Reactivity of reduced [2Fe-2S] ferredoxins parallels host susceptibility to nitroimidazoles

Abstract

The kinetics of the electron transfer reaction between reduced [2Fe-2S] ferredoxins and select nitroimidazole antimicrobial agents is reported. The ferredoxins from the protozoan Trichomonas vaginalis and the cyanobacterium Anabaena sp. strain 7120 were studied because they are the proximal electron donors to nitroimidazoles in these two organisms with significantly different nitroimidazole susceptibilities. The rates of electron transfer from Anabaena ferredoxin to all nitroimidazoles were 1 to 2 orders of magnitude lower than for T. vaginalis ferredoxin. Quantitative structure-activity analysis of the kinetic data showed that the size of the alkyl substituent on the N-1 position of the imidazole ring strongly influenced the magnitude of the electron transfer rate constant. This implies that the distance between the iron-sulfur cluster and the nitro group of the imidazole is the critical variable in determining the rate of electron transfer. A correlation between the magnitude of the one-electron transfer rate constant with the susceptibility of the host organism to the cytotoxic effects of nitroimidazoles was also discovered. These results demonstrate that reductive activation is the most crucial step in determining the toxicity of nitroimidazoles.

FIG. 1.

Concentration dependence of the pseudo-first-order rate constant for ferredoxin oxidation (k_obs) on nitroimidazole concentration. Data points for oxidation by compound 2 of Anabaena ferredoxin (□) and T. vaginalis ferredoxin (⋄) and by compound 4 of Anabaena (▵) and T. vaginalis ferredoxin (○) were obtained from the average of five experiments. Standard errors of the mean are shown as vertical bars. Fits to concentration dependences were obtained by nonlinear least-squares fits.

FIG. 2.

Marcus plot of logarithm of the second-order rate constant for oxidation of Anabaena (□) or T. vaginalis ferredoxin (○) against the reduction potential of the nitroimidazole. Data points are the average of five experiments. Standard errors of the mean are shown as vertical bars.

FIG. 3.

Plot of the logarithm of the second-order rate constant for oxidation of Anabaena (□) or T. vaginalis ferredoxin (○) against the partition coefficient K of the nitroimidazole (x axis). Data points are the average of five experiments. Standard errors of the mean are shown as vertical bars.

FIG. 4.

Plot of the logarithm of the second-order rate constant for oxidation of Anabaena (□) or T. vaginalis ferredoxin (○) against the value of the Taft steric parameter E_s of the N-1 substituent of the nitroimidazole. Data points are the average of five experiments. Standard errors of the mean are shown as vertical bars.

FIG. 5.

Surface of T. vaginalis ferredoxin showing cavity that exposes the bridging sulfur atom to solvent (arrow). Surface map was calculated using the VMD program by rolling a probe with a 1.4-Å radius over the atoms of the protein.