Peptidoglycan

Peptidoglycan, also known as murein, is a large, mesh-like macromolecule composed of sugars and amino acids that forms an essential structural layer of the bacterial cell wall. Found in nearly all members of the domain Bacteria, this polymer envelops the cytoplasmic membrane and protects cells from osmotic lysis while preserving characteristic shapes such as rods and cocci. Its continuous synthesis and turnover are crucial for bacterial growth, division, environmental resilience, and taxonomic differentiation.

Structural Organisation

Peptidoglycan consists of alternating residues of the amino sugars N-acetylglucosamine (NAG or GlcNAc) and N-acetylmuramic acid (NAM or MurNAc) linked by β(1→4) glycosidic bonds. Attached to each NAM unit is a short peptide chain, typically a tetrapeptide or pentapeptide, whose precise amino acid composition varies with bacterial species. These peptides include several characteristic D-amino acids, which are rare in most biological macromolecules.
Representative peptide structures include:

• Gram-negative bacteria (e.g., Escherichia coli): L-alanine, D-glutamic acid, meso-diaminopimelic acid, and D-alanine.
• Gram-positive bacteria (e.g., Staphylococcus aureus): L-alanine, D-glutamine, L-lysine, and D-alanine, with an additional five-glycine interbridge linking peptide chains.

Peptidoglycan strands are crosslinked through the activity of DD-transpeptidase enzymes, creating a robust three-dimensional lattice. This extensive crosslinking confers rigidity, defines cellular morphology, and counteracts cytoplasmic turgor pressure.

Gram-Positive and Gram-Negative Architectures

The thickness of the peptidoglycan layer is a major determinant of Gram staining behaviour:

• Gram-positive bacteria possess a thick peptidoglycan layer (20–80 nm), accounting for up to 40–90% of the cell wall’s dry mass. This substantial layer also contributes to attachment functions and serotyping characteristics.
• Gram-negative bacteria have a much thinner layer (7–8 nm), representing about 10% of the cell wall mass, situated between the inner membrane and an outer membrane.

Despite differences in thickness, the peptidoglycan meshwork is porous enough to permit the passage of particles approximately 2 nm in diameter. Because morphology alone is insufficient to distinguish Gram types, Gram staining, introduced by Hans Christian Gram in 1884, remains the standard diagnostic method: Gram-positive cells stain purple, whereas Gram-negative cells stain pink.

Functional Roles

Peptidoglycan is indispensable for:

• Maintaining cell shape, as new wall material added during growth preserves the overall morphology.
• Preventing osmotic lysis, acting as a mechanical counterbalance to internal turgor pressure.
• Facilitating binary fission, where coordinated synthesis and hydrolysis enable septum formation.
• Protecting against environmental stress, including mechanical damage and osmotic variance.

During growth and division, cells must dynamically remodel the cell wall through enzymatic clipping, insertion of new material, and recrosslinking, maintaining structural integrity throughout.
Organisms lacking peptidoglycan—such as mycoplasmas and L-form bacteria—do not undergo binary fission but instead proliferate through budding or other alternative mechanisms.

Evolutionary Significance

The emergence of rigid peptidoglycan walls was likely a pivotal evolutionary innovation, enabling early bacteria to withstand environmental challenges and colonise diverse habitats. This structural development allowed the bacterial lineage to undergo extensive radiation across the geosphere and hydrosphere.

Biosynthesis

Peptidoglycan synthesis is a multistage process involving cytosolic precursor formation, membrane-associated transformations, and extracellular polymerisation:
Stage One: Cytosolic Precursor Synthesis

1. Glutamine donates an amino group to fructose-6-phosphate, forming glucosamine-6-phosphate (GlmS-catalysed).
2. Acetylation produces N-acetylglucosamine-6-phosphate (GlmM).
3. Isomerisation yields N-acetylglucosamine-1-phosphate (GlmU).
4. Uridylation forms UDP-N-acetylglucosamine (UDP-GlcNAc), the precursor for NAG.
5. Conversion of UDP-GlcNAc to UDP-MurNAc (MurA and MurB).
6. Sequential addition of amino acids generates UDP-MurNAc-pentapeptide (MurC, MurD, MurE), requiring ATP.

Stage Two: Membrane-Associated Steps

1. Transfer of UDP-MurNAc-penta onto undecaprenyl phosphate (bactoprenol) creates lipid I (MraY).
2. Addition of GlcNAc yields lipid II, a disaccharide-pentapeptide unit (MurG).
3. MurJ flippase translocates lipid II across the cytoplasmic membrane.

Stage Three: Polymerisation and Crosslinking

1. Transglycosylation, catalysed by glycosyltransferase (GTase), elongates the glycan chain by linking new disaccharide units.
2. Transpeptidation, catalysed by DD-transpeptidase (penicillin-binding proteins), crosslinks peptide side chains. Certain PBPs possess both GTase and TPase activities.

These biosynthetic steps are key targets of many antibacterial agents, including β-lactams, which inhibit transpeptidation.

Pseudopeptidoglycan

Some Archaea, particularly members of the Methanobacteriales and Methanopyrus, synthesise pseudopeptidoglycan (pseudomurein). This polymer consists of N-acetylglucosamine and N-acetyltalosaminuronic acid joined by β(1→3) linkages, rendering the structure resistant to lysozyme. Although functionally analogous to bacterial peptidoglycan, its distinct chemistry reflects independent evolutionary origins.

Recognition by the Immune System

Peptidoglycan is a major pathogen-associated molecular pattern (PAMP). Recognition mechanisms are evolutionarily conserved among animals and include pattern-recognition receptors such as:

• NOD-like receptors
• Peptidoglycan recognition proteins (PGRPs)