Biochemical fates of α hemoglobin bound to α hemoglobin-stabilizing protein AHSP

Alpha hemoglobin-stabilizing protein (AHSP) is an erythroid protein that binds free α hemoglobin (αHb) to maintain its structure and limit its pro-oxidant activity. Prior studies have defined two different αHb·AHSP complexes. Binding of AHSP to Fe(II) αHb induces an unusual configuration in which the F helix of the globin becomes disordered and the heme ring becomes solvent-exposed. Over time, this intermediate oxidizes to form a stable hemichrome in which the proximal (F8) and distal (E7) histidines are coordinated to the heme iron atom. The addition of βHb to either Fe(II) or Fe(III) αHb·AHSP displaces AHSP to generate tetrameric (α₂β₂) HbA species. The biochemical properties and in vivo significance of the two αHb·AHSP complexes are poorly understood. Here we show that Fe(III) αHb·AHSP forms from auto-oxidation of oxygenated αHb bound to AHSP and that this process is greatly accelerated at physiologic temperature and oxygen pressures. In contrast to free Fe(III) αHb hemichromes, AHSP-bound Fe(III) αHb does not precipitate and can be recycled into functional HbA. This requires enzymatic reduction of AHSP-bound αHb, either prior to or after extraction by β subunits. In contrast, reaction of Fe(II) αHb-AHSP with αHb generates functional HbA directly. Our findings support a model in which AHSP can either stabilize αHb transiently en route to HbA formation during normal erythropoiesis or convert excessive free αHb into a more chemically inert state from which recovery of αHb is possible by redox cycling.