Conformational Differences Observed Between the Apo And Nucleotide Bound Form of an Enzyme Responsible for Aminoglycoside Antibiotic Resistance. A. Leung, W.-C. Hon, G.D. Wright, and A.M. Berghuis, McMaster University, Hamilton, Ontario, Canada

Antibiotic resistance is a growing clinical problem as bacterial infections become more difficult to treat. The continual emergence of antibiotic resistant bacterial strains has urged scientists to search for novel and more potent drugs. A detailed understanding of the mechanism of antibiotic resistance will provide valuable information for future development of new compounds that can combat antibiotic resistance.

Aminoglycosides (e.g. gentamicin, amikacin, streptomycin) belong to a class of bactericidal antibiotics widely used in clinical settings. Resistance to this type of antibiotics is achieved through chemical modification which causes the aminoglycoside to lose its affinity for its target. Three classes of aminoglycoside modifying enzymes are found in pathogenic bacteria: O-phosphotransferases, N-acetyltransferases and O-adenyltransferases. We have successfully solved the three-dimensional structure of one of the aminoglycoside phosphorylating enzymes (APH), APH(3')-IIIa. This enzyme transfers the (-phosphate group from ATP to the 3' and/or 5' hydroxyl of a wide range of aminoglycosides. The enzyme with an ADP molecule bound to its active site was solved to 2.2 Å by multiwavelength anomalous dispersion method. The apoenzyme was solved to 3.2 Å by molecular replacement. The comparison of the conformational differences observed between the apo and nucleotide bound form of the enzyme will be presented.