FIGURE 2. Two major sialic acids in mammalian cells.

Sialic acids share a nine-carbon backbone, a carboxylic acid at the C1 position, and various -glycosidic linkages to the underlying sugar chain (R) from the C2 position. Various substitutions at the C4, C5, C7, C8 and C9 positions combine with linkage variation to generate the diversity of sialic acids in nature. The structures shown here are the most prevalent sialic acids found in mammalian cells. The only difference between the two is the additional oxygen atom in the N-glycolyl group of Neu5Gc (blue). This modification of the precursor sialic acid Neu5Ac cannot occur in humans because of a genetic mutation that occurred during human evolution.