Chloramphenicol acetyltransferase (CAT) is a bacterial enzyme that plays a direct role in antibiotic resistance. This enzyme enables bacteria to overcome the effects of chloramphenicol, an antibiotic once widely used to treat various bacterial infections. By understanding how CAT functions, we gain insight into the sophisticated ways bacteria adapt and survive in the presence of antimicrobial drugs, posing ongoing challenges for public health and treatment strategies.
Understanding Chloramphenicol Acetyltransferase
CAT is a bacterial enzyme responsible for detoxifying the antibiotic chloramphenicol. Chloramphenicol itself is a broad-spectrum antibiotic that was historically used to treat a range of bacterial infections. It works by interfering with a bacterium’s ability to produce proteins, which are essential for its growth and survival.
Bacteria have developed mechanisms to protect themselves from antibiotics as a survival strategy. The evolution of enzymes like CAT allows bacteria to inactivate chloramphenicol. This adaptation provides a selective advantage, enabling resistant bacterial strains to thrive even when exposed to the antibiotic.
The Mechanism of Chloramphenicol Inactivation
CAT inactivates chloramphenicol by catalyzing a specific chemical reaction. The enzyme transfers an acetyl group from a molecule called acetyl-coenzyme A (acetyl-CoA) to the chloramphenicol molecule. This transfer occurs at the 3-hydroxyl group of chloramphenicol.
The acetylation of chloramphenicol modifies its structure. This structural change prevents the antibiotic from effectively binding to the bacterial ribosome, its intended target. Chloramphenicol normally binds to the 23S ribosomal RNA of the 50S ribosomal subunit, inhibiting peptidyl transferase activity and thus blocking protein synthesis.
This allows the bacterial cell to continue synthesizing proteins, thus rendering the antibiotic ineffective. A histidine residue, His195, in the CAT enzyme, plays a direct role in this catalytic mechanism by acting as a general base catalyst. His195 abstracts a proton from the chloramphenicol’s hydroxyl group, initiating a nucleophilic attack on the carbonyl carbon of the acetyl-CoA molecule. The resulting intermediate, 3-O-acetyl-chloramphenicol, is unable to bind to the ribosome.
Structural Insights and CAT Diversity
Chloramphenicol acetyltransferase is structured as a trimer, composed of three identical protein subunits. Each monomer has a molecular weight of approximately 25,000 Daltons. These subunits associate to form a trimeric structure.
The active site where chloramphenicol binds is located in a deep pocket at the boundary between adjacent subunits of the trimer. While most residues forming this binding pocket belong to one subunit, the catalytically active histidine residue belongs to the neighboring subunit. Acetyl-CoA, the other substrate, accesses the active site through a tunnel within the protein.
CAT enzymes are categorized into three main classes: CATI, CATII, and CATIII. Despite variations in their specific sequences and structures, all three classes perform the same 3-O-acetylation of chloramphenicol. For instance, CATI has been found to inactivate not only chloramphenicol but also bind to other antibiotics like fusidic acid. Structural differences among these classes might influence their efficiency or substrate specificity.
The Significance of CAT in Bacterial Resistance
The presence of chloramphenicol acetyltransferase (CAT) in bacteria is a significant factor in the challenge of antibiotic resistance. When bacteria possess the genes encoding CAT, they gain the ability to inactivate chloramphenicol. This allows them to survive treatment with this antibiotic, leading to treatment failures in infected individuals.
The spread of CAT genes among bacterial populations, often carried on mobile genetic elements like plasmids, contributes to the wider problem of multidrug-resistant strains. This resistance mechanism undermines the effectiveness of chloramphenicol, limiting its use in clinical settings. The ongoing prevalence of CAT-mediated resistance highlights the continuous need for new antimicrobial drugs and innovative strategies to overcome bacterial defense mechanisms.