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 protohe mes (Fe) in either the alpha- or beta-subunits were substituted with p rotoporphyrins IX (PP) alpha(PP)2beta(Fe)2 and alpha(Fe)2beta(PP)2 hav e been prepared. The structural and functional properties of these hyb rid Hbs were investigated by measuring oxygen equilibrium curves and p roton nuclear magnetic resonance spectra. The equilibrium constants of the first ligand, K1, observed for alpha(PP)2beta(Fe)2 were much smal ler than K1 values of HbA. The effects of pH and inositol hexaphosphat e on K1 were substantially diminished. On the other hand, K1 values of alpha(Fe)2beta(PP)2 were similar to those of HbA, including the pH an d inositol hexaphosphate effects. The deoxy forms of alpha(PP)2beta(Fe )2 and alpha(Fe)2beta(PP)2 showed exchangeable proton resonances at 11 and 14 parts/million arising from the hydrogen bonds at the alpha1bet a2 contact in a deoxy-like structure. In the liganded form, these sign als were dependent upon solution conditions. As K1 became larger, the reduction in the intensity of these signals was observed for both liga nded forms. The resonance position of E11 Val originating from the bet a subunits of alpha(PP)2beta(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 m ay be linked to the metal-proximal His interaction in the a subunits. However, the first oxygen binding to the a heme is linked minimally to the metal-proximal His interaction in the beta subunits but may be co rrelated instead to the position of E11 Val relative to the porphyrin plane in the beta subunits.