Index >> Cellular Components >> Peptidoglycan Biosynthesis

The biosynthesis of this peptidyl chain does not involve ribosomes, m-RNA, etc. The amino acids are added sequentially to UDP-NAMA. The peptide sequence of L-alanine, D-glutamate, a di amino acid (lysine, mesodiaminopimelic acid, ornithine) ends with D-alanine, which is a general feature of most bacterial peptidoglycans.

In the second set of reactions, the UDP-NAMA-pentapeptide is transferred to a lipid carrier (C55 isoprenoid alcohol) in the cell membrane. Subsequently, two other additions are made to the lipid bound NAMA-pentapeptide before polymerization into the peptidoglycan. The first is the addition of NAG from UDP-n-acetyl glucosamine to yield a disaccharide unit and the second is the addition of amino acids to form an interpeptide cross link. The amino acid composition and the number of amino acids it cross link vary with different bacteria. In S.aureus, this consists of a pentaglycine bridge.

It is now known that penicillin binds to the transpeptidase, carboxypeptidase and to a few other proteins known as penicillin binding proteins and prevents their functioning. Penicillin is also involved in the induction of lytic enzyme and cell lysis in the presence of penicillin results due to a mechanical damage caused by the induction of the lytic enzyme. Inhibition of protein synthesis and the induction of the lytic enzyme by antibiotics such as streptomycin can prevent lysis of Gram-positive cells in the presence of penicillin. It is for this reason that today a combination of penicillin and streptomycin is not permitted.