Cobalt hemoglobin: pH dependence of Adair constants and the Bohr effects

Precise oxygen equilibrium curves have been obtained for cobalt hemoglobin at pH values from 5.5 to 8.2. The Hill plots are symmetric having asymptotes with slopes of unity. At pH 7.0, cobalt hemoglobin has p0.5 = 116 toor (15.45 kPa), pm = 117 torr (15.58 kPa) and a Hill coefficient of n = 1.72. The values of n decrease slightly with either decrease or increase of pH; the protein is almost non-cooperative at pH greater than 8.2. The Adair constants have been calculated with a non-linear least-squares program. From deltalnpm/deltapH a maximum of 2.5 Bohr protons was calculated at physiological pH values. The majority of alkaline Bohr protons are released after binding of the first and the third oxygen with maxima at pH 7.6 and 7.3, respectively. The acid Bohr effect was also observed with the majority of the protons taken up following the first and third oxygen bound. Smaller alkaline Bohr effects were obtained by differential titration and at higher pH than that calculated from oxygen equilibria. The discrepancy can be largely attributed to the binding of salt components to cobalt hemoglobin.