Weiss Mitchell J, Zhou Suiping, Feng Liang, Gell David A, Mackay Joel P, Shi Yigong, Gow Andrew J
The Children's Hospital of Philadelphia, University of Pennsylvania, Philadelphia, PA 19104, USA.
Ann N Y Acad Sci. 2005;1054:103-17. doi: 10.1196/annals.1345.013.
Hemoglobin (Hb) synthesis is coordinated by homeostatic mechanisms to limit the accumulation of free alpha or beta subunits, which are cytotoxic. Alpha hemoglobin-stabilizing protein (AHSP) is an abundant erythroid protein that specifically binds free alphaHb, stabilizes its structure, and limits its ability to participate in chemical reactions that generate reactive oxygen species. Gene ablation studies in mice demonstrate that AHSP is required for normal erythropoiesis. AHSP-null erythrocytes are short-lived, contain Hb precipitates, and exhibit signs of oxidative damage. Loss of AHSP exacerbates beta-thalassemia in mice, indicating that altered AHSP expression or function could modify thalassemia phenotypes in humans, a topic that is beginning to be explored in clinical studies. We used biochemical, spectroscopic, and crystallographic methods to examine how AHSP stabilizes alphaHb. AHSP binds the G and H helices of alphaHb on a surface that largely overlaps with the alpha1-beta1 interface of HbA. This result explains previous findings that betaHb can competitively displace AHSP from alphaHb to form HbA tetramer. Remarkably, binding of AHSP to oxygenated alphaHb induces dramatic conformational changes and converts the heme-bound iron to an oxidized hemichrome state in which all six coordinate positions are occupied. This structure limits the reactivity of heme iron, providing a mechanism by which AHSP stabilizes alphaHb. These findings suggest a biochemical pathway through which AHSP might participate in normal Hb synthesis and modulate the severity of thalassemias. Moreover, understanding how AHSP stabilizes alphaHb provides a theoretical basis for new strategies to inhibit the damaging effects of free alphaHb that accumulates in beta-thalassemia.
血红蛋白（Hb）的合成由稳态机制协调，以限制具有细胞毒性的游离α或β亚基的积累。α血红蛋白稳定蛋白（AHSP）是一种丰富的红系蛋白，它特异性结合游离的αHb，稳定其结构，并限制其参与产生活性氧的化学反应的能力。对小鼠的基因敲除研究表明，正常红细胞生成需要AHSP。缺乏AHSP的红细胞寿命较短，含有Hb沉淀物，并表现出氧化损伤的迹象。AHSP的缺失会加重小鼠的β地中海贫血，这表明AHSP表达或功能的改变可能会改变人类地中海贫血的表型，这一话题已开始在临床研究中进行探索。我们使用生化、光谱和晶体学方法来研究AHSP如何稳定αHb。AHSP在一个与HbA的α1-β1界面大部分重叠的表面上结合αHb的G螺旋和H螺旋。这一结果解释了先前的发现，即βHb可以竞争性地将AHSP从αHb上置换下来，形成HbA四聚体。值得注意的是，AHSP与氧合αHb的结合会引起显著的构象变化，并将血红素结合的铁转化为氧化高铁血红素状态，其中所有六个配位位置都被占据。这种结构限制了血红素铁的反应性，提供了一种AHSP稳定αHb的机制。这些发现提示了一条生化途径，通过该途径AHSP可能参与正常Hb的合成并调节地中海贫血的严重程度。此外，了解AHSP如何稳定αHb为抑制β地中海贫血中积累的游离αHb的破坏作用的新策略提供了理论基础。
