Role of the Water−Metal Ion Bridge in Mediating Interactions between Quinolones and Escherichia coli Topoisomerase IV

ABSTRACT: Although quinolones have been in clinical use for decades, the mechanism underlying drug activity and resistance has remained elusive. However, recent studies indicate that clinically relevant quinolones interact with Bacillus anthracis (Gram-positive) topoisomerase IV through a critical water− metal ion bridge and that the most common quinolone resistance mutations decrease drug activity by disrupting this bridge. As a first step toward determining whether the water−metal ion bridge is a general mechanism of quinolone− topoisomerase interaction, we characterized drug interactions with wild-type Escherichia coli (Gram-negative) topoisomerase IV and a series of ParC enzymes with mutations (S80L, S80I, S80F, and E84K) in the predicted bridge-anchoring residues. Results strongly suggest that the water−metal ion bridge is essential for quinolone activity against E. coli topoisomerase IV. Although the bridge represents a common and critical mechanism that underlies broad-spectrum quinolone function, it appears to play different roles in B. anthracis and E. coli topoisomerase IV. The water−metal ion bridge is the most important binding contact of clinically relevant quinolones with the Gram-positiv