Characterization of a Cobalt-Substituted Globin-Coupled Oxygen Sensor Histidine Kinase from sp. Fw109-5: Insights into Catalytic Regulation by Its Heme Coordination Structure.

Heme-based gas sensors are an emerging class of heme proteins. GcHK, a globin-coupled histidine kinase from sp. Fw109-5, is an oxygen sensor enzyme in which oxygen binding to Fe(II) heme in the globin sensor domain substantially enhances its autophosphorylation activity. Here, we reconstituted GcHK with cobalt protoporphyrin IX (Co-GcHK) in place of heme (Fe-GcHK) and characterized the spectral and catalytic properties of the full-length proteins. Spectroscopic analyses indicated that Co(III) and Co(II)-O complexes were in a 6-coordinated low-spin state in Co-GcHK, like Fe(III) and Fe(II)-O complexes of Fe-GcHK. Although both Fe(II) and Co(II) complexes were in a 5-coordinated state, Fe(II) and Co(II) complexes were in high-spin and low-spin states, respectively. The autophosphorylation activity of Co(III) and Co(II)-O complexes of Co-GcHK was fully active, whereas that of the Co(II) complex was moderately active. This contrasts with Fe-GcHK, where Fe(III) and Fe(II)-O complexes were fully active and the Fe(II) complex was inactive. Collectively, activity data and coordination structures of Fe-GcHK and Co-GcHK indicate that all fully active forms were in a 6-coordinated low-spin state, whereas the inactive form was in a 5-coordinated high-spin state. The 5-coordinated low-spin complex was moderately active-a novel finding of this study. These results suggest that the catalytic activity of GcHK is regulated by its heme coordination structure, especially the spin state of its heme iron. Our study presents the first successful preparation and characterization of a cobalt-substituted globin-coupled oxygen sensor enzyme and may lead to a better understanding of the molecular mechanisms of catalytic regulation in this family.