Neuraminic acid (5-amino-3,5-dideoxy-D-
glycero-D-
galacto-non-2-ulosonic acid) is a 9-carbon
monosaccharide (a
nonose), a derivative of a
ketononose. Neuraminic acid may be visualized as the product of an
aldol-condensation product of
pyruvic acid and D-
mannosamine (2-amino-2-deoxy-mannose). Neuraminic acid does not occur naturally, but many of its derivatives are found widely distributed in animal tissues and in bacteria, especially in
glycoproteins and
gangliosides. The
N- or
O-substituted derivatives of neuraminic acid are collectively known as
sialic acids, the predominant form in mammalian cells being
N-acetylneuraminic acid. 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.