Unusual Dynamics of Ligand Binding to the Heme Domain of the Bacterial CO Sensor Protein RcoM-2

The aerobic Gram-negative bacterium Burkholderia xenovorans expresses two highly homologous carbon monoxide (CO)-responsive transcriptional regulators, RcoM-1 and RcoM-2, which display extraordinarily high CO affinities, even under oxygenic conditions. To gain insight into the origin and perspectives of this feature, we characterized the ligand-binding properties of the N-terminal, heme-binding Per/Arnt/Sim sensor domain of RcoM-2 by time-resolved spectroscopy. We show that upon photodissociation of the heme-ligand bond, CO geminately rebinds to the heme with picosecond time constants and more than 99% rebinding yield, an unprecedented property of native heme proteins. Remarkably, the rebinding kinetics speeds up when the protein motions are slowed by cooling or solvent viscosity. This indicates that the origin of the observed efficient rebinding is a protein-imposed CO configuration in the heme pocket that is highly favorable for binding, a feature strongly in contrast to that of hemoglobins. The binding of CO to the ferrous heme from the solvent requires dissociation of the methionine axial heme ligand. From the kinetics of ligand binding and the extreme stability of the CO complex, we deduce that the dissociation constant for CO is lower than 100 pM. Finally, we show that when the ferric complex is exposed to CO gas or a CO-releasing molecule under oxygenic conditions formation of the ferrous carbonyl complex can occur on a time scale of minutes in the presence of a redox mediator. These findings pave the way for possible applications of the RcoM-2 heme domain as a CO sensor and/or scavenger.