Reversible Hexacoordination of α-Hemoglobin-stabilizing Protein (AHSP)/α-Hemoglobin Versus Pressure

• Published in: The Journal of biological chemistry 2007-03, Vol.282 (9), p.6398
• Main Authors: Djemel Hamdane, Corinne Vasseur-Godbillon, Véronique Baudin-Creuza, Gaston Hui Bon Hoa, Michael C. Marden
• Format: Article
• Language: English
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M610543200

Using high hydrostatic pressure or hydrogen peroxide as perturbing agents, we demonstrate a protective effect of the chaperone AHSP for the α-chains of Hb. High pressure induces an irreversible aggregation of the ferrous deoxy α-chains, whereas the AHSP/α-Hb complex shows reversible hexacoordination of the α-Hb without protein aggregation. Upon pressure release, the relaxation kinetics of the transition from the hexacoordinated to pentacoordinated form of α-Hb in the presence of AHSP exhibit a biphasic shape. High pressure did not induce dissociation of α-Hb from its chaperone, as evidenced by the ligand binding kinetics that show a unique rate for the AHSP/α-Hb complex. For both free α-Hb and the AHSP/α-Hb complex, the bimolecular rate constant of CO binding (k CO on ) versus pressure exhibits a bell shape, attributed to the transition of the rate-determining step from the chemical barrier to the migration of CO within the protein matrix. These results reveal a plasticity of the α-Hb active site in the presence of the chaperone and indicate that the AHSP was still active at 300 MPa. The ferric state of the AHSP/α-Hb complex shows hexacoordination even at atmospheric pressures, indicating a His-Fe-His binding scheme as previously observed in neuroglobin and cytoglobin. The reaction with hydrogen peroxide of ferric α-Hb within the complex also demonstrates a protection against aggregation.