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Chloramphenicol exerts its antibiotic effect by binding to the ribosomal peptidyl transferase in bacteria and thus preventing protein biosynthesis. A lot of bacteria (also clinically relevant ones) are resistant to chloramphenicol. The resistance arises from a covalent modification of the antibiotic. The enzyme chloramphenicol acetyl transferase (E.C.2.3.1.28) transfers an acetyl group from acetyl coenzyme A to the primary hydroxyl on C6 of chloramphenicol. The modified antibiotic no longer binds to ribosomes.

In monoacetylated chloramphenicol the acetyl group may shift non-enzymatically (dependent on pH) to the C7-OH group. This isomer may again be acetylated by the transferase on C6.

	Acetyl-CoA     C N O P S

Often chloramphenicol acetyl transferase is coded by plasmids. One of these is a type III enzyme coded by plasmid R387 from Escherichia coli. It consists of 213 amino acids (which for consensus reasons are numbered 6-78 and 80-219) and has a relative molar mass of 25 000.

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