Towards Understanding and Overcoming the Antibiotic Resistance Conferred by Acetyltransferases.

Abstract

Aminoglycoside (AG) antibiotics have been widely applied to the treatment of bacterial infections since the discovery of streptomycin. Having enjoyed over 60 years of clinical success, AGs have encountered problems with bacterial resistance, as do all antibiotics. The covalent chemical modification of the AG structure by AG-modifying enzymes (AMEs) poses a large threat to the future applicability of AGs. Chloramphenicol (CAM), another natural product with excellent antibacterial properties, suffers from a similar resistance problem. The modification of CAM by chloramphenicol acetyltransferase (CAT) renders it inactive. This dissertation focuses on acetyltransferases conferring resistance to antibiotics, discussing progress towards understanding and overcoming a major hurdle in our ability to combat bacterial infections.

Our laboratory reported the unusual regio-versatility of the AG N-acetyltransferase (AAC), Eis, from Mycobacterium tuberculosis (Mtb). We sought to understand the order, number, and regio-specificity of the acetylations carried out by Eis by NMR spectroscopy. We found that Eis not only acetylates multiple positions, but that the positions acetylated and order varies based on the particular AG scaffold. Furthermore, Eis is capable of acetylating amines that have never been reported. We also investigated other anti-TB drugs to determine if it was possible that Eis could cause resistance across drug classes. We found that capreomycin (CAP), a cyclic peptide antibacterial agent, could also be acetylated by Eis.

Using our knowledge of AMEs, we sought to develop novel AGs with improved/maintained activity and the ability to avoid modification by AMEs. A series of molecules were synthesized and tested against numerous bacterial strains. These studies and the knowledge gained regarding Eis will serve as a guide to the development of novel AGs targeting Mtb and other pathogens.

Additionally, we determined the first X-ray crystal structure of CATI with its natural substrate, CAM, bound in the active site, along with a structure of the unbound form of CATI. Comparison to a structure with fusidic acid (FA) bound and CATIII with CAM bound allowed for a deeper understanding of the broader substrate preference of CATI. We hope that the insights provided in our studies may one day aid in the development of novel CAM analogs.