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Structure of Peptidoglycan
Peptidoglycan is a vital structural component of the bacterial cell wall, providing mechanical strength and shape to the cell. It consists of repeating units of two sugars—N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)—linked by β-1,4 glycosidic bonds. These sugar chains are cross-linked by short peptide chains, forming a mesh-like polymer that surrounds the bacterial plasma membrane.
Cytoplasmic Phase – Precursor Synthesis
Peptidoglycan biosynthesis begins in the cytoplasm with the formation of soluble precursors. In this stage, uridine diphosphate (UDP), an ester of pyrophosphoric acid, binds to NAG and NAM to form UDP-NAG and UDP-NAM, respectively. UDP-NAG is then enzymatically converted into UDP-NAM. A pentapeptide chain—typically composed of L-alanine, D-glutamic acid, meso-diaminopimelic acid (or L-lysine), and two D-alanine residues—is sequentially added to UDP-NAM, forming UDP-NAM-pentapeptide. This compound serves as the foundational building block for later stages of peptidoglycan synthesis.
Membrane-Associated Phase – Lipid Carrier Transfer
In the membrane-associated phase, the UDP-NAM-pentapeptide is transferred to bactoprenol phosphate, a lipid carrier embedded in the inner leaflet of the plasma membrane. This transfer results in the formation of Lipid I. Next, a NAG unit is added to Lipid I, generating Lipid II, which now contains the complete disaccharide-pentapeptide structure. A flippase enzyme then translocates Lipid II across the membrane, delivering the precursor to the periplasmic side for incorporation into the growing cell wall.
Periplasmic Phase – Polymerization and Cross-Linking
In the final phase, located in the periplasm, glycosyltransferases incorporate the disaccharide-pentapeptide units into the existing peptidoglycan mesh by catalyzing the formation of glycosidic bonds. The last step, known as transpeptidation, is carried out by transpeptidase enzymes. This reaction releases the subunit from bactoprenol and forms peptide cross-links between adjacent stem peptides, reinforcing the structural integrity of the peptidoglycan layer and completing the biosynthesis process.
