BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

PART V. MOLECULAR PHYSIOLOGY

CHAPTER 32. BACTERIAL CELL ENVELOPES

Summary

Bacterial Cells are encased in Cell walls that protect them against osmotic lysis. The Cell wall of Gram-positive Bacteria is typically 250 Å thick and consists of peptidoglycan and teichoic acid, whereas The Cell surface of Gram-negative bacteria has a more complex architecture. The cell wall Functions as a single giant, sac-like macromolecule. The peptidoglycan of S. aureus comprises three repeating units: NAG-NAM, a tetrapeptide composed of D- and L-residues, and a pentaglycine chain. Pentaglycine bridges cross-link the individual polysaccharide chains. The Biosynthesis of peptidoglycans utilizes activated sugars in the form of UDP-NAG and UDP-NAM. This biosynthetic pathway proceeds in five stages:

1) a peptide unit is assembled onto the NAM residue attached to UDP;

2) the NAM-peptide is transferred to a lipid carrier;

3) NAG and the pentaglycine bridge are attached to the NAM-peptide unit;

4) the NAG-NAM-peptide unit is transferred from the lipid carrier to the growing polysaccharide chain;

5) the amino group of the pentaglycine bridge on one polysaccharide chain attacks the terminal D-Ala-D-Ala peptide bond in a tetrapeptide belonging to another chain, thereby forming a cross-link between the polysaccharide chains.

Penicillin kills bacteria by inhibiting cell wall synthesis. Structurally resembling acyl-D-Ala-D-Ala, penicillin features a highly reactive peptide bond. It irreversibly inhibits glycopeptide transpeptidase, thereby blocking cross-linking in the peptidoglycan layer. Some bacteria are resistant to penicillin due to the presence of penicillinase, an enzyme that hydrolyzes and thus inactivates the antibiotic.

Unlike Gram-positive bacteria, Gram-negative bacteria possess an outer membrane situated over the peptidoglycan layer. Furthermore, a periplasmic space is located between The Plasma Membrane and the peptidoglycan layer in Gram-negative cells. The outer membrane is composed of highly distinctive lipopolysaccharides. These amphipathic compounds contain lipid A, anchored in the outer leaflet of the outer membrane, as well as outwardly projecting Oligosaccharides and O-antigen side chains.

The O-antigen side chains, which incorporate unusual sugars, are synthesized by the polymerization of tetrasaccharide units on a lipid carrier. The structural Water/126.html">Diversity of the O-antigen side chains enables Gram-negative bacteria to evade the host's immune defense. Small polar (but not hydrophobic) molecules diffuse through water-filled channels in the outer membrane; these channels are formed by transmembrane Proteins known as porins. Larger molecules, such as maltose or vitamin B₁₂, cross the membrane via specific transport systems. A major protein component of the outer membrane is a small lipoprotein that anchors the membrane to the underlying peptidoglycan layer. Cell envelope proteins are synthesized on Ribosomes attached to the plasma membrane. The precursors of these proteins contain N-terminal signal sequences that are cleaved off after the precursor protein is translocated across the membrane.