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

Alpha hemoglobin-stabilizing protein (AHSP) is an erythroid protein that binds free alpha hemoglobin (alphaHb) to maintain its structure and limit its pro-oxidant activity. Prior studies have defined two different alphaHb.AHSP complexes. Binding of AHSP to Fe(II) alphaHb 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 betaHb to either Fe(II) or Fe(III) alphaHb.AHSP displaces AHSP to generate tetrameric (alpha(2)beta(2)) HbA species. The biochemical properties and in vivo significance of the two alphaHb.AHSP complexes are poorly understood. Here we show that Fe(III) alphaHb.AHSP forms from auto-oxidation of oxygenated alphaHb bound to AHSP and that this process is greatly accelerated at physiologic temperature and oxygen pressures. In contrast to free Fe(III) alphaHb hemichromes, AHSP-bound Fe(III) alphaHb does not precipitate and can be recycled into functional HbA. This requires enzymatic reduction of AHSP-bound alphaHb, either prior to or after extraction by beta subunits. In contrast, reaction of Fe(II) alphaHb-AHSP with betaHb generates functional HbA directly. Our findings support a model in which AHSP can either stabilize alphaHb transiently en route to HbA formation during normal erythropoiesis or convert excessive free alphaHb into a more chemically inert state from which recovery of alphaHb is possible by redox cycling.