| N-Acetyl-neuraminic acid (sialic acid)
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| UDP-GlcNAc is the precursor of N-acetyl-neuraminic acid (NANA) or sialic acid. These are 9-carbon sugar acids that are common components of N- and O-linked oligosaccharides. Sialic acid is produced by the condensation of an amino sugar with phosphoenolpyruvate. Cytidine monophosphate (CMP)-sialic acid is the sugar nucleotide donor for glycoconjugate biosynthesis. It is formed by reaction of free sialic acid with cytidine triphosphate (CTP) (see Fig. 25.9). CMP-sialic acid (CMP-NANA) is the only nucleotide-monophosphate sugar donor in glycoconjugate metabolism. In microorganisms, keto-deoxy-octulosonic acid, a component of the cell wall, is activated as CMP-KDO. With few exceptions, all amino sugars in eukaryotic glycoconjugates are acetylated; thus, they are neutral and do not contribute charge to glycoconjugates.
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