Roles of Heme Axial Ligands in the Regulation of CO Binding to CooA

CooA is a CO-dependent transcription factor of the bacterium Rhodospirillum rubrum that contains a six-coordinate heme. It has as its heme axial ligands Pro 2 and Cys 75 in the ferric state and Pro 2 and His 77 in the ferrous state. To probe the regulation of CO binding and the ligand switching mechanism in CooA, we have prepared site-directed mutants in which the residues contributing the axial ligands are substituted. The properties of these mutants were investigated by resonance Raman and CO titration methods. Wild-type CooA binds CO with a modest dissociation constant ( K d ) of 11 μ m , this value being typical for gas-sensing heme proteins. The K d value was greatly decreased in the P2H mutant, indicating that Pro 2 coordination fine tunes CO sensing in CooA. The bound CO in P2H gives rise to a ν(Fe-CO) stretching Raman line at 490 cm -1 , which is similar to that in wild-type CooA. Thus, Pro 2 is the ligand that is replaced by exogenous CO. In the H77A mutant, equilibrium CO binding is biphasic, and at high CO pressures two CO molecules occupy both axial sites. The ν(Fe-CO) stretching Raman line for the first CO molecule was observed at 497 cm -1 . Some of the His 77 mutants showed an additional ν(Fe-CO) line at 525 cm -1 . The binding affinity of the second CO molecule correlates with the five-coordinate component in the ferrous His 77 mutants and also with the acidity of the side chain at position 77. Thus, we propose the Cys 75 -His 77 ligand switch is controlled by His 77 acting as a proton reservoir.