Dynamics of α-Hb chain binding to its chaperone AHSP depends on heme coordination and redox state
Résumé
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.
Mots clés
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.