Lipid Linked Oligosaccharides Are Precursors for Bacterial Cell Wall Synthesis

Like plants, many bacteria have thick, rigid extracellular walls that protect them from osmotic lysis. The pep-tidoglycan that gives bacterial envelopes their strength and rigidity is an alternating linear copolymer of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (Mur2Ac), linked by (^1n4) glycosidic bonds and cross-linked by short peptides attached to the Mur2Ac (Fig. 20-33). During assembly of the polysaccharide backbone of this complex macromolecule, both GlcNAc and Mur2Ac are activated by attachment of a uridine

W-Acetylglucosamine (GlcNAc)

W-Acetylmuramic acid (Mur2Ac)

W-Acetylglucosamine (GlcNAc)

W-Acetylmuramic acid (Mur2Ac)

Reducing end

Reducing end

Pentaglycine cross-link

FIGURE 20-33 Peptidoglycan structure. This is the peptidoglycan of the cell wall of Staphylococcus aureus, a gram-positive bacterium. Peptides (strings of colored spheres) covalently link N-acetylmuramic acid residues in neighboring polysaccharide chains. Note the mixture of l and d amino acids in the peptides. Gram-positive bacteria such as S. aureus have a pentaglycine chain in the cross-link. Gram-negative bacteria, such as E. coli, lack the pentaglycine; instead, the terminal D-Ala residue of one tetrapeptide is attached directly to a neighboring tetrapeptide through either L-Lys or a lysine-like amino acid, di-aminopimelic acid.

Pentaglycine cross-link

FIGURE 20-33 Peptidoglycan structure. This is the peptidoglycan of the cell wall of Staphylococcus aureus, a gram-positive bacterium. Peptides (strings of colored spheres) covalently link N-acetylmuramic acid residues in neighboring polysaccharide chains. Note the mixture of l and d amino acids in the peptides. Gram-positive bacteria such as S. aureus have a pentaglycine chain in the cross-link. Gram-negative bacteria, such as E. coli, lack the pentaglycine; instead, the terminal D-Ala residue of one tetrapeptide is attached directly to a neighboring tetrapeptide through either L-Lys or a lysine-like amino acid, di-aminopimelic acid.

FIGURE 20-34 Synthesis of bacterial peptidoglycan.

In the early steps of this pathway (1 through @), N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (Mur2Ac) are activated by attachment of a uridine nucleotide (UDP) to their anomeric carbons and, in the case of Mur2Ac, of a long-chain isoprenyl alcohol (dolichol) through a phosphodiester bond. These activating groups participate in the formation of glycosidic linkages; they serve as excellent leaving groups. After (5, © assembly of a disaccharide with a peptide side chain (10 amino acid residues), © this precursor is transferred to the nonreducing end of an existing pepti-doglycan chain, which serves as a primer for the polymerization reaction. Finally, © in a transpeptida-tion reaction between the peptide side chains on two different peptidoglycan molecules, a Gly residue at the end of one chain displaces a terminal D-Ala in the other chain, forming a cross-link. This transpeptidation reaction is inhibited by the penicillins, which kill bacteria by weakening their cell walls.

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