Human KDN (deaminated neuraminic acid) and its elevated expression in cancer cells: mechanism and significance.

Sialic acids are a family of nine-carbon carboxylated sugars having a nonulosonate skeletal structure (Fig. 35.1). This structure is uniquely different from that of other sugar units of animal glycans. The most popular sialic acid is N-acetylneuraminic acid (Neu5Ac), which is universally found on cell surface glycocalyx and in secreted glycoproteins of vertebrates and some invertebrates. Sialic acids have low acid–base dissociation constants and give a negative charge on the cell surface under a wide range of physiological pH. In nature, more than 50 kinds of sialic acids are known. Nearly all of them are derived from Neu5Ac by a substitution on the hydroxyl groups (e.g., O-acetyl-Neu5Ac) and/or a hydroxylation of the N-acetyl group (e.g., N-glycolylneuraminic acid, Neu5Gc). Each modified sialic acid has properties different from those of Neu5Ac and is believed to contribute to specific physiological functions. In animal cells, sialic acids are most frequently the terminal sugars of cell surface glycolipids and glycoproteins, and any change that occurs on sialic acids can considerably influence the biological properties of a cell.