Structural studies by proton magnetic relaxation of stereochemical probes. II. Allosteric effects in human methaemoglobin.

The haem-iron accessibility to solvent molecules in human aquomet- and fluoromethaemoglobin was studied by the magnetic relaxation of protons from a stereochemical probe (methanol in deuterated solutions) in its dependence on allosteric effects induced by inositol hexaphosphate and pH between 5.5 and 8.5. The exchange of methanol with bulk solvent was observed only when inositol hexaphosphate was bound to aquomethaemoglobin, which is consistent with a widening of the haemcrevice compared to the conformation in the absence of inositol hexaphosphate. An increase in alkalinity in the physiological range of the Bohr effect results in a gradual impedence of the solvent dynamics inside the haem-pocket. The fast-relaxation phase of methyl protons indicates that a large number of methanol molecules are under the strong influence of the protein; this effect is considerably smaller with inositol hexaphosphate bound to aquomethaemoglobin. The hypothesis which implies a proton from the coordinated water molecule is responsible for the observed relaxation rates has been critically discussed. The model with a water molecule exchanging between a position next to the sixth-ligand site of the haem-iron and the bulk solvent is further substantiated experimentally. This model has been found to be the simplest and most self consistent in the interpretation of all these proton magnetic relaxation data.