Molecular mechanism of AHSP-mediated stabilization of alpha-hemoglobin.
Hemoglobin A (HbA), the oxygen delivery system in humans, comprises two alpha and two beta subunits. Free alpha-hemoglobin (alphaHb) is unstable, and its precipitation contributes to the pathophysiology of beta thalassemia. In erythrocytes, the alpha-hemoglobin stabilizing protein (AHSP) binds alphaHb and inhibits its precipitation. The crystal structure of AHSP bound to Fe(II)-alphaHb reveals that AHSP specifically recognizes the G and H helices of alphaHb through a hydrophobic interface that largely recapitulates the alpha1-beta1 interface of hemoglobin. The AHSP-alphaHb interactions are extensive but suboptimal, explaining why beta-hemoglobin can competitively displace AHSP to form HbA. Remarkably, the Fe(II)-heme group in AHSP bound alphaHb is coordinated by the distal but not the proximal histidine. Importantly, binding to AHSP facilitates the conversion of oxy-alphaHb to a deoxygenated, oxidized [Fe(III)], nonreactive form in which all six coordinate positions are occupied. These observations reveal the molecular mechanisms by which AHSP stabilizes free alphaHb.
Feng
Liang
L
Department of Molecular Biology, Lewis Thomas Laboratory, Princeton University, Princeton, NJ 08544, USA.
Gell
David A
DA
Zhou
Suiping
S
Gu
Lichuan
L
Kong
Yi
Y
Li
Jianqing
J
Hu
Min
M
Yan
Nieng
N
Lee
Christopher
C
Rich
Anne M
AM
Armstrong
Robert S
RS
Lay
Peter A
PA
Gow
Andrew J
AJ
Weiss
Mitchell J
MJ
Mackay
Joel P
JP
Shi
Yigong
Y
D000818:Animals; D001665:Binding Sites / Q000502:physiology; D001798:Blood Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D002417:Cattle; D018360:Crystallography, X-Ray; D004795:Enzyme Stability / Q000502:physiology; D006441:Hemoglobin A / Q000737:chemistry* / Q000378:metabolism*; D006639:Histidine / Q000378:metabolism; D006801:Humans; D007501:Iron / Q000378:metabolism; D046911:Macromolecular Substances / Q000737:chemistry; D051379:Mice; D008958:Models, Molecular; D018832:Molecular Chaperones / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D019906:Nuclear Magnetic Resonance, Biomolecular; D010108:Oxyhemoglobins / Q000378:metabolism; D011485:Protein Binding / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000502:physiology; D021122:Protein Subunits / Q000378:metabolism; D051381:Rats; D017386:Sequence Homology, Amino Acid; D012689:Sequence Homology, Nucleic Acid; D034421:Sus scrofa
C468768:AHSP protein, human; D001798:Blood Proteins; D046911:Macromolecular Substances; D018832:Molecular Chaperones; D010108:Oxyhemoglobins; D021122:Protein Subunits; D006639:Histidine; D006441:Hemoglobin A; C029774:oxyhemoglobin A; D007501:Iron