Sandbox 156: Difference between revisions
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Found in bacteria, the CAT III enzyme is responsible for conferring resistance of the antibiotic chloramphenicol to the cell. Chloramphenicol arrests protein synthesis by binding to the bacterial ribosomes and causing the inhibition of | Found in bacteria, the CAT III enzyme is responsible for conferring resistance of the antibiotic chloramphenicol to the cell. Chloramphenicol arrests protein synthesis by binding to the bacterial ribosomes and causing the inhibition of | ||
[http://en.wikipedia.org/wiki/Peptidyl_transferase peptidyl transferase] activity<ref name= | [http://en.wikipedia.org/wiki/Peptidyl_transferase peptidyl transferase] activity<ref name=”Day”>PMID: 1544895</ref>. However, when CAT III catalyzes the acetylation of chloramphenicol, the antibiotic can no longer bind to the ribosomes and is rendered inactive. The genes for the enzyme are commonly found on the plasmid of the bacteria and have been found in a numerous bacterial species<ref>PMID: 2268277</ref>. | ||
The multifunctional enzyme consists of three identical subunits with three active sites at the subunit interfaces. The side chains of one subunit allow [http://en.wikipedia.org/wiki/Van_der_Waals_force van der Waals interactions] and two hydrogen bonds with chloramphenicol, causing binding of the substrate. The opposing subunit provides a histidine (His-195) residue essential for catalysis<ref>PMID: 8407936</ref>. Water molecules in the cavity provide a bridging hydrogen bond between the 1-hydroxyl of chloramphenicol and the hydroxyl of a threonine (Thr-174) residue. The active site of CAT III performs two acetylations of chloramphenicol and can accommodate the presence of the first intermediates quite well<ref name= | The multifunctional enzyme consists of three identical subunits with three active sites at the subunit interfaces. The side chains of one subunit allow [http://en.wikipedia.org/wiki/Van_der_Waals_force van der Waals interactions] and two hydrogen bonds with chloramphenicol, causing binding of the substrate. The opposing subunit provides a histidine (His-195) residue essential for catalysis<ref>PMID: 8407936</ref>. Water molecules in the cavity provide a bridging hydrogen bond between the 1-hydroxyl of chloramphenicol and the hydroxyl of a threonine (Thr-174) residue. The active site of CAT III performs two acetylations of chloramphenicol and can accommodate the presence of the first intermediates quite well<ref name=”Murray”>PMID:2015231</ref>. | ||
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[[Image:Picture2.jpg|500x400 px|center]] | [[Image:Picture2.jpg|500x400 px|center]] | ||
In the first step of the reaction, Histidine-195 abstracts a proton from the 3-hydroxyl of chloramphenicol, promoting a [http://en.wikipedia.org/wiki/Nucleophilic_attack nucleophilic attack] from the [http://en.wikipedia.org/wiki/Oxyanion oxyanion] to the thioester bond of the acetyl-CoA. The intermediate produced, 3-acetylchloramphenicol, then rearranges non-enzymatically to 1-acetylchloramphenicol. Regeneration of the 3-hydroxyl allows another round of CAT III catalyzed nucleophilic attack and a 1,3-diacetylchloramphenicol product is formed<ref name= | In the first step of the reaction, Histidine-195 abstracts a proton from the 3-hydroxyl of chloramphenicol, promoting a [http://en.wikipedia.org/wiki/Nucleophilic_attack nucleophilic attack] from the [http://en.wikipedia.org/wiki/Oxyanion oxyanion] to the thioester bond of the acetyl-CoA. The intermediate produced, 3-acetylchloramphenicol, then rearranges non-enzymatically to 1-acetylchloramphenicol. Regeneration of the 3-hydroxyl allows another round of CAT III catalyzed nucleophilic attack and a 1,3-diacetylchloramphenicol product is formed<ref name="Murray"/>. | ||
==Structure== | ==Structure== | ||
