Studies on cobalt myoglobins and hemoglobins. X. Determination of microscopic oxygen-equilibrium constants of iron-cobalt hybrid hemoglobins and their parent hemoglobins

Oxygen equilibrium curves of tetrameric hemoglobin, α(Fe)₂β(Co)₂, carrying ferrous protoporphyrin in the α subunits and cobaltous protoporphyrin in the β subunits, its complementary form α(Co)₂β(Fe)₂ and their parent hemoglobins, i.e. Fe-hemoglobin (α(Fe)₂β(Fe)₂) and Co-hemoglobin (α(Co)₂β(Co)₂), were measured at different pH values between 6·5 and 7·9 by the automatic recording method. Each hybrid hemoglobin gave two equilibrium curves, one for Fe-containing subunits and the other for Co-containing subunits, which were measured at the isosbestic points of Co-porphyrin and Fe-porphyrin, respectively. Generalized Adair equations in which the α and β subunits were treated as non-equivalent were fitted simultaneously to the six equilibrium curves by a least-squares method to estimate the values of 12 microscopic equilibrium constants, eight of which are independent. It was assumed that interactions between the subunits depend on their ligation state but not on the kind of metal carried by them. From these parameter values the oxygen saturation of the α and β subunits were calculated as a function of average saturation and principal pathways of oxygenation on the microscopic oxygenation scheme were derived. In Fe-hemoglobin the β subunits have higher oxygen affinities than the α subunits, the difference becoming smaller as pH decreases. In Co-hemoglobin the affinity difference is slight, the α subunits having somewhat higher affinities. The present results on the affinity difference between subunits were compared with earlier conclusions drawn from other kinds of experiment. The results obtained so far are not mutually consistent and inherent problems are discussed.