Structure of a crystal form of human methemoglobin indicative of fiber formation

Human methemoglobin was crystallized in a unique unit cell and its structure was solved by molecular replacement. The hexagonal unit cell has unit-cell parameters a = b = 54.6, c = 677.4 A, with symmetry consistent with space group P6122. The unit cell has the second highest aspect ratio of all unit cells contained in the PDB. The 12 molecules in the unit cell describe a right-handed helical filament having no polarity, which is different from the filament composed of HbS fibers, which is the only other well characterized fiber of human hemoglobin. The filaments reported here can be related to canonical sickle-cell hemoglobin filaments and to an alternative sickle-cell filament deduced from fiber diffraction by slight modifications of intermolecular contacts.