Studies on nitrosyl hemes in Ni(II)-Fe(II) hybrid hemoglobins

Subunit heterogeneity within a particular subunit in hemoglobin A have been explored with electron paramagnetic resonance spectroscopy using the nitrosyl hemes in Ni-Fe hybrid Hb under various solution conditions. Our previous studies on the crystal structure of NiHb demonstrated the presence of subunit heterogeneity within α-subunit. To further cross check this hypothesis, we made a hybrid Hb in which either the α- or β-subunit contains iron, which alone can bind to NO. By this way dynamic exchange between penta- and hexa-coordinated forms within a subunit was confirmed. Upon the addition of inositol hexa phosphate (IHP) to these hybrids, R to T state transition is observed for [α2(Fe-NO)β2(Ni)] but such a direct transformation is less marked in [α2(Ni)β2(Fe-NO)]. Hence the bond between Nε and Fe is fundamental to the structure-function relation in Hb, as the motion of this nitrogen triggers the vast transformation, which occurs in the whole molecule on attachment of NO.