Peptidoglicà

Peptidoglicà, known in English as peptidoglycan, is a large, mesh-like polymer that forms the primary structural component of most bacterial cell walls. It consists of repeating disaccharide units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), with NAM residues bearing short peptide stems. These peptides are cross-linked to one another, creating a strong, rigid lattice that helps the cell maintain shape and resist osmotic pressure.

The organization of peptidoglicà varies between major groups of bacteria. In Gram-positive bacteria, the layer is relatively thick and often contains teichoic and lipoteichoic acids that extend through the cell wall. In Gram-negative bacteria, the peptidoglicà layer is thinner and located between the inner cytoplasmic membrane and an outer membrane that contains lipopolysaccharides. Cross-linking between peptide stems can occur directly or via peptide bridges, typically involving amino acids such as D-alanine and dipeptide cross-links. Enzymes called penicillin-binding proteins (PBPs) catalyze transpeptidation, the key cross-linking step.

Biosynthesis begins in the cytoplasm, where the NAM peptide is built on a UDP-NAM primer. The lipid carrier undecaprenyl phosphate ferries the unit across the membrane, where polymerization by transglycosylases and subsequent cross-linking by transpeptidases strengthen the wall. The process is tightly regulated and modulated by autolysins that remodel the wall during growth.

Peptidoglicà is a common target for antibiotics, including β-lactams and glycopeptides, which disrupt cross-linking or elongation. It is absent or highly modified in some organisms (e.g., Mycoplasma; certain archaea), contributing to differences in cell envelope biology.