CooA is a heme-containing transcription factor that responds to the presence of carbon monoxide. This protein forms homodimers and is a homolog of cAMP receptor protein.[1] CooA regulates the expression of carbon monoxide dehydrogenase, an enzyme that catalyzes the oxidation of CO to CO2. The most well-studied CooA homolog comes from Rhodospirillum rubrum (RrCooA), but the CooA homolog from Carboxydothermus hydrogenoformans (ChCooA) has been studied as well.[2] The main difference between these two CooA homologs is the ferric heme coordination. For RrCooA, the ferric heme iron is bound to a cysteine and the amine of the N-terminal proline, while, in the ferrous state, a ligand switch occurs where a nearby histidine displaces the thiolate.[3][4][5] For ChCooA, the heme iron is ligated by a histidine and the N-terminal amine in both the ferric and ferrous states.[6] For both homologs, CO displaces the amine ligand and activates the protein to bind to its target DNA sequence.[7][8] Several structures of CooA exist: RrCooA in the ferrous state (1FT9),[9] ChCooA in the ferrous, imidazole-bound state (2FMY),[10] and ChCooA in the ferrous, CO-bound state (2HKX).[11]
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