peptidoglukaani

Peptidoglycan is a complex polymer that forms a rigid, mesh‑like wall surrounding the cytoplasmic membrane of most bacteria. The backbone consists of repeating disaccharide units of N‑acetylglucosamine and N‑acetylmuramic acid linked by β‑1,4 glycosidic bonds. Attached to the lactyl group of each muramic acid is a short peptide chain, typically comprising a dipeptide of L‑meso‑diaminopimelic acid and L‑lysine in Gram‑negative bacteria or L‑lysine in Gram‑positive species. Cross‑linking of these peptides by trans‑peptidation reactions, catalyzed by penicillin‑binding proteins, gives the peptidoglycan a three‑dimensional structure that provides mechanical strength and maintains cell shape.

In Gram‑positive bacteria the peptidoglycan layer is thick, comprising up to 90% of the cell wall, and is often decorated with teichoic acids. In Gram‑negative bacteria the layer is thinner, but the peptidoglycan is anchored to an outer membrane that contains lipopolysaccharide. The differences in layer thickness and composition are exploited by antibiotics: β‑lactams, such as penicillin, inhibit the trans‑peptidase step, preventing cross‑linking and weakening the wall; glycopeptide antibiotics, like vancomycin, bind the D‑alanine–D‑alanine terminus of the stem peptide, blocking trans‑glycosylation.

Peptidoglycan is the target of many innate immune sensors. The pattern recognition receptor NOD‑1 detects mesodiaminopimelic acid–containing fragments, while NOD‑2 recognizes muramyl dipeptide, a minimal motif present in all peptidoglycans. These immune responses help to differentiate self from bacterial cells and drive inflammatory signaling.

The enzyme complex that synthesizes peptidoglycan is essential for bacterial survival, making it a prime target for antimicrobial drug discovery. Mutations in genes coding for the enzymes that build the peptidoglycan backbone or its cross‑linking can confer antibiotic resistance, as seen in methicillin‑resistant Staphylococcus aureus and vancomycin‑resistant Enterococcus species. Because of its universal presence and absence in eukaryotic cells, peptidoglycan serves as a cornerstone of bacterial physiology, immunology, and pharmacology.