Oxygen equilibrium properties of asymmetric nickel(II)-iron(II) hybrid hemoglobin

Asymmetric Ni(II)-Fe(II) hybrid hemoglobin, XL[alpha(Fe)beta(Fe)][alpha(Ni)beta(Ni)], in which the alpha 1 beta 1 dimer containing ferrous protoporphyrin IX and the complementary alpha 2 beta 2 dimer containing Ni(II) protoporphyrin IX were cross-linked between Lys-82 beta 1 and Lys-82 beta 2 by reaction with bis(3,5-dibromosalicyl) fumarate, was synthesized and characterized. We have previously shown that (i) Ni(II) protoporphyrin IX, which binds neither oxygen nor carbon monoxide, mimics a fixed deoxyheme with respect to its effect on the oxygen equilibrium properties of the counterpart iron subunits in both symmetric Ni(II)-Fe(II) hybrid Hbs [Shibayama, N., Morimoto, H., & Miyazaki, G. (1986) J. Mol. Biol. 192, 323-329] and (ii) the cross-linking used in this study little affects the oxygen equilibrium properties of hemoglobin [Shibayama, N., Imai, K., Hirata, H., Hiraiwa, H., Morimoto, H., & Saigo, S. (1991) Biochemistry 30, 8158-8165]. These remarkable features of our model allowed us to measure the oxygen equilibrium curves for the first two steps of oxygen binding to the alpha 1 beta 1 dimer within the hemoglobin tetramer. At all pH values examined, the affinities of this asymmetric hybrid for the first oxygen molecule are as low as those of native hemoglobin. The hybrid did not show cooperative oxygen binding at pH 6.4, while significant cooperativity was observed with rising pH; i.e., the Hill coefficient was increased from 1.41 to 1.53 upon a pH change from 7.4 to 8.4. The electronic absorption spectrum of Ni(II) protoporphyrin IX in the alpha 2 subunit was changed upon carbon monoxide (or oxygen) binding to the alpha 1 beta 1 dimer.(ABSTRACT TRUNCATED AT 250 WORDS)