Impact of Nitric Oxide (NO) Treatment on the Spectroscopic Properties and DNA Binding Activity of the Carbon Monoxide-Sensing Heme Transcription Factor, CooA

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CooA is a carbon monoxide (CO)-sensing heme transcription factor that regulates CO metabolism in several bacteria including Carboxydothermus hydrogenoformans (Ch). The goal of this research was to investigate the spectroscopic properties and DNA binding activity of Fe(III) Ch CooA that was reacted with NO. Based on electronic absorbance spectra, addition of NO to Fe(III) CooA resulted in the formation of a 6-coordinate (6-C) Fe(III)-NO intermediate species, which slowly converted to a 5-coordinate (5-C) Fe(II)-NO complex. A fluorescence anisotropy assay revealed appreciable DNA binding activity by the 6-C Fe(III)-NO form and lesser activity by the 5-C adduct. DNA binding by 5-C Fe(II)-NO CooA was unexpected because this species has a much different coordination structure than the native 6-C Fe(II)-CO form or the 6-C Fe(III)-NO adduct. Based on these findings, we propose that the most important factor required for activating CooA’s DNA binding function is the presence of a gas molecule (CO or NO) that is bound to the correct (i.e. distal) face of the heme. The authors wish to thank the Valparaiso University College of Arts and Sciences for financial support.