Electronic absorption, EPR, and resonance raman spectroscopy of CooA, a CO-sensing transcription activator from R-rubrum, reveals a five-coordinate NO-heme

Electronic absorption, EPR, and resonance Raman spectroscopies revealed tha t CooA, the GO-sensing transcriptional regulator from Rhodospirillum rubrum , reacts with NO to form a five-coordinate NO-heme. NO must therefore displ ace both of the heme ligands from six-coordinate, low-spin Fe(II)CooA in fo rming five-coordinate Fe(II)CooA(NO). CO in contrast, displaces a single he me ligand from Fe(II)CooA to form six-coordinate Fe(II)CooA(CO). Of a serie s of common heme-binding ligands, only CO and NO were able to bind to the h eme of wild-type CooA; imidazole, azide anion, and cyanide anion had no eff ect on the heme absorption spectrum. Although NO binds to the heme and disp laces the endogenous ligands, NO was not able to induce CooA to bind to its target DNA. The mechanism of GO-dependent activation of CooA is thus more complex than simple displacement of a ligand from the heme iron since NO do es not trigger DNA binding. These observations suggest that the CooA heme s ite discriminates between NO and the biologically relevant signal, CO.