|MadSci Network: Biochemistry|
Penicillin and related antibiotics are indeed irreversible inhibitors of a key enzyme in cell wall synthesis in bacteria, in particular Gram-positive bacteria.
The cell wall in Gram positive bacteria is made up of linear polysaccharides strands containing alternating residues of two sugars, N-acetylglucosamine and N-acetylmuramic acid. To give the cell wall the necessary strength the adjacent polysaccharide strands must be linked together, "cross-linked", with short peptide chains.
The peptide chains contain segments of repeating D-alanine residues, and are attached to the polysaccharides by an enzyme called transpeptidase. Penicillin inhibits correct cell wall synthesis by inhibiting the transpeptidase. Penicillin belongs to a class of antibiotics called beta-lactams. The structure of penicillin looks very much like the structure of D-Alanine-D-Alanine, and binds readily to the active site of the transpeptidase. The enzyme forms a covalent bond bewteen the penicillin and an active site residue (serine), and the enzyme is irreversibly inhibited.
Because we do not have cell walls like these bacteria, penicillin is generally not toxic to us at levels that are effective in combating bacterial infections. However, some people do develop allergies to penicillin and related beta-lactam antibiotics. These allergies can sometimes lead to severe allergic shock reactions.
You can read more about penicillin at: http://www.elmhurst.edu/ ~chm/vchembook/652penicillin.html and: http://www.chemsoc.org/exemplarchem/entries/2002/stanley/01_Contents/ Contents.htm
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