Peptidoglycan is a unique and widely present component of bacterial cell walls. This complex molecule provides bacteria with structural integrity and shape, acting as a protective outer layer that allows them to withstand internal pressure and environmental challenges. Its presence specifically in bacteria makes it a distinctive and important subject in microbiology.
What Peptidoglycan Is
Peptidoglycan forms a mesh-like layer that surrounds the bacterial cytoplasmic membrane. This polymer is composed of repeating units of two alternating amino sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). These sugar units are linked together by β-(1,4) glycosidic bonds, forming linear glycan strands.
Attached to each N-acetylmuramic acid (NAM) unit is a short peptide chain, typically consisting of three to five amino acids. These peptide chains cross-link with peptide chains from adjacent glycan strands, creating a three-dimensional, interconnected network. This cross-linking provides the rigidity and strength necessary for the bacterial cell wall.
While the fundamental structure is similar, there are differences in the peptidoglycan layers between Gram-positive and Gram-negative bacteria. Gram-positive bacteria possess a significantly thicker peptidoglycan layer, ranging from 20 to 80 nanometers, and can even reach up to 100 nm. In contrast, Gram-negative bacteria have a much thinner peptidoglycan layer, typically only 2 to 8 nanometers thick, which is located between two lipid bilayer membranes.
The Peptidoglycan Synthesis Pathway
The creation of peptidoglycan is a complex, multi-stage process involving numerous enzymes, occurring in different compartments of the bacterial cell. This pathway can be broadly divided into three main stages: the cytoplasmic stage, the membrane stage, and the extracellular stage.
Cytoplasmic Stage
The cytoplasmic stage begins with the synthesis of UDP-N-acetylglucosamine (UDP-GlcNAc) from fructose-6-phosphate. Subsequently, UDP-GlcNAc is converted into UDP-N-acetylmuramic acid (UDP-MurNAc) through the action of MurA and MurB enzymes.
Following the formation of UDP-MurNAc, a pentapeptide chain is sequentially added to it. This process is carried out by a series of ATP-dependent enzymes known as Mur ligases. For instance, MurC adds L-alanine, MurD adds D-glutamic acid, and MurE adds a diamino acid, such as meso-diaminopimelic acid (mDAP) in most Gram-negative bacteria or L-lysine in most Gram-positive bacteria. Finally, MurF adds the dipeptide D-alanyl-D-alanine, completing the UDP-MurNAc-pentapeptide precursor.
Membrane Stage
The membrane stage commences with the transfer of the disaccharide-pentapeptide precursor, UDP-MurNAc-pentapeptide, to a lipid carrier molecule called undecaprenyl phosphate. This transfer, catalyzed by the membrane-bound enzyme MraY, forms Lipid I on the inner side of the cytoplasmic membrane. Undecaprenyl phosphate is an isoprene lipid that acts as a shuttle, transporting sugar intermediates across the plasma membrane.
Lipid I is then modified by the addition of an N-acetylglucosamine (GlcNAc) unit from UDP-GlcNAc, a reaction catalyzed by the membrane-associated glycosyltransferase MurG. This step results in the formation of Lipid II, which is the disaccharide-pentapeptide unit linked to the lipid carrier. Subsequently, a specific flippase, MurJ, translocates Lipid II from the inner side of the membrane to the outer side, making it available for incorporation into the growing peptidoglycan network.
Extracellular (Transpeptidation/Transglycosylation) Stage
The extracellular stage of peptidoglycan synthesis occurs outside the cytoplasmic membrane, where Lipid II units are integrated into the existing cell wall structure. This involves two primary enzymatic activities: transglycosylation and transpeptidation. Transglycosylases link the sugar (glycan) chains of Lipid II units to the growing peptidoglycan polymer. This process elongates the linear polysaccharide strands.
Concurrently, transpeptidases, also known as penicillin-binding proteins (PBPs), form the peptide cross-bridges between adjacent glycan strands. These enzymes catalyze the formation of an amide bond between the peptide chains, providing the necessary three-dimensional cross-linking for cell wall rigidity. PBPs are classified into high molecular weight (HMW) and low molecular weight (LMW) types, with HMW PBPs like PBP1A and PBP1B having both transglycosylase and transpeptidase activities, while Class B HMW PBPs primarily exhibit transpeptidase activity.
Peptidoglycan Synthesis as an Antibiotic Target
Targeting peptidoglycan synthesis is an effective strategy for antibacterial drugs because this structure is unique to bacteria and absent in human cells, minimizing host toxicity. Disrupting this synthesis pathway compromises the structural integrity of the bacterial cell wall, leading to cell lysis and bacterial death.
Beta-lactam antibiotics, a widely used class including penicillins and cephalosporins, inhibit the transpeptidation step of peptidoglycan synthesis. These antibiotics bind to penicillin-binding proteins (PBPs), which are the transpeptidases responsible for forming the peptide cross-links in the bacterial cell wall. By irreversibly binding to PBPs, beta-lactams prevent the cross-linking, resulting in a weakened cell wall that cannot withstand internal osmotic pressure, leading to bacterial cell bursting.
Glycopeptide antibiotics, such as vancomycin, operate by a different mechanism, preventing the incorporation of new units into the growing peptidoglycan chain. Vancomycin binds specifically to the D-alanyl-D-alanine (D-Ala-D-Ala) terminus of the peptidoglycan precursors. This binding physically obstructs the transpeptidases and transglycosylases from accessing their substrates, thereby inhibiting both the cross-linking and elongation of the peptidoglycan strands. The emergence of antibiotic resistance, where bacteria modify the D-Ala-D-Ala target, presents challenges to the effectiveness of these drugs.