Proton nuclear magnetic resonance and spectrophotometric studies of nickel(II)-iron(II) hybrid hemoglobins.

Abstract

Ni(II)-Fe(II) hybrid hemoglobins, alpha(Fe)2 beta(Ni)2 and alpha(Ni)2 beta(Fe)2 have been characterized by proton nuclear magnetic resonance with Ni(II) protoporphyrin IX (Ni-PP) incorporated in apoprotein, which serves as a permanent deoxyheme. alpha(Fe)2 beta(Ni)2, alpha(Ni)2 beta(Fe)2, and NiHb commonly show exchangeable proton resonances at 11 and 14 ppm, due to hydrogen-bonded protons in a deoxy-like structure. Upon binding of carbon monoxide (CO) to alpha(Fe)2 beta(Ni)2, these resonances disappear at pH 6.5 to pH 8.5. On the other hand, the complementary hybrid alpha(Ni)2 beta(Fe-CO)2 showed the 11 and 14 ppm resonances at low pH. Upon raising pH, the intensities of both resonances are reduced, although these changes are not synchronized. Electronic absorption spectra and hyperfine-shifted proton resonances indicate that the ligation of CO in the beta(Fe) subunits induced changes in the coordination and spin states of Ni-PP in the alpha subunits. In a deoxy-like structure, the coordination of Ni-PP in the alpha subunits is predominantly in a low-spin (S = 0) four-coordination state, whereas in an oxy-like structure the contribution of a high-spin (S = 1) five-coordination state markedly increased. Ni-PP in the beta subunits always takes a high-spin five-coordination state regardless of solution conditions and the state of ligation in the partner alpha(Fe) subunits. In the beta(Ni) subunits, a significant downfield shift of the proximal histidyl N delta H resonance and a change in the absorption spectrum of Ni-PP were detected, upon changing the quaternary structure of the hybrid.(ABSTRACT TRUNCATED AT 250 WORDS)