Structure of peptidoglycan
Peptidoglycan is a tough, protective layer located outside the cytoplasmic membrane. In Gram-negative bacteria, it is additionally covered by an outer membrane composed of lipopolysaccharides, which acts as a barrier to many antibiotics.
The molecular backbone consists of long chains formed by alternating amino sugars:
- N-acetylglucosamine (NAG);
- N-acetylmuramic acid (NAM).
A short oligopeptide is attached to each NAM molecule. These peptide tails of adjacent polymer chains are linked together by cross-bridges, forming a rigid three-dimensional mesh.
Stage 1: Monomer synthesis
The process begins in the cytoplasm and on the inner surface of the cell membrane. Glucose is converted into NAG and NAM. Then, a peptide chain is assembled on the NAM molecule with the participation of MurC, MurD, MurE enzymes.
First, L-alanine and D-glutamate are added sequentially. The third amino acid depends on the bacterial type: in most Gram-positive bacteria, it is L-lysine, whereas in Gram-negative bacteria (and some Gram-positive ones), it is diaminopimelic acid (DAP). Since DAP does not occur in the human body, it is an attractive target for novel antibiotics.
Finally, the dipeptide D-Ala-D-Ala is added to the chain, synthesized from L-alanine via the enzyme D-Ala-D-Ala synthetase.
Next, NAG is added to the complex, and a connecting polypeptide — usually a pentaglycine bridge — is attached to the third amino acid (lysine or DAP) in Gram-positive bacteria.
Stage 2: Transport and polymerization
The synthesized hydrophilic murein monomer cannot spontaneously cross the lipophilic cytoplasmic membrane. A carrier molecule, bactoprenol, is used for this purpose.
Bactoprenol binds the monomer, transports it to the outer surface of the membrane, and inserts it into the growing polymer chain. After transglycosylase enzymes release bactoprenol, it returns to the cytoplasm to pick up a new building block.
The process of linear chain elongation (linking the delivered NAM monomer to the NAG of the existing polymer) is catalyzed by the enzyme peptidoglycan glycosyltransferase.
Stage 3: Transpeptidation (cross-linking)
The final stage provides structural strength to the cell wall. Parallel peptidoglycan chains are cross-linked to one another.
This process is catalyzed by transpeptidase enzymes. They bind to the terminal D-Ala-D-Ala dipeptide, cleave off one (terminal) D-alanine residue, and form a temporary complex with the peptidoglycan. Then, the free amino group of an adjacent chain attacks this complex, forming a covalent cross-link.
Transpeptidases are the primary targets for $\beta$-lactam antibiotics (such as benzylpenicillin), which is why these enzymes are known as penicillin-binding proteins (PBPs).
Briefly: answers to topical questions
Which antibiotics inhibit the cytoplasmic stage of peptidoglycan monomer synthesis?
Murein monomer synthesis inhibitors act during the early stages of cell wall synthesis by disrupting the formation of its building blocks in the cytoplasm.
- Fosfomycin (Fosfomycin) — irreversibly blocks the enzymatic condensation of uridine diphosphate N-acetylglucosamine, preventing the formation of murein monomers.
- Fosmidomycin (Fosmidomycin) — belongs to the group of agents that disrupt the early stages of cell wall synthesis.
Which antibiotic blocks monomer transport by disrupting the bactoprenol cycle?
Transport of murein monomers across the membrane is disrupted by bacitracin, a peptide antibiotic that targets the lipid carrier.
- Bacitracin (Bacitracin) — blocks the dephosphorylation (regeneration of activity) of the lipid carrier bactoprenol pyrophosphate, causing it to lose its transport function and halting cell wall synthesis.
Which classes of antibiotics inhibit the transpeptidation stage of cell wall synthesis?
The polymer cross-linking (transpeptidation) stage is inhibited by drugs that bind to penicillin-binding proteins and suppress transpeptidase activity.
- Carbapenems (Carbapenems) — bind to specific penicillin-binding proteins and suppress transpeptidase, disrupting the formation of peptidoglycan cross-links.
How does the cell wall structure of Gram-negative bacteria differ from that of Gram-positive bacteria?
The cell walls of Gram-positive and Gram-negative bacteria differ significantly in the thickness of the peptidoglycan layer and the presence of additional protective structures.
| Sign | Gram-positive | Gram-negative |
|---|---|---|
| Wall thickness | 20-60 nm | 10-20 nm |
| Peptidoglycan as % of dry wall weight | 40-90% | 5-10% |
| Teichoic acids | + | - |
| Periplasmic space | - | + |
| Outer membrane | - | + |
| Gram stain color | blue-purple | pink |
Remember the core composition: "NAM and NAG build the backbone."
Class and exam questions
Which enzymes are called penicillin-binding proteins (PBPs) and why?
These are transpeptidases — enzymes that cross-link peptidoglycan chains. Beta-lactam antibiotics (penicillins) block their function, which is why they are called PBPs.
What is the function of bactoprenol?
It acts as a lipophilic carrier. Monomer components are hydrophilic and cannot independently cross the cell membrane to the outside.
What is the difference in the third amino acid of the peptide chain among various bacteria?
Most Gram-positive bacteria use L-lysine, whereas Gram-negative bacteria use diaminopimelic acid (DAP).
What else to study on this topic
- Mechanism of action of beta-lactam antibiotics
- Structure of the outer membrane in Gram-negative bacteria
- Monomer synthesis enzymes (MurA-MurF)
- The bactoprenol cycle and drugs affecting it
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