Living Textbook MC610

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Bacterial Cell Wall

Function :

  • A protective external support to the cell, to preserve the protoplast's integrity. Must also allow for semi-permeability of nutrients.
  • Mammalian cells have no cell wall, but Bacterial cells need a cell wall since they have a hypotonic environment in which the cell functions.
  • The mammalian cell is in a controlled environment; very complex processes are taking place. Microorganisms are usually in a hostile environment, may need to endure changes in external osmolarity.
  • In Gram positive bacteria, the osmotic pressure is 20 atmosphere, while in Gram negative bacteria, the osmotic pressure reaches 5 to 6 atmospheres.
  • If no cell wall, Bacterial cells would rupture, leading to cell death.

Composition of cell wall:

It varies from one species to the other. In general, Gram positive bacteria cell wall differs significantly from Gram negative bacteria, due to the difference in how they operate in the host cells, where the former are exacting in their needs, while the latter are more adaptive.

  • Peptidoglycan, which is the backbone that gives it rigidity. It is made up of polysaccharide chain that is cross-linked: a ß1 N-Acetyl Glucosamide (NAG), cross linked with N-Acetyl muramic acid.
  • Hydrophillic components, protrude outside the cell. Always negatively charged, thus allows ion exchange such as Magnesium (important for many membrane-bound enzymes) in and out of the cell.
  • Proteins, form pores that will allow certain nutrients to pass through (usually up to a molecular weight of 600 - 700).

Gram postive bacteria has a cell wall that is 15-50 nm thickness, made up of 50% peptidoglycan and 30-40% teichoic acid (TA), 10% protein, polysaccharide or lipids (linked to TA).

Gram negative bacteria which are more adaptive and can survive on basic nutrients and their cell wall is much more complex. It contains mostly lipopolysaccharides, phospholipids and proteins in the outer layer with no TA, with only 5% or less peptidoglycan, and other components that include glucose, galactose, heptoses and 3-deoxy-D-manno-octulosonic acid (KDO).

 

Cell Wall Biosynthesis

Stage I - Formation of Starting Material:

The cell starts by combining N-Acetylglucosamine-1-P (NAG)and Uridene Triphosphate (UTP) to yield Uridene diphosphate N-Acetylglucosamine (UDPNAG) (step 1) catalyzed by the enzyme UDPNAG Synthase (GlmU) and releasing a diphosphate molecule. UDPNAG is then combined with Phosphoenolyruvate (PEP) in the presence of PEP Transferase (MurA) releasing a phosphate molecule (step 2). The resulting UDPNAG-enolpyruvate is then reduced to Uridine Diphospho-N-Acetyl Muramic Acid (UDPNAM) using the cofactor NADPH and the enzyme UDPNAG-enolpyruvate Reductase (MurB) (step 3). The tripeptide form of the UDPNAM is then formed in three successive steps using L-Alanine, D-Glutamic Acid and L-Lysine respectively, each step requiring its enzymes (MurC, D and E) and cofactors. The tripeptide is then combined with D-Alanyl-D-Alanine in presence of Ligase (MurF) to form the pentapeptide. The D-Alanine-D-Alanine is formed in a side reaction starting with two L-Alanines that are racemated to the D-form via the enzyme Racemase, followed by formation of the dipeptide in presence of the enzyme Synthase.

Stage II - Cell Wall Biosynthesis:

The UDPNAM-pentapeptide is then transferred to the cell membrane utilizing a Translocase enzyme, where it is bound to a Bactoprene molecule to form Diphospho-NAM-pentapeptide-Bactoprene complex (lipid I) and releasing a UMP molecule that is recycled and reused in Stage I. Glycosidation of the complex using N-Acetlyglucosamine-1-phosphate follows to yield Diphospho-NAGNAM-pentapeptide-Bactoprene complex (lipid II) that is then converted to the decapeptide form in presence of 5 Glycines, a step that is catalyzed by tRNA Glycine. Finally the decapeptide is released from the Bactoprene in presence of the enzyme Pyrophosphate that also releases Bactoprene that can be recycled and reused at the start of Stage II. This step involves the polymerization of the chain where two molecules of NAGNAM-decapeptide are combined via a 1,4-glycosidic bond.

Stage III - Cross-Linking:

The final step in the cell wall biosynthesis involves cross linking between two molecules of the decapeptide, where a peptide bond is formed between the terminal Glycine of one molecule and the fourth D-Alanine on the other releasing a molecule of D-Alanine. This step is catalyzed by a group of enzymes known as Penicillin Binding Proteins (PBPs).

There are a number of classes of PBPs:
PBP A and B are the most important, leading to the elongation of the peptidoglycan.  They have a transpeptidase and transglycosidase activity, that if inhibited will cause cell lysis.
PBP C are carboxypeptidases that remove the last D-alanine.