| Summary: | The ability of microorganisms to adapt to low oxygen concentrations confers a considerable growth advantage. Furthermore, the colonization of microaerobic environments by pathogenic bacteria, such as Pseudomonas aeruginosa is a significant clinical issue. The cbb3 cytochrome c oxidases are members of the heme copper oxidase superfamily that regulate microaerobic respiration in diverse Proteobacteria. Cytochrome cbb3 oxidases are composed of four non-identical subunits encoded by one or two ccoNOQP operons. Surprisingly, the CcoP subunit contains a low potential hexacoordinate heme that binds CO in the reduced state following displacement of the distal endogenous ligand. The biochemical significance of CcoP is poorly understood but the cbb3 complex reports the redox status of the cell leading to transcriptional activation of genes involved in energy metabolism via the sensory kinase RegB/RoxS. By expressing the diheme subunit CcoP from the non-pathogenic Pseudomonad, Pseudomonas stutzeri in Escherichia coli, we have now examined the biochemical properties of the CO binding c-type heme. We characterized wild-type and mutant CcoP using mediated redox potentiometry, UVVisible, magnetic circular dichroism, and electron paramagnetic spectroscopies and have clearly identified two low-spin His/His coordinated c-type hemes, with redox potentials of + 185 mV and -15 mV. Examination of the spectral characteristics and oxidase activity of both cbb3 oxidase isoforms from the clinically relevant P. aeruginosa suggested that the cbb3 -1 oxidase has an important metabolic function at high oxygen tensions and the cbb3-2 oxidase has a more significant role under oxygen limiting conditions. In conclusion, our data suggests a prominent function for the CcoP subunit of cytochrome cbb3 oxidases in the adaptive ability of Pseudomonads to colonize diverse environments. Further understanding of this adaptive biochemistry may reveal drugable targets for P. eeruginosa.
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