Spectroscopic description of the two nitrosyl-iron complexes responsible for fur inhibition by nitric oxide

Ferric uptake regulation protein (Fur) is a global regulator, ubiquitous in Gram negative bacteria, that acts as a transcriptional repressor when it binds ferrous ion. Fur is involved in responses to several types of stress related to iron metabolism, such as stress induced by nitric oxide (NO) generated by macrophages against bacterial invasion. NO was recently shown to react with Fe2+ ions in FeFur (iron substituted Fur protein) leading to an Fur bound iron−nitrosyl complex, unable to bind DNA, and characterized by a g = 2.03 EPR signal, associated with an S = 1/2 ground state. This electronic configuration could arise from either a mononitrosyl−iron {Fe(NO)}7 or a dinitrosyl−iron {Fe(NO)2}9 complex. The use of several spectroscopic tools such as EPR, ENDOR, FTIR, Mossbauer, and UV−visible spectroscopies as well as mass spectrometry analysis was necessary to characterize the iron−nitrosyl species in Fur. Furthermore, changes of C132 and C137 into glycines by site directed mutagenesis reveal that neither...