An Overiew of Peptidoglycan Stucture
Peptidoglycan is also commonly referred as the cell wall. Bacteria are divided into two main categories depending on composition of their cell wall. Cells can be Gram positive, characterized by a thick layer of peptidoglycan and lack of an outer membrane or cells can be Gram negative. Gram negative cells have a thin layer of peptidoglycan between an inner and outer cell membrane. E. coli are Gram negative. Peptidoglycan is composed of repeating units of N-acetylglucosamine (NAG)-N-acetylmuramic acid (NAM) pentapeptide. Peptidoglycan strands are crosslinked through their pentapeptide chains, which gives the structure its flexibility and strength.
The Three Main Reaction Areas of Peptidoglyan Biosynthesis
The synthesis of peptidogylan is highly regulated and takes place in three different places in the cell.
- Cytoplasmic Reactions: The goal of the cytoplasmic reactions is to form the repeating NAM-NAG pentapeptide unit and to attach this unit to a transmembrane carrier. The transmembrane carrier is known as bactoprenol or undecaprenyl.
- Membrane Reactions: Very few specifics are known about these reactions. Bactoprenol is responsible for flipping the repeating monomer units from the cytoplasm to the periplasm. Bactoprenol remains in the membrane at all times.
- Periplasmic Space Reactions: The primary goal of the periplasmic space reactions is to catalyze the formation of glycan chains, cross-link peptide subunits, and bind newly made material to the exisiting cell wall.
Genes and Gene Products Involved in Peptidoglycan Biosynthesis
The genes encoding enzymes involved in peptidoglycan biosynthesis are found in five clusters on the E. coli chromosome: mra, mrb, mrc, mrd, and mre. mr stands for murein biosynthetic gene cluster.
Control of cell shape and width remains elusive. This is because peptidoglycan biosynthesis is a complex process in which growth information must be translated into size of peptidoglycan. There are three proposed hypotheses for how this translation occurs:
- Change in growth rate is sensed as a change in cytoplasmic turgor pressure
- Concentrations of a second messenger or signaling molecules
- Recognition by cytoskelton
Antibiotics and Inhibition of Cell Wall Biosynthesis
The mechanism of action of many common antibiotics is to interfere with some reactions of peptidoglycan biosynthesis. Listed below are examples:
Bacitracin: interferes bactoprenol returning to the cytoplasmic side of the membrane.
Vancomycin: Prevents cross-link of glycan chains.