The Selective Inhibitory Action of the Antibiotic Tropodithietic Acid

Abstract

Recent stagnation in the development of antibiotics and the spread of resistances among bacteria have prompted greater efforts in the search for novel drugs from traditional actinomycetes sources and from alternative species. At the same time, research into the mode of action of novel drugs must keep up in order to bridge the gap between antibiotic discovery and effective clinical usage. Tropodithietic acid (TDA), a natural product from the roseobacter clade of marine bacteria, is a promising broad spectrum antibiotic that has been shown to collapse the proton motive force of target bacteria but not of roseobacter. A roseobacter TdaR3 protein is key to TDA immunity and is homologous to the widely conserved gene chaC, a putative glutathione-specific γ-glutamylcyclotransferase in the bacterial glutamate acid resistance pathway in Escherichia coli. The results presented here provide elucidation of TDA’s effect on these proteins. We show that both E. coli ChaC and TdaR3 act as γ-glutamylcyclotransferases by lysing glutathione into 5-oxo-proline and a Cys-Gly dipeptide. However, TDA inhibits ChaC more potently than it does TdaR3, with a ChaC Ki of 182 ± 40 μM versus a TdaR3 Ki of 313 ± 59 μM. TDA is shown to lower the Vmax and Km of the E. coli ChaC, indicating an uncompetitive inhibition mechanism. These results suggest that ChaC is a molecular target of TDA and provide an explanation of roseobacter immunity to TDA.