Competition with xenon elicits ligand migration and escape pathways in myoglobin

Evidence for ligand migration toward the xenon-binding cavities in myoglobin comes from a number of laser photolysis studies of MbO2 including mutants and from cryo- and time-resolved crystallography of MbCO. To explore ligand migration in greater detail, we investigated the rebinding kinetics of both MbO2 and MbCO under a xenon partial pressure ranging from 1 to 16 atm over the temperature range (293-77 K). Below 180 K xenon affects to a significant, but minor, extent the thermodynamic parameters for rebinding from the primary docking site in each Mb taxonomic substate. Above 200 K the ligand migrates to the proximal Xe1 site but when the latter is occupied by xenon a new kinetic process appears. It is attributed to rebinding from transient docking sites located on the path between the primary and the secondary docking site of both ligands. Ligand escape exhibits a more complicated pattern than expected. At room temperature O2 and CO escape appears to take place exclusively from the primary site. In contrast, at T approximately 250 K, roughly 50% of the CO molecules that have escaped from the protein originate from the Xe1 secondary site.