Zhou Suiping, Olson John S, Fabian Marian, Weiss Mitchell J, Gow Andrew J
Division of Hematology, The Children's Hospital of Philadelphia, 3615 Civic Center Boulevard, Philadelphia, PA 19104, USA.
J Biol Chem. 2006 Oct 27;281(43):32611-8. doi: 10.1074/jbc.M607311200. Epub 2006 Aug 10.
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.
α-血红蛋白稳定蛋白（AHSP）是一种红细胞蛋白，它结合游离的α-血红蛋白（αHb）以维持其结构并限制其促氧化活性。先前的研究已经定义了两种不同的αHb-AHSP复合物。AHSP与亚铁（II）αHb的结合会诱导一种不寻常的构象，其中珠蛋白的F螺旋变得无序，血红素环暴露于溶剂中。随着时间的推移，这种中间体氧化形成稳定的高铁血红素，其中近端（F8）和远端（E7）组氨酸与血红素铁原子配位。向亚铁（II）或高铁（III）αHb-AHSP中添加βHb会取代AHSP，生成四聚体（α2β2）HbA物种。两种αHb-AHSP复合物的生化特性和体内意义尚不清楚。在这里，我们表明高铁（III）αHb-AHSP由与AHSP结合的氧合αHb自动氧化形成，并且该过程在生理温度和氧压力下大大加速。与游离的高铁（III）αHb高铁血红素不同，与AHSP结合的高铁（III）αHb不会沉淀，并且可以循环回功能性HbA。这需要在β亚基提取之前或之后对与AHSP结合的αHb进行酶促还原。相比之下，亚铁（II）αHb-AHSP与βHb的反应直接生成功能性HbA。我们的研究结果支持了一个模型，其中AHSP可以在正常红细胞生成过程中在形成HbA的途中短暂稳定αHb，或者将过量的游离αHb转化为化学惰性更强的状态，通过氧化还原循环可以从中恢复αHb。
