Binding and Electron Transfer between Cytochrome b5 and the Hemoglobin α- and β-Subunits through the Use of [Zn, Fe] Hybrids Academic Article uri icon

abstract

  • We have measured the binding affinity (K(A) and electron transfer (ET) rate constants (k) for the complex of hemoglobin (Hb) and cytochrome b5 (b5), using triplet quenching titrations of mixed-metal [ZnM, Fe3+(N3- )] Hb hybrids and of fully substituted Zn-mesoporphyrin (ZnM)Hb by b5 (trypsin-solubilized, bovine) (pH values 6.0 and 7.0). The use of the mixed- metal Hb hybrids with Zn in one chain type allows us to selectively monitor the 3ZnP → Fe3+P ET reaction of Fe3+b5 with either the α-chains or the β-chains. The self-consistent analysis of the results for the mixed- metal hybrids and those for the (ZnM)Hb allows us to determine the reactivity and affinity constants for the interactions of b5 with the individual subunits of T-state Hb. The results confirm that ET occurs within a complex between b5 and Hb, not through a simple bimolecular collision process. At pH 6.0, the binding affinity constant of the α-chains (K(α) ≃ 2.0 x 104 M-1) is ~4-fold larger than that of the β-chains (K(β) = 4.9 x 103 M-1); the intracomplex ET rate constant of the α-chains (k(α) ≃ 1500 s-1) is ~2-fold larger than that of the β-chains (k(β) ≃ 850 s-1). The binding affinity and ET rate constant of the α-chains both decrease as the pH is increased from 6.0 to 7.0; the binding affinity of the β-chains is essentially the same at pH 6.0 and 7.0, while the ET reactivity decreases. The kinetic results are consistent with a docking model in which each subunit binds a molecule of b5. However, they permit an alternative in which b5 reacts with the α-chains by binding at a site which spans the α1/β2 dimer interface.

published proceedings

  • Journal of the American Chemical Society

author list (cited authors)

  • Naito, N. R., Huang, H. e., Sturgess, A. W., Nocek, J. M., & Hoffman, B. M

citation count

  • 23

complete list of authors

  • Naito, Naomi R||Huang, He||Sturgess, Annie Willie||Nocek, Judith M||Hoffman, Brian M

publication date

  • November 1998