murein

Murein, also called peptidoglycan, is a polymer that forms the cell wall of most bacteria. It consists of long chains of repeating disaccharide units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). Each NAM residue carries a short canonical peptide, typically L-alanine, D-glutamic acid, meso-diaminopimelic acid (or L-lysine), and D-alanine. These peptide chains are cross-linked by peptide bridges, giving the mesh-like, rigid structure characteristic of the bacterial cell wall. The glycan strands are arranged in a layered network that varies among species.

In Gram-positive bacteria the peptidoglycan layer is thick and may contain teichoic and teichuronic acids; in Gram-negative bacteria the layer is thinner and located in the periplasm between the inner cytoplasmic membrane and the outer membrane, which contains lipopolysaccharide.

Biosynthesis begins in the cytoplasm with formation of UDP-N-acetylmuramyl-pentapeptide; linked to the lipid carrier bactoprenol; the lipid-linked subunit is flipped across the membrane by flippases such as MurJ; polymerization by transglycosylases adds subunits to the growing glycan chains, and cross-linking by transpeptidases (penicillin-binding proteins) forms the mature network.

Function and clinical relevance: Murein provides rigidity, determines cell shape, and protects against osmotic lysis. It is the target of many antibiotics, notably beta-lactams that inhibit PBPs and vancomycin that blocks cross-linking by binding the D-Ala-D-Ala terminus. Gram-positive bacteria tend to be more affected by glycopeptides due to their thicker peptidoglycan layer; Gram-negatives are more accessible to beta-lactams provided outer membrane permeability.

Immune recognition: fragments of murein, such as muramyl dipeptide, can be sensed by host pattern-recognition receptors (NOD1 and NOD2), contributing to innate immune responses.