Reactivity of nitric oxide with the [4Fe–4S] cluster of dihydroxyacid dehydratase from Escherichia coli

Although the NO (nitric oxide)-mediated modification of iron–sulfur proteins has been well-documented in bacteria and mammalian cells, specific reactivity of NO with iron–sulfur proteins still remains elusive. In the present study, we report the first kinetic characterization of the reaction between NO and iron–sulfur clusters in protein using the Escherichia coli IlvD (dihydroxyacid dehydratase) [4Fe–4S] cluster as an example. Combining a sensitive NO electrode with EPR (electron paramagnetic resonance) spectroscopy and an enzyme activity assay, we demonstrate that NO is rapidly consumed by the IlvD [4Fe–4S] cluster with the concomitant formation of the IlvD-bound DNIC (dinitrosyl–iron complex) and inactivation of the enzyme activity under anaerobic conditions. The rate constant for the initial reaction between NO and the IlvD [4Fe–4S] cluster is estimated to be (7.0±2.0)×10⁶ M⁻²·s⁻¹ at 25 °C, which is approx. 2–3 times faster than that of the NO autoxidation by O₂ in aqueous solution. Addition of GSH failed to prevent the NO-mediated modification of the IlvD [4Fe–4S] cluster regardless of the presence of O₂ in the medium, further suggesting that NO is more reactive with the IlvD [4Fe–4S] cluster than with GSH or O₂. Purified aconitase B [4Fe–4S] cluster from E. coli has an almost identical NO reactivity as the IlvD [4Fe–4S] cluster. However, the reaction between NO and the endonuclease III [4Fe–4S] cluster is relatively slow, apparently because the [4Fe–4S] cluster in endonuclease III is less accessible to solvent than those in IlvD and aconitase B. When E. coli cells containing recombinant IlvD, aconitase B or endonuclease III are exposed to NO using the Silastic tubing NO delivery system under aerobic and anaerobic conditions, the [4Fe–4S] clusters in IlvD and aconitase B, but not in endonuclease III, are efficiently modified forming the protein-bound DNICs, confirming that NO has a higher reactivity with the [4Fe–4S] clusters in IlvD and aconitase B than with O₂ or GSH. The results suggest that the iron–sulfur clusters in proteins such as IlvD and aconitase B may constitute the primary targets of the NO cytotoxicity under both aerobic and anaerobic conditions.