Redox properties and coordination structure of the heme in the CO-sensing transcriptional activator CooA

The CO-sensing transcriptional activator CooA contains a six-coordinate pro toheme as a CO sensor. Cys(75) and His(77) are assigned to the fifth ligand of the ferric and ferrous hemes, respectively. In this study, we carried o ut alanine-scanning mutagenesis and EXAFS analyses to determine the coordin ation structure of the heme in CooA. Pro(2) is thought to be the sixth liga nd of the ferric and ferrous hemes in CooA, which is consistent with the cr ystal structure of ferrous CooA (Lanzilotta, W, N., Schuller, D. J., Thorst einsson, M. V., Kerby, R, L,, Roberts, G. P., and Poulos, T. L. (2000) Not. Struct Biol. 7, 876-880). CooA exhibited anomalous redox chemistry, i.e. h ysteresis was observed in electrochemical redox titrations in which the obs erved reduction and oxidation midpoint potentials were -320 mV and -260 mV, respectively. The redox-controlled ligand exchange of the heme between Cys (75) and His(77) is thought to cause the difference between the reduction a nd oxidation midpoint potentials.