aminoglycoside acetyltransferase

aminoglycoside acetyltransferase
Name	aminoglycoside acetyltransferase
EC number	2.3.1.81
CAS number	9076-58-8

aminoglycoside acetyltransferase. These enzymes are a primary mechanism of bacterial resistance to aminoglycoside antibiotics, critically undermining the efficacy of these important drugs. They function by transferring an acetyl group from acetyl-CoA to specific amino groups on the aminoglycoside molecule, thereby neutralizing its antibacterial activity. The widespread dissemination of these enzymes among pathogenic bacteria represents a major challenge in infectious disease management and antimicrobial stewardship.

Structure and classification

Aminoglycoside acetyltransferases are typically monomeric proteins that belong to the broader GCN5-related N-acetyltransferase (GNAT) superfamily. Their three-dimensional structure commonly features a characteristic β-sheet core flanked by α-helices, which forms the binding pocket for both acetyl-CoA and the incoming aminoglycoside substrate. Based on their regioselectivity—the specific position on the aminoglycoside they modify—they are systematically classified into four major types: AAC(1), AAC(2'), AAC(3), and AAC(6'). This classification system, maintained by experts at institutions like the Pasteur Institute, is crucial for tracking resistance patterns. The AAC(6')-Ib variant, in particular, is one of the most frequently encountered enzymes in clinical isolates from hospitals worldwide.

Mechanism of action

The catalytic mechanism involves a conserved glutamate or aspartate residue that acts as a general base, deprotonating the target amino group on the aminoglycoside to enhance its nucleophilicity. This activated group then performs a nucleophilic attack on the carbonyl carbon of the acetyl moiety bound to coenzyme A. The reaction results in the formation of an acetylated, inactive aminoglycoside and releases CoA-SH. This acetylation sterically hinders the drug's binding to its primary target, the bacterial 16S ribosomal RNA within the 30S ribosomal subunit, effectively blocking protein synthesis. The efficiency of this transfer is a key factor in the level of resistance conferred to bacteria like Pseudomonas aeruginosa and Acinetobacter baumannii.

Substrate specificity and resistance profiles

Substrate specificity varies dramatically between enzyme classes, directly determining the spectrum of aminoglycosides they can inactivate. For instance, AAC(3) enzymes commonly modify gentamicin and tobramycin, while many AAC(6') enzymes confer resistance to amikacin and tobramycin. Some bifunctional enzymes, such as AAC(6')-Ie-APH(2'')-Ia, combine acetylation with phosphorylation activity, providing high-level resistance to almost all clinically available aminoglycosides. This broad-spectrum resistance is especially problematic in pathogens like Enterococcus faecium and Staphylococcus aureus. Surveillance networks, including those coordinated by the World Health Organization and the Centers for Disease Control and Prevention, monitor these profiles to inform treatment guidelines.

Genetic basis and distribution

The genes encoding these enzymes, such as aacA, aacB, and aacC, are often located on mobile genetic elements like plasmids, transposons, and integrons. This genetic mobility facilitates rapid horizontal gene transfer between different bacterial species, even across genera. The widespread use of aminoglycosides in human medicine and agriculture has exerted strong selective pressure, leading to the global dissemination of these resistance determinants. Notable outbreaks have been linked to the spread of specific genes, such as aac(6')-Ib-cr, which also confers resistance to fluoroquinolones, in hospitals from New York City to Shanghai.

Clinical significance

The presence of aminoglycoside acetyltransferases renders standard aminoglycoside therapy ineffective, forcing clinicians to resort to alternative, often more toxic or expensive, antimicrobial agents. This directly contributes to increased morbidity, mortality, and healthcare costs. These enzymes are particularly concerning in treating infections caused by multidrug-resistant Gram-negative bacilli in settings like intensive care units and among immunocompromised patients. Their detection often requires specialized molecular diagnostics, such as PCR assays, to guide appropriate antibiotic selection, a practice emphasized by societies like the Infectious Diseases Society of America.

Inhibition and therapeutic strategies

Research into inhibiting these enzymes focuses on designing molecules that compete with either the aminoglycoside or acetyl-CoA for binding to the active site. Some approaches involve developing novel aminoglycoside derivatives that evade acetylation, such as plazomicin, which was approved by the U.S. Food and Drug Administration. Another strategy explores the use of adjuvant compounds that, when co-administered with an aminoglycoside, inhibit the acetyltransferase and restore the drug's activity. Ongoing efforts by pharmaceutical companies and academic consortia, including those funded by the National Institutes of Health, aim to outpace the evolution of resistance through these innovative therapeutic strategies.

Category:Enzymes Category:Antibiotic resistance