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Oxygen equilibrium study and light absorption spectra of Ni(II)Fe(II) hybrid hemoglobins

1986; Elsevier BV; Volume: 192; Issue: 2 Linguagem: Inglês

10.1016/0022-2836(86)90367-0

1089-8638

Naoya Shibayama, Hideki Morimoto, Gentaro Miyazaki,

Porphyrin and Phthalocyanine Chemistry

Ni(II)Fe(II) hybrid hemoglobins, in which hemes in either the α or β subunit are substituted with Ni(II) protoporphyrin IX, have been prepared and characterized. Since Ni(II) protoporphyrin IX binds neither oxygen nor carbon monoxide, the oxygen equilibrium properties of the Fe subunit in these hybrid hemoglobins were specifically determined. K1 values, namely the equilibrium constants for the first oxygen molecule to bind to hemoglobin, agreed well for these hybrid hemoglobins with the K1 value of native hemoglobin A in various conditions. Therefore, Ni(II) protoporphyrin IX in these hybrid hemoglobins behaves like a permanently deoxygenated heme. Both NeFe hybrid hemoglobins bound oxygen non-co-operatively at low pH values. When the pH was raised, α2(Fe)β2(Ni) showed co-operativity, but the complementary hybrid, α2(Ni)β2(Fe), did not show co-operativity even at pH 8.5. The light absorption spectra of Ni(II)Fe(II) hybrid hemoglobins indicated that the co-ordination states of Ni(II) protoporphyrin IX in the a subunits responded to the structure of the hybrid, whereas those in the β subunits were hardly changed. In a deoxy-like structure (the structure that looks like that observed in deoxyhemoglobin), four-co-ordinated Ni(II) protoporphyrin IX was dominant in the α(Ni) subunits, while under the conditions that stabilized an oxy-like structure (the structure that looks like that observed in oxyhemoglobin), five-co-ordinated Ni(II) protoporphyrin IX increased. The small change observed in the absorption spectrum of the β(Ni) subunits is not related to the change of the co-ordination number of Ni(II) protoporphyrin IX. Non-co-operative binding of oxygen to the β subunits in α2(Ni)β2(Fe) accompanied the change of absorption spectrum in the α(Ni) subunits. We propose a possible interpretation of this unique feature.