peptideglycan

Peptideglycan, more commonly called peptidoglycan, is a large, rigid polymer that forms the cell wall of most bacteria. It consists of long chains of alternating sugars, N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc), linked by beta-1,4-glycosidic bonds. Attached to MurNAc are short peptide stems, typically consisting of four amino acids, which cross-link adjacent glycan chains through peptide bridges. This cross-linking creates a thick, protective mesh that gives bacterial cells their shape and helps withstand osmotic pressure.

In Gram-positive bacteria, peptidoglycan forms a thick, multilayered layer that is accompanied by teichoic and lipoteichoic acids, contributing to surface charge and rigidity. In Gram-negative bacteria, a thinner peptidoglycan layer lies in the periplasmic space between the inner cytoplasmic membrane and the outer membrane, which contains lipopolysaccharides and porins. The specific amino acid composition of the peptide stems and the cross-linking pattern vary among species, with common templates including diaminopimelic acid (DAP) or lysine in cross-bridges.

Biosynthesis of peptidoglycan proceeds via cytoplasmic precursor synthesis, producing UDP-MurNAc-pentapeptide, which is linked to a lipid carrier (undecaprenyl phosphate) and transferred to the exterior leaflet of the cytoplasmic membrane. Polymerization occurs through transglycosylation to elongate glycan strands, followed by transpeptidation to form cross-links. These processes are mediated by enzymes such as penicillin-binding proteins.

Peptidoglycan is essential for bacterial viability and is a major target for antibiotics, including beta-lactams that inhibit cross-linking and glycopeptides that block precursor incorporation. Resistance mechanisms include altered PBPs, reduced precursor synthesis, and efflux. Peptideglycan is absent in archaea and eukaryotes, making it a key distinguishing feature of bacteria.