Peptidoglycan

Peptidoglycan

File:Peptidoglycan.png

Diagram of peptidoglycan structure

Peptidoglycan, also known as murein, is a polymer that forms a protective structural layer in the cell walls of bacteria. It is a crucial component that provides mechanical strength and rigidity to the bacterial cell wall, allowing bacteria to maintain their shape and resist osmotic pressure.

Structure

Peptidoglycan is composed of long chains of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) residues. These chains are cross-linked by short peptides, which provide the structural integrity necessary for the cell wall's function. The peptide cross-links vary among different bacterial species, but they typically include unusual amino acids such as D-alanine and D-glutamic acid.

File:Peptidoglycan layer.png

Cross-section of a bacterial cell wall showing peptidoglycan layer

Function

The primary function of peptidoglycan is to maintain the shape of the bacterial cell and to protect it from osmotic lysis. The rigid structure of peptidoglycan prevents the cell from bursting in hypotonic environments, where water influx could otherwise cause the cell to swell and rupture.

Biosynthesis

Peptidoglycan biosynthesis is a complex process that involves several stages:

1. Synthesis of precursors: NAG and NAM are synthesized in the cytoplasm and linked to a carrier molecule called bactoprenol. 2. Polymerization: The NAG-NAM disaccharide units are transported across the cell membrane and polymerized into long glycan chains. 3. Cross-linking: The glycan chains are cross-linked by transpeptidase enzymes, which form peptide bonds between the peptide side chains of adjacent glycan strands.

Antibiotic Target

Peptidoglycan is a major target for antibiotics such as penicillin and vancomycin. These antibiotics inhibit the enzymes involved in the cross-linking of peptidoglycan strands, weakening the cell wall and leading to cell lysis. The effectiveness of these antibiotics highlights the importance of peptidoglycan in bacterial survival.

Variations in Bacteria

Peptidoglycan structure varies between Gram-positive bacteria and Gram-negative bacteria.

• Gram-positive bacteria have a thick peptidoglycan layer, which is responsible for retaining the crystal violet stain used in the Gram staining procedure.
• Gram-negative bacteria have a thinner peptidoglycan layer located between the inner cytoplasmic membrane and the outer membrane, which contains lipopolysaccharides.

Related pages

• Bacterial cell wall
• Antibiotic resistance
• Gram staining
• Bacterial morphology