Peptidoglycanstrukturen

Peptidoglycans, also called murein, form the mesh-like backbone of most bacterial cell walls. They are polymers of repeating disaccharide units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by β(1→4) glycosidic bonds. Each NAM carries a short peptide stem that typically contains D-amino acids; the stems are cross-linked to neighboring stems, creating a strong, three-dimensional lattice.

Cross-links between peptide stems are formed by enzymes known as transpeptidases or penicillin-binding proteins. In many Gram-negative bacteria, cross-links connect the penultimate D-alanine of one stem to meso-diaminopimelic acid (DAP) of another stem. In many Gram-positive bacteria, cross-links may be established via short peptide bridges (for example, pentaglycine in Staphylococcus aureus). The exact pattern of cross-linking influences cell-wall rigidity and porosity.

Biosynthesis occurs in several steps: cytoplasmic synthesis of UDP-NAM-pentapeptide is linked to the lipid carrier bactoprenol to form lipid II. The lipid II molecule is flipped across the inner membrane, after which glycan chains are elongated by glycosyltransferases and cross-linked by transpeptidases in the periplasm or cell wall region.

The peptidoglycan layer provides structural integrity, determines cell shape, and protects against osmotic lysis. It is a prime target for antibiotics; β-lactams inhibit transpeptidases, weakening the wall, while lysozyme cleaves the glycosidic bond between NAG and NAM. Variation in cross-linking and composition can affect antibiotic susceptibility.

Most bacteria contain peptidoglycan, whereas archaea generally do not and may use pseudopeptidoglycan or other polymers. Some bacteria exhibit modified or reduced peptidoglycan, influencing their interactions with antibiotics.