BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

PART V. MOLECULAR PHYSIOLOGY

Class="center">Interrelation between Information, Conformation, and METABOLISM under Physiological Conditions

Scanning electron micrograph of retinal rod Cells. A single photon is sufficient to stimulate a rod Cell

CHAPTER 32. BACTERIAL CELL WALLS

Unlike animal cells, bacterial cells are surrounded by a Cell wall that provides a defined shape and mechanical support. The Plasma Membrane alone is unable to withstand the high osmotic pressure generated within a bacterial cell due to the high concentration of metabolites, which can reach up to 20 atm. Bacterial cells lacking cell walls undergo lysis in a normal environment.

Bacterial cell walls are of considerable medical interest. In fact, it is precisely The Cell walls and their associated substances that determine bacterial virulence. For instance, the administration of isolated bacterial cell walls to experimental animals successfully reproduces the symptoms of many microbial diseases. Specific bacterial Antigens are localized on the cell walls. By administering an extract of cell walls from certain Bacteria to an animal, Immunity against these bacteria can be induced. Finally, when the Synthesis of cell walls is inhibited, bacteria die. This is the exact MECHANISM OF ACTION of penicillin and several Other Antibiotics.

Fig. 32.1. Electron micrograph of an isolated cell wall of Bacillus licheniformis

For over half a century, bacteria have been classified as Gram-positive or Gram-negative based on their response to the Gram stain. The underlying basis of this empirically discovered difference is now understood. These classes of bacteria differ in the type of cell envelope (Fig. 32.2). The plasma membrane of Gram-positive bacteria is surrounded by a massive cell wall, typically 250 Å thick, composed of peptidoglycan and teichoic acid. In Gram-negative bacteria, the cell envelope has a more complex Structure: the plasma membrane is surrounded by a peptidoglycan cell wall 30 Å thick, which is further enclosed by an outer membrane 80 Å thick, consisting of a mosaic of Proteins, Lipids, and lipopolysaccharides.

Fig. 32.2. Schematic representation of the cell envelopes of Gram-positive (A) and Gram-negative (B) bacteria

32.1. The Cell Wall Is a Giant Saccular Macromolecule

Let us examine the Structure and Biosynthesis of the cell wall of Staphylococcus aureus, a Gram-positive bacterium responsible for purulent infections in Tissues such as the Skin, bones, and Lungs. The macromolecule that forms the cell wall of this bacterium is called peptidoglycan because it consists of peptide and carbohydrate units. In peptidoglycan, linear polysaccharide chains are cross-linked by short Peptides. This Abundance of cross-links results in a single giant saccular macromolecule. When isolated, cell walls retain their original shape (i.e., the shape of the bacterium they enclosed).

The peptidoglycan is composed of three repeating units (Fig. 32.3): 1) a disaccharide of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by a β-1,4-glycosidic bond; 2) a tetrapeptide consisting of L-Alanine, D-glutamine, L-Lysine, and D-alanine; and 3) a pentaglycine peptide bridge. The tetrapeptide is quite unusual in two respects: it contains D-Amino Acids, which are never found in proteins, and, furthermore, its constituent D-glutamine residue forms a peptide bond with the γ-carboxyl group of its own side chain.

Fig. 32.3. The main structural unit of peptidoglycan from Staphylococcus aureus

In the intact proteoglycan, NAG and NAM alternate sequentially to form a linear polysaccharide chain. The pentaglycine peptide links NAM residues belonging to different polysaccharide chains. The amino group of the (Gly)₅ peptide forms a peptide bond with the carboxyl group of D-alanine, whereas the carboxyl group of (Gly)₅ forms a peptide bond with the ε-amino group of the L-lysine side chain.