Ump

^{^Dolichol

^{^Dolichol

Existing peptidoglycan "primer"

Mature, fully cross-linked peptidoglycan

Penicillins

Another peptidoglycan chain

Existing peptidoglycan "primer"

Mature, fully cross-linked peptidoglycan

Penicillins

Another peptidoglycan chain nucleotide at their anomeric carbons. First, GlcNAc 1-phosphate condenses with UTP to form UDP-GlcNAc (Fig. 20-34, step ©), which reacts with phospho-enolpyruvate to form UDP-Mur2Ac (step ©); five amino acids are then added (step ©). The Mur2Ac-pentapep-tide moiety is transferred from the uridine nucleotide to the membrane lipid dolichol, a long-chain isoprenoid alcohol (see Fig. 10-22f) (step ©), and a GlcNAc residue is donated by UDP-GlcNAc (step ©). In many bacteria, five glycines are added in peptide linkage to the amino group of the Lys residue of the pentapeptide (step ©). Finally, this disaccharide decapeptide is added to the nonreducing end of an existing peptidoglycan molecule (step ©). A transpeptidation reaction crosslinks adjacent polysaccharide chains (step ©), contributing to a huge, strong, macromolecular wall around the bacterial cell. Many of the most effective antibiotics in use today act by inhibiting reactions in the synthesis of the peptidoglycan (Box 20-1).

Many other oligosaccharides and polysaccharides are synthesized by similar routes in which sugars are activated for subsequent reactions by attachment to nu-cleotides. In the glycosylation of proteins, for example (see Fig. 27-34), the precursors of the carbohydrate moieties include sugar nucleotides and lipid-linked oligosaccharides.

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