Peptidoglycan is a unique, net-like heteropolymer forming the essential structural scaffold of bacterial cell walls, conferring rigidity against turgor pressure while enabling growth and division. This macromolecule plays a central role in bacterial physiology and survival. Its study bridges classical polysaccharides (such as laminarin, agarose, and inulin) with a peptide-crosslinked glycan framework that is specific to prokaryotic organisms and absent in eukaryotic cells.
Molecular Structure
Peptidoglycan (also called murein) consists of linear glycan chains composed of alternating β-(1→4)-linked N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc). Each MurNAc residue carries a D-lactyl group at the C3 position, to which a short peptide stem is covalently attached. In Gram-negative bacteria, this stem typically follows the sequence L-Ala¹–D-iGlu²–meso-diaminopimelic acid³–D-Ala⁴–D-Ala⁵, whereas Gram-positive organisms commonly substitute meso-diaminopimelic acid with L-lysine at the third position.
Adjacent peptide stems are cross-linked, usually between the third amino acid of one stem and the fourth D-alanine of another, generating a continuous three-dimensional lattice. This structure forms a single, enormous macromolecule with a molecular mass approaching 109 Da, surrounding the cytoplasmic membrane as a protective mesh. The layer reaches approximately 30–100 nm in thickness in Gram-positive bacteria, while remaining much thinner (2–7 nm) in Gram-negative species, where it is located between the inner and outer membranes.
Properties and Variations
The covalently cross-linked lattice of peptidoglycan enables bacterial cells to withstand internal turgor pressures ranging from approximately 2 to 15 bar while retaining sufficient elasticity to permit growth and division. During each generation, up to 25% of the peptidoglycan mass may be remodeled, reflecting continuous turnover and controlled enzymatic degradation and resynthesis.
Peptidoglycan also serves as a pathogen-associated molecular pattern (PAMP) recognized by innate immune receptors such as NOD1 and NOD2, and it is a substrate for lysozyme-mediated hydrolysis. Structural variations are widespread and biologically significant, including peptide stem amidation in species such as Bacillus subtilis, O-acetylation conferring resistance to lysozyme, and genus-specific peptide compositions, as observed in Bifidobacterium. Gram-positive bacteria frequently embed teichoic acids within the peptidoglycan matrix, whereas Gram-negative organisms anchor the layer to the outer membrane via Braun’s lipoprotein.