Proton-linked subunit heterogeneity in ferrous nitrosylated human adult hemoglobin: an EPR study

The effect of pH on the X-band electron paramagnetic resonance (EPR) spectrum of ferrous nitrosylated human adult tetrameric hemoglobin (HbNO) as well as of ferrous nitrosylated monomeric α- and β-chains has been investigated, at −163 °C. At pH 7.3, the X-band EPR spectrum of tetrameric HbNO and ferrous nitrosylated monomeric α- and β-chains displays a rhombic shape. Lowering the pH from 7.3 to 3.0, tetrameric HbNO and ferrous nitrosylated monomeric α- and β-chains undergo a transition towards a species characterized by a X-band EPR spectrum with a three-line splitting centered at 334 mT. These pH-dependent spectroscopic changes may be taken as indicative of the cleavage, or the severe weakening, of the proximal HisF8-Fe bond. In tetrameric HbNO, the pH-dependent spectroscopic changes depend on the acid-base equilibrium of two apparent ionizing groups with pKa values of 5.8 and 3.8. By contrast, the pH-dependent spectroscopic changes occurring in ferrous nitrosylated monomeric α- and β-chains depend on the acid-base equilibrium of one apparent ionizing group with pKa values of 4.8 and 4.7, respectively. The different pKa values for the proton-linked spectroscopic transition(s) of tetrameric HbNO and ferrous nitrosylated monomeric α- and β-chains suggest that the quaternary assembly drastically affects the strength of the proximal HisF8-Fe bond in both subunits. This probably reflects a 'quaternary effect', i.e., structural changes in both subunits upon tetrameric assembly, which is associated to a relevant variation of functional properties (i.e., proton affinity).