Peptidoglycan is a polymer that forms the cell wall of bacteria, providing structural strength. It is composed of alternating sugars, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), linked to form a mesh-like layer with cross-linking peptide chains. This structure surrounds the cytoplasmic membrane, protecting the cell from environmental stress and preventing it from bursting due to internal pressure. The integrity of this wall is necessary for bacterial survival, as it maintains the cell’s shape.
The synthesis of peptidoglycan involves enzymatic reactions within the cytoplasm and at the cell membrane. Precursor molecules are created inside the cell, transported across the membrane, and assembled on the exterior surface. This continuous construction process allows for cell growth and division, and its mechanism makes it a target for many antibiotics.
The Peptidoglycan Structure
The structure of peptidoglycan is a molecular scaffold built from two components: long glycan chains and short peptide chains that create cross-links. The backbone consists of alternating sugar derivatives, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), forming extended polymers. Attached to each NAM sugar is a stem peptide, a short chain of amino acids that connects adjacent glycan backbones through a process called cross-linking.
This design creates a single molecule surrounding the bacterium, much like a chain-link fence where glycan strands are horizontal wires and peptide cross-links are vertical connectors. This arrangement provides the tensile strength needed to withstand significant internal turgor pressure.
The thickness of the peptidoglycan layer is a defining characteristic used to classify bacteria. In Gram-positive bacteria, the cell wall is thick and composed of many layers of peptidoglycan. Conversely, Gram-negative bacteria possess a much thinner peptidoglycan layer situated between two cell membranes. This structural difference accounts for the varied susceptibility of bacteria to certain antibiotics.
Cytoplasmic Precursor Synthesis
The process of building the peptidoglycan wall begins in the cytoplasm, where its fundamental building blocks are assembled. This initial phase is dedicated to constructing a standardized unit, the UDP-NAM-pentapeptide, which will later be integrated into the growing cell wall.
The synthesis pathway starts with the formation of UDP-N-acetylmuramic acid (UDP-NAM) from a precursor sugar, UDP-N-acetylglucosamine (UDP-GlcNAc). This conversion is carried out by two enzymes, MurA and MurB. Once UDP-NAM is formed, the cell begins the sequential addition of amino acids to its lactyl group, a process managed by a series of enzymes known as Mur ligases.
These Mur ligase enzymes (MurC, MurD, MurE, and MurF) add a specific amino acid or a small peptide in a step-wise fashion, with each addition requiring energy from ATP. The process involves the addition of:
- L-alanine
- D-glutamic acid
- A diamino acid like meso-diaminopimelic acid
- A dipeptide of D-alanine-D-alanine
The result of this cytoplasmic assembly is the precursor molecule UDP-MurNAc-pentapeptide. This molecule contains the NAM sugar and the complete five-amino-acid side chain, ready for transport to the cell membrane.
Membrane Transport and Final Assembly
Once the UDP-NAM-pentapeptide precursor is synthesized in the cytoplasm, it must be moved to the exterior of the cell. This journey begins at the inner surface of the cytoplasmic membrane, where the precursor is transferred to a lipid carrier molecule called bactoprenol. This attachment creates a large, membrane-anchored intermediate, which is then flipped across the membrane to the outer surface. This translocation exposes the building block, making it available for polymerization.
With the precursor now on the outside of the cell, the final assembly can commence. This phase involves two enzymatic reactions. The first is transglycosylation, where the newly delivered NAM-NAG monomer unit is attached to the growing end of an existing glycan chain, lengthening the wall one unit at a time.
The second and final reaction is transpeptidation, which gives the cell wall its strength. This process is catalyzed by Penicillin-Binding Proteins (PBPs), which form peptide cross-links between the amino acid side chains of adjacent glycan strands. The PBPs cleave the terminal D-alanine from one pentapeptide and use the energy from that bond to form a new peptide bond with a neighboring chain.
This cross-linking creates the strong, cohesive mesh that encases the bacterium. The coordinated action of transglycosylation and transpeptidation ensures that new material is seamlessly integrated, allowing the cell to expand its wall during growth or build a new septum during cell division without compromising its integrity.
The Role of Biosynthesis in Antibiotic Action
The peptidoglycan biosynthesis pathway is a primary target for many antibiotics because it is a process unique to bacteria. By interrupting specific steps in this construction sequence, these drugs can weaken the bacterial cell wall. This leads to a loss of structural integrity, causing the bacterium to rupture and die from its own internal pressure, a process known as osmotic lysis.
Different classes of antibiotics interfere with different stages of the synthesis process. The antibiotic bacitracin works by disrupting the transport phase. It interferes with the lipid carrier molecule bactoprenol, preventing it from flipping the peptidoglycan precursors across the cytoplasmic membrane to the exterior of the cell.
The most famous class of antibiotics, the beta-lactams, which includes penicillin, targets the final step of assembly. These drugs are structurally similar to the D-Ala-D-Ala portion of the pentapeptide side chain. They bind to the active site of the Penicillin-Binding Proteins (PBPs), the enzymes responsible for transpeptidation.
This binding inactivates the PBPs, preventing them from forming the peptide cross-links that give the wall its strength. Without these cross-links, the newly synthesized glycan chains are not properly secured, resulting in a fragile cell wall. As the bacterium attempts to grow, the weakened wall cannot withstand the physical stresses, leading to cell lysis.