Structure of imidazole methemoglobin

Crystals of horse methemoglobin shatter when soaked in crystallization buffer containing high concentrations of imidazole. By using less than saturating concentrations of imidazole, a stable imidazole derivative of crystalline methemoglobin was prepared and analyzed by X-ray difference Fourier techniques. Both subunits of imidazole methemoglobin show extensive, but different, changes in tertiary structure. Many of the tertiary structural changes observed in the transition from deoxyhemoglobin to methemoglobin are amplified in the transition from methemoglobin to imidazole methemoglobin. Unlike all other ligands that have been examined, imidazole only partially enters the ligand pocket and does not occupy the usual ligand site distal to pyrrole II. The position of the imidazole is on a possible pathway for entrance of smaller diatomic ligands from the solvent into the heme pocket. The extent of imidazole binding of the α-hemes and β-hemes is about 25% and 45%, respectively. An explanation for this difference in occupancy is suggested, involving steric interaction of the distal histidine and phenylalanine CD4 in each subunit. This structural hypothesis may have implications for the kinetics of ligand binding.