muropeptide

Muropeptide is the minimal structural unit of peptidoglycan, the cell wall polymer that gives many bacteria their rigid shape. A muropeptide consists of a disaccharide made of N-acetylglucosamine and N-acetylmuramic acid, to which a short peptide stem is attached. The peptide stem length and composition vary among species, and cross-linking between stems forms the peptidoglycan mesh.

Typically the disaccharide-peptide unit is the repeating unit of the peptidoglycan, with the stem peptide often containing amino acids such as L-alanine, D-glutamic acid, meso-diaminopimelic acid or L-lysine and D-alanine; subsequent cross-links can connect stem peptides of adjacent units. In many Gram-negative bacteria the stem is L-Ala-D-Glu-m-DAP-D-Ala; in Gram-positive bacteria there may be additional residues or cross-bridges.

Muropeptides are released during cell wall synthesis and remodeling; lysozymes and autolysins cleave the glycan strands to produce muropeptide fragments. Antibiotics targeting transpeptidation (beta-lactams) can alter muropeptide profiles by inhibiting crosslinking and triggering accumulation of particular fragments.

Many muropeptides act as signaling molecules. In bacteria they are intermediates in peptidoglycan turnover; in animals they function as pathogen-associated molecular patterns that activate innate immunity through NOD-like receptors, notably muramyl dipeptide (MDP) recognized by NOD2 and iE-DAP recognized by NOD1.

Because of their role in immune sensing and peptidoglycan biology, muropeptides are studied in microbiology and immunology; they have also been explored as vaccine adjuvants.