muramic

Muramic acid, commonly known as N-acetylmuramic acid (NAM), is a modified monosaccharide that is a central component of bacterial peptidoglycan, the polymer that provides strength to most bacterial cell walls. NAM is the N-acetylated derivative of muramic acid and is linked to N-acetylglucosamine (NAG) in a β-1,4 glycosidic bond, forming the repeating disaccharide units of the glycan backbone.

Each NAM residue carries a lactyl group at the C3 position to which a short amino acid peptide stem is attached. In most bacteria this stem is four or five residues long and can cross-link with stems on neighboring NAM units, creating a cross-linked network that provides rigidity to the cell wall. The exact peptide composition varies among species, commonly including diaminopimelic acid or lysine.

Biosynthesis and function: NAM-containing units are synthesized in the cytoplasm and transported across the cell membrane for incorporation into the growing peptidoglycan. The process is essential for cell viability and is a target of many antibiotics, particularly β-lactams, which inhibit the cross-linking step.

Distribution and significance: NAM is found in the peptidoglycan of virtually all bacteria and is absent in archaea, fungi, and plants. Because it is a hallmark of bacterial cell walls, NAM-containing structures are used in microbiological study and detection. Enzymes such as lysozyme can cleave the glycosidic bond between NAM and NAG, compromising cell wall integrity.

Terminology: NAM stands for N-acetylmuramic acid; muramic acid can describe the same moiety in discussions of peptidoglycan. The historical term murein refers to the entire peptidoglycan polymer.