The porphyrin-iron hybrid hemoglobins. Absence of the Fe-His bonds in one type of subunits favors a deoxy-like structure with low oxygen affinity.

Protoporphyrin-protoheme hybrid hemoglobins (Hb), in which the protohemes (Fe) in either the alpha- or beta-subunits were substituted with protoporphyrins IX (PP) alpha(PP)2 beta(Fe)2 and alpha(Fe)2 beta(PP)2 have been prepared. The structural and functional properties of these hybrid Hbs were investigated by measuring oxygen equilibrium curves and proton nuclear magnetic resonance spectra. The equilibrium constants of the first ligand, K1, observed for alpha(PP)2 beta(Fe)2 were much smaller than K1 values of HbA. The effects of pH and inositol hexaphosphate on K1 were substantially diminished. On the other hand, K1 values of alpha(Fe)2 beta(PP)2 were similar to those of HbA, including the pH and inositol hexaphosphate effects. The deoxy forms of alpha(PP)2 beta(Fe)2 and alpha(Fe)2 beta(PP)2 showed exchangeable proton resonances at 11 and 14 parts/million arising from the hydrogen bonds at the alpha 1 beta 2 contact in a deoxy-like structure. In the liganded form, these signals were dependent upon solution conditions. As K1 became larger, the reduction in the intensity of these signals was observed for both liganded forms. The resonance position of E11 Val originating from the beta subunits of alpha(PP)2 beta(Fe-CO)2 also varied in accordance with K1. We compare properties of PP-Fe hybrids with those of Co-Fe and Ni-Fe hybrids and conclude that the first oxygen binding to the beta heme may be linked to the metal-proximal His interaction in the alpha subunits. However, the first oxygen binding to the alpha heme is linked minimally to the metal-proximal His interaction in the beta subunits but may be correlated instead to the position of E11 Val relative to the porphyrin plane in the beta subunits.