Peptidoglycan

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Peptidoglycan is found in both Gram-negative and Gram-positive bacteria. It is present in larger amounts in Gram-positive bacteria as it appears as a multimolecular layer and can be found in association with additional compounds. In Gram-negative bacteria, it is found in either 1 or 2 layers between the inner and outer membrane[1].

Peptidoglycan is a macromolecule made up of alternating residues of N-acetyl-β-D-glucosamine (NAG) and N-acetylmuramic acid (NAM) joined together by a glycosidic bond[2]. There is then a pentapeptide attached to the NAM amino acid which forms cross-links with other polysaccharide chains forming a 3D mesh-like layer. These cross-links are formed using the enzyme glycopeptide transpeptidase and provide peptidoglycan with stability. This stability allows it to provide mechanical support and prevent osmotic lysis from occurring in bacteria[3].

Penicillin inhibits the effect of peptidoglycan in bacteria by binding to serine in the enzyme glycopeptide transpeptidase. As a result, it can no longer catalyse the formation of cross-links. The enzyme is irreversibly inhibited which therefore results in the termination of bacterial growth[4].

Penicillin inhibits the transpeptidase linking together the polysaccharide chains and cells become osmotically sensitive and burst.

References

  1. http://www.chem.qmul.ac.uk/iupac/misc/glycp.html#3.9
  2. http://faculty.ccbcmd.edu/courses/bio141/lecguide/unit1/prostruct/cw.html
  3. Berg J., Tymoczko J and Stryer L. (2012) Biochemistry, 7th edition, New York: WH Freeman: pg 252
  4. http://www.ncbi.nlm.nih.gov/books/NBK7986/
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