino group bears either an acetyl or a glycolyl group. The hydroxyl substituents may vary considerably: acetyl, lactyl, methyl, sulfate, and phosphate groups have been found.

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Sialic acid-rich glycoproteins bind selectin in humans and other organisms. Cancer cells that can metastasize often have a lot of sialic acid-rich glycoproteins. This helps these late-stage cancer cells enter the blood stream.

Sialic acid-rich oligosaccharides on the glycoconjugates found on surface membranes help keep water at the surface of cells. The sialic acid-rich regions contribute to creating a negative charge on the cells' surface. Since water is a polar molecule with partial positive charges on both hydrogen atoms, it is attracted to cell surfaces and membranes. This also contributes to cellular fluid uptake.

Sialic acid can "hide" mannose antigens on the surface of host cells or bacteria from mannose-binding lectin. This prevents activation of complement.

The term "sialic acid" (from the Greek σιαλοσ (sialos) 'saliva') was first introduced by Swedish biochemist, Gunnar Blix, in 1952.

See also

• Sialidosis
• Sialoglycoprotein
• Sialyltransferase

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