peptoglycaan

Peptoglycaan (often spelled peptidoglycan) is the structural polymer that forms the cell wall of most bacteria. It consists of long chains of alternating N-acetylglucosamine and N-acetylmuramic acid residues linked by β-1,4 glycosidic bonds. Each NAM sugar carries a short peptide stem, typically containing L-alanine, D-glutamic acid, meso-diaminopimelic acid (or L-lysine in some species), and D-alanine. Cross-linking between peptide stems, carried out by penicillin-binding proteins and related enzymes, creates a strong mesh-like lattice that provides rigidity and shape.

In Gram-positive bacteria, the peptidoglycan layer is thick and exposed to the exterior, whereas in Gram-negative bacteria it is comparatively thin and located between the inner cytoplasmic membrane and an outer membrane. Synthesis occurs in stages: cytoplasmic production of UDP-NAM-pentapeptide, transport across the cytoplasmic membrane by the lipid carrier bactoprenol, polymerization by transglycosylases, and cross-linking by transpeptidases.

Peptoglycaan is essential for bacterial viability and is a major antibiotic target. Beta-lactam antibiotics inhibit transpeptidases, preventing cross-link formation, while glycopeptides block incorporation of the NAM-NAG unit or its termini in some organisms. Bacterial resistance can involve altered PBPs, reduced permeability, or modifications of the target.

Fragments of peptoglycan released during growth can activate host immune receptors, including NOD-like receptors such as NOD1 and NOD2, contributing to inflammation. The polymer is unique to bacteria or bacterial-like cell walls, distinguishing it from polymers found in other domains of life.