Coordination and redox state-dependent structural changes of the heme-based oxygen sensor GcHK associated with intraprotein signal transduction.

The heme-based oxygen sensor histidine kinase GcHK is part of a two-component signal transduction system in bacteria. O binding to the Fe(II) heme complex of its N-terminal globin domain strongly stimulates autophosphorylation at His in its C-terminal kinase domain. The 6-coordinate heme Fe(III)-OH and -CN complexes of GcHK are also active, but the 5-coordinate heme Fe(II) complex and the heme-free apo-form are inactive. Here, we determined the crystal structures of the isolated dimeric globin domains of the active Fe(III)-CN and inactive 5-coordinate Fe(II) forms, revealing striking structural differences on the heme-proximal side of the globin domain. Using hydrogen/deuterium exchange coupled with mass spectrometry to characterize the conformations of the active and inactive forms of full-length GcHK in solution, we investigated the intramolecular signal transduction mechanisms. Major differences between the active and inactive forms were observed on the heme-proximal side (helix H5), at the dimerization interface (helices H6 and H7 and loop L7) of the globin domain and in the ATP-binding site (helices H9 and H11) of the kinase domain. Moreover, separation of the sensor and kinase domains, which deactivates catalysis, increased the solvent exposure of the globin domain-dimerization interface (helix H6) as well as the flexibility and solvent exposure of helix H11. Together, these results suggest that structural changes at the heme-proximal side, the globin domain-dimerization interface, and the ATP-binding site are important in the signal transduction mechanism of GcHK. We conclude that GcHK functions as an ensemble of molecules sampling at least two conformational states.