Antiobiotics exploit the special chemistry of bacteria

We have repeatedly emphasized that all life has very similar chemistry. From the biochemical point of view the most important division is that separating prokaryotes from eukaryotes. Prokaryotes, which include bacteria, evolved fi rst and have simple cells with no nucleus. Eukaryotes, which include plants, mammals, and all other multicellular creatures, evolved later and have more complex cells, including nuclei. Even then, much of the biochemistry on both sides of the divide is the same. When medicinal chemists are looking for ways to attack bacteria, one approach is to inter fere with chemistry carried out by prokaryotes but not by us. The most famous of these attacks is aimed at the construction of the cell walls of some bacteria that contain ‘unnatural’ (R)- (or D-) amino acids. Bacterial cell walls are made from glycopeptides of an unusual kind. Polysaccharide chains are cross-linked with short peptides containing (R)-alanine (D-Ala). Before they are linked up, one chain ends with a glycine molecule and the other with D-Ala D-Ala. In the final step in the cell wall synthesis, the glycine attacks the D-Ala–D-Ala sequence to form a new peptide bond by displacing one D-Ala residue.

The antibiotic penicillin works by interfering with this step—although this was not even suspected when penicillin was discovered. Penicillin inhibits the enzyme that catalyses the D-Ala transfer in a very specifi c way. It fi rst binds specifi cally to the enzyme (so it must be a mimic of the natural substrate) and it then reacts with the enzyme and inactivates it by block ing a vital OH group at the active site. If we emphasize the peptide nature of penicillin and compare it with D-Ala–D-Ala, the mimicry may become clearer.

Penicillin imitates D-Ala and binds to the active site of the enzyme, encouraging the OH group of a serine residue to attack the reactive strained β-lactam. This same OH group of the same serine residue would normally be the catalyst for the D-Ala–D-Ala cleavage used in the building of the bacterial cell wall. The reaction with penicillin ‘protects’ the serine and irreversibly inhibits the enzyme. The bacterial cell walls cannot be completed, and the bacterial cells literally burst under the pressure of their contents. Penicillin does not kill bacteria whose cell walls are already complete but it does prevent new bacteria being formed.

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متلازمة المبنى المريض.. هكذا يتحول تكييف الهواء إلى خطر صحي ؟؟

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كيف تتعامل مع حرارة الطقس أثناء القيادة؟

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المزيد

مواضيع ذات صلة

Lipids

Most sugars are embedded in complex carbohydrates

Glycosides in nature

Sugars can be fixed in one shape by acetal formation

Sugars normally exist in cyclic forms with much stereochemistry

Amino acids combine to form peptides and proteins

Proteins are made of amino acids

Cyclic nucleosides and stereochemistry

HIV and AIDS are treated with modifi ed nucleosides Modified nucleosides

The stimulants in tea and coffee are methylated purines

Life begins with nucleic acids

Aldol reactions catalysed by proline