Molecular Mechanism of AHSP-Mediated Stabilization of α-Hemoglobin

ized by high-level production of hemoglobin A (HbA), which is composed of two ␣ and two ␤ subunits, each bound to a heme moiety and produced in a concerted * the bound heme; oxygen binds cooperatively and re-* versibly to ferrous iron from the opposite side of the 1 Department of Molecular Biology heme plane. The elaborate oxygen binding mechanism Lewis Thomas Laboratory of HbA allows the concentration of oxygen in the blood Princeton University to nearly reach that in open air. Although the syntheses Princeton, New Jersey 08544 of ␣-and ␤-hemoglobin are coordinated for efficient While free ␤-hemoglobin (␤Hb) 3 The Children's Hospital of Philadelphia and forms a relatively stable homo-tetramer, free ␣-hemoglo-The University of Pennsylvania bin (␣Hb) exists as a structurally unstable monomer and Philadelphia, Pennsylvania 19104 is prone to oxidation and precipitation, likely contribut-4 Center for Heavy Metals Research and ing to the pathophysiology of ␤ thalassemia and other Center for Structural Biology School of Chemistry Molecular chaperones have long been proposed to University of Sydney help stabilize free globin and hemoglobin subunits. Ef-NSW 2006 fort to identify such chaperones led to the discovery Australia of the ␣-hemoglobin stabilizing protein (AHSP), which specifically interacts with and stabilizes free ␣Hb in the absence of available ␤ subunit (Kihm et al., 2002). When Summary available, ␤Hb binds more avidly to the ␣ subunit, displacing AHSP and forming tetrameric HbA (Gell et al., Hemoglobin A (HbA), the oxygen delivery system in 2002; Kihm et al., 2002). AHSP is an abundant, erythroid-humans, comprises two ␣ and two ␤ subunits. Free specific protein that protects free ␣Hb from precipitation ␣-hemoglobin (␣Hb) is unstable, and its precipitation both in solution and in live cells. AHSP knockout mice contributes to the pathophysiology of ␤ thalassemia. exhibit reticulocytosis, abnormal erythrocyte morphol-In erythrocytes, the ␣-hemoglobin stabilizing protein ogy with intracellular inclusion bodies, and increased (AHSP) binds ␣Hb and inhibits its precipitation. The reactive oxygen species (ROS) with subsequent cellular crystal structure of AHSP bound to Fe(II)-␣Hb reveals oxidative damage (Kihm et al., 2002; Y. Kong et al., sub-that AHSP specifically recognizes the G and H helices mitted). of ␣Hb through a hydrophobic interface that largely Although AHSP plays an essential role in modulating recapitulates the ␣ 1-␤ 1 interface of hemoglobin. The the stability of ␣Hb and the assembly of HbA, the under-AHSP-␣Hb interactions are extensive but suboptimal, lying molecular mechanisms are undetermined. A recent explaining …