Background: AHSP is an erythroid molecular chaperone of the α-hemoglobin chains (α-Hb). Upon AHSP binding, native ferric α-Hb undergoes an unprecedented structural rearrangement at the heme site giving rise to a 6th coordination bond with His(E7).
Methods: Recombinant AHSP, WT α-Hb:AHSP and α-Hb(HE7Q):AHSP complexes were expressed in Escherichia coli. Thermal denaturation curves were measured by circular dichroism for the isolated α-Hb and bound to AHSP. Kinetics of ligand binding and redox reactions of α-Hb bound to AHSP as well as α-Hb release from the α-Hb:AHSP complex were measured by time-resolved absorption spectroscopy.
Results: AHSP binding to α-Hb is kinetically controlled to prevail over direct binding with β-chains and is also thermodynamically controlled by the α-Hb redox state and not the liganded state of the ferrous α-Hb. The dramatic instability of isolated ferric α-Hb is greatly decreased upon AHSP binding. Removing the bis-histidyl hexacoordination in α-HbH58(E7)Q:AHSP complex reduces the stabilizing effect of AHSP binding. Once the ferric α-Hb is bound to AHSP, the globin can be more easily reduced by several chemical and enzymatic systems compared to α-Hb within the Hb-tetramer.
Conclusion: α-Hb reduction could trigger its release from AHSP toward its final Hb β-chain partner producing functional ferrous Hb-tetramers. This work indicates a preferred kinetic pathway for Hb-synthesis.
General significance: The cellular redox balance in Hb-synthesis should be considered as important as the relative proportional synthesis of both Hb-subunits and their heme cofactor. The in vivo role of AHSP is discussed in the context of the molecular disorders observed in thalassemia.
Keywords: AHSP; AHSP(WT); AHSP:α-Hb; Alpha-hemoglobin stabilizing protein (AHSP); Chaperone; Cyt b5; Erythropoiesis; GST; Hb; Heme hexacoordination; Hemoglobin; MetHb; Ngb; PBS; RBCs; ROS; alpha hemoglobin-stabilizing protein; complex formed between WT α-Hb chain and recombinant human wild type AHSP; ferric heme; glutathione S-transferase; hemin; human adult hemoglobin; neuroglobin; oxidized Hb; phosphate-buffered saline; reactive oxygen species; recombinant human wild type AHSP with an N-terminal Gly-Pro-Leu-Gly-Ser peptide; recombinant soluble domain of human membrane-bound cytochrome b5; red blood cells.
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