High pressure enhances hexacoordination in neuroglobin and other globins

The techniques of high applied pressure and flash photolysis have been combined to study ligand rebinding to neuroglobin (Ngb) and tomato Hb, globins that may display a His-Fe-His hexacoordination in the absence of external ligands. High pressure induces a moderate decrease in the His association rate and a large decrease in His dissociation rate, thus leading to an enhancement of the overall His affinity. The overall structural difference between penta- and hexacoordinated globins may be rather small and can be overcome by external modifications such as high pressure. Over the pressure range 0.1-700 MPa (7 kbar), the globins may show a loss of over a factor of 100 in the amplitude of the bimolecular rebinding phase after photodissociation. The kinetic data show that pressure induces a moderate increase of the rate for ligand binding from the correlated pair state (just after photodissociation) and a large (factor of 1000) decrease in rate for migration through the protein. The effect on the ligand migration phase was similar for both the external ligands (such as oxygen) as for the internal (histidine) ligand, suggesting the dominant role of protein fluctuations, rather than specific chemical barriers. Thus high pressure efficiently closes the protein migration channels; however, contrary to the effect of high viscosity, high pressure induces a greater decrease in rate for ligand migration toward the exterior (heme to the solvent) versus inward migration, as if the presence of the ligand itself induces an additional steric constraint.