Bacterial and Algal Cell Walls Contain Structural Heteropolysaccharides

The rigid component of bacterial cell walls is a heteropolymer of alternating (^1n4)-linked N-acetylgluco-samine and N-acetylmuramic acid residues (Fig. 7-22). The linear polymers lie side by side in the cell wall, cross-linked by short peptides, the exact structure of which depends on the bacterial species. The peptide cross-links weld the polysaccharide chains into a strong sheath that envelops the entire cell and prevents cellular swelling and lysis due to the osmotic entry of water. The enzyme lysozyme kills bacteria by hydrolyzing the (^1n4) gly-cosidic bond between N-acetylglucosamine and N-acetylmuramic acid (see Fig. 6-24). Lysozyme is notably present in tears, presumably as a defense against bacterial infections of the eye. It is also produced by certain bacterial viruses to ensure their release from the host

Staphylococcus aureus

Staphylococcus aureus

Route Entry Staph Aureus Into Host

W-Acetylglucosamine (GlcNAc)

W-Acetylmuramic acid (Mur2Ac)

W-Acetylglucosamine (GlcNAc)

W-Acetylmuramic acid (Mur2Ac)

Site of cleavage by lysozyme

Reducing end

Site of cleavage by lysozyme

Reducing end

Pentaglycine Amino

Pentaglycine cross-link

FIGURE 7-22 Peptidoglycan. Shown here 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 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, diaminopimelic acid.

Pentaglycine cross-link

FIGURE 7-22 Peptidoglycan. Shown here 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 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, diaminopimelic acid.

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