CooA, a member of the bacterial CRP/FNR (cyclic AMP receptor protein/fumarate nitrate reductase) transcriptional regulator family, is a homodimer in which each monomer contains a b-type heme. In Rhodospirillum rubrum CooA is a transcpriton factor that enables the bacterium to sense and grow on CO(g)as a sole energy source.  Incredibly, CooA senses both the redox state of the cell and CO(g), the heme undergoes reduction and only the reduced form binds the effector.  Binding of the CO(g) effector to Fe(II) heme initiates allosterty.  Heme-dependent gas sensors are an expanding group of allosteric proteins yet it has been difficult to characterize the global structural changes and thermodynamics that accompany effector binding among members of this group. 

Our best understanding of structure-function relationships in heme containing gas sensors is based on studies of CooA.  We are working to address the many questions about allosteric regulation in CooA and heme-dependent sensors still to be answered.  The most notable is how the signaling molecule cooperates with protein structural elements to transmit the allosteric signal and alter global protein structure.  As a group we are probing allosteric dynamics with isothermal calorimetry (ITC), fluorescence anisotropy, spin labels and hydrogen/deuterium exchange mass spectrometry (H/DX-MS).  

CooA Coordination States

Heme coordination sates of CooA.  In the Fe(III) state, the heme is ligated by Cys75 and Pro2 (from the opposite monomer). When the heme is reduced, the Cys75 ligand is replaced with His77.  CooA is activated when CO(g) binds to the reduced heme, displacing the Pro2 ligand and enabling DNA binding.