Abstract
CooA is a transcriptional activator found in a purple non-sulfur photosynthetic bacterium, Rhodospirillum rubrum and is responsible for the expression of the coo operons in response to CO. CooA is a heme-containing and CO-sensing transcriptional activator whose activity is regulated by CO. CooA shows several unique features for the coordination structure of the heme compared with other heme proteins, as described below. First, a proline residue is coordinated to the ferric and ferrous hemes as an axial ligand. CooA is the first and only example of the coordination of a proline residue to a metal ion in metaloproteins. Second, a redox-controlled ligand exchange occurs between Cys75 and His77 during the change in the oxidation state of the heme in CooA. Cys75that iscoordinated to the ferric heme in CooA is replaced by His77 upon the reduction of the heme iron, andvice versa. Third, CO can easily react with the ferrous heme in CooA to form the CO-bound form under physiologicalconditions, though the ferrous heme is saturated coordinationally with two endogenous axial ligand. The ligand exchange proceedsbetween Pro2 and CO during formation of the CO-bound CooA. I will also discuss some properties of a CooA homologue we have found recently. [Jpn J Physiol 55 Suppl:S12 (2005)]
