sialic acid

Meaning: Sialic acids are a family of nine-carbon acidic monosaccharides. The most common form in humans is N-acetylneuraminic acid. They usually sit at the outer, terminal ends of glycan chains on glycoproteins and glycolipids (such as gangliosides) on the cell surface, and on secreted glycoproteins such as mucins. The carboxyl group is deprotonated at physiological pH, so each sialic acid adds a negative charge. That charge helps cells and molecules repel one another, and the exposed terminal sugar acts as a recognition signal.

On the MCAT: The usual setting is influenza. Viral hemagglutinin binds sialic acid on host cell glycans to start infection. Viral neuraminidase, a glycosidase, cleaves sialic acid so new virions can detach from the cell. Other questions ask you to predict that removing sialic acid lowers the net negative charge of a cell surface or glycoprotein, which could change electrophoretic migration or cell-cell repulsion. Others use it as an example of carbohydrate-based cell recognition.

Common confusion: The word "acid" and the nitrogen-containing name lead students to treat it as an amino acid. It is a carbohydrate, a sugar acid with a nine-carbon backbone and an N-acetyl group. Its acidity comes from a carboxylic acid group, which is ionized at pH 7.4, not protonated.