Biochemistry - Bacterial Cell Walls
Most prokaryotes have a hard cell wall that is 3–25 nm thick to shield it from osmotic pressure and mechanical damage. Peptidoglycan, a combination of oligosaccharides and proteins, makes up the cell wall. The linear chains of alternate N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (NAM) connected b1-4 make up the oligosaccharide component. A tetrapeptide containing d-amino acids is linked to the lactic acid group on NAM by an amide bond. Other short peptides form covalent cross-links between adjacent parallel peptidoglycan chains via the tetrapeptide side-chains. The peptidoglycan cell wall's strength and rigidity are a result of the considerable cross-linking in the structure. A unique target for the action of some antibiotics, such as penicillin, is provided by the presence of d-amino acids in the peptidoglycan, which renders the cell wall resistant to the activity of proteases, which act on the more frequently present l-amino acids. Penicillin weakens the cell wall by blocking the enzyme that creates the covalent cross-links in the peptidoglycan. The lysozyme enzyme, found in tears, mucus, and other bodily secretions, can hydrolyze the b1-4 glycosidic bond between NAM and GlcNAc. Depending on whether they accept the Gram stain, bacteria can be categorised as either Gram-positive or Gram-negative. Gram-positive bacteria, like Bacillus polymyxa, have a cell wall that surrounds their plasma membrane that is thick (25 nm), whereas Gram-negative bacteria, like E. coli, have a cell wall that is thinner (3 nm) and have a second outer membrane. Due to porin proteins, which create pores in the lipid bilayer, this outer membrane is much more permeable to the passage of relatively large molecules (molecular weight >1000 Da) than the plasma membrane. The periplasm, which is a region where the cell's secreted proteins are located, lies between the outer membrane and the cell wall.
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November 2022
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