Influence of allosteric effectors and temperature on oxygen binding properties and the Bohr effect of bovine hemoglobin.

The O2 binding properties of bovine Hb were examined. The increase in Cl- and DPG concentration enhanced P50. A reduction in n(max) was observed at high Cl- concentration, while DPG had little effect on n(max). An increase in Cl- concentration enhanced the Bohr effect, the magnitude of which reached a maximum at 0.1 M Cl- and 20 degrees C. This concentration is nearly equal to that at the highest slope of the log P50 vs. log [Cl-] plot, and also equal to the physiological Cl- concentration (0.1 M) of bovine blood. Furthermore, the influence of Cl- concentration on the Bohr effect is independent of temperature. On the other hand, in the absence of Cl-, bovine Hb is sensitive to DPG; an increase in DPG concentration enhanced the Bohr effect, which reached a maximum at 3 mM DPG and 20 degrees C. This concentration is nearly equal to that at the highest slope of the log P50 vs. log [DPG] plot. At low DPG concentrations, the DPG effect on the Bohr effect became small with increasing temperature, whereas at high DPG concentrations, the DPG effect was insensitive to temperature changes. At the physiological concentration of DPG (0.5 mM), increases in both Cl- concentration and temperature diminished the DPG effect. At the physiological concentrations of Cl- and DPG, the Bohr effect was -0.36 at 37 degrees C. The deltaH value at the physiological concentrations of Cl- and DPG was approximately -5.8 kcal/mol at pH 7.4. These results indicate that Cl- and temperature are important determinants of the O2 binding properties of bovine Hb.

Contribution of cooperativity and the Bohr effect to efficient oxygen transport by hemoglobins from five mammalian species.

By using published experimental values of the standard oxygen (O2) equilibrium curve and the in vivo arterial and venous O2 pressure (PO2) of fetal and maternal blood in five mammalian species (human, cow, pig, sheep, and horse), we investigated the relationship between the efficiency of O2 delivery and the effectiveness of the Bohr shift, and discussed the significance of cooperativity for mammalian Hb. The O2 delivery of fetal blood was more efficient than that of maternal blood, and the effectiveness of the Bohr shift at both O2 loading and release sites of fetal blood was high. A linear relationship was observed between the efficiency of O2 delivery and the effectiveness of the Bohr shift at O2 loading sites of the five mammalian species. In both fetal and maternal blood, the theoretically obtained optimal P50 value for O2 delivery (optP50(OD)) was nearly equal to the optimal P50 value for the effectiveness of the Bohr shift at the O2 loading site (optP50(BS)(loading)). This phenomenon was favorable for fetal blood to uptake O2 from maternal blood with the aid of the Bohr shift and to deliver a large amount of O2 to the tissues. The optP50s for the effectiveness of the Bohr shift at given arterial PO2 (PaO2) and venous PO2 (PvO2) were derived as follows: optP50(BS)(loading) = PaO2((n+1)/(n-1))(1/n), and optP50(BS)(release) = PvO2((n+1)/(n-1))(1/n). The relationship between in vivo PO2s and n, PaO2/PvO2 = ((n+1)/(n-1))(2/n), was derived by letting optP50 for the efficiency of O2 delivery be equal to that for the effectiveness of the Bohr shift.

Significance of affinity and cooperativity in oxygen binding to hemoglobin of horse fetal and maternal blood.

The physiological significance of the position and shape of the oxygen equilibrium curve (OEC) of horse hemoglobin (Hb) is considered from the viewpoint of oxygen (O2) transport efficiency and the effectiveness of the Bohr effect. In horse fetal and maternal bloods, their physiological O2 affinities are nearly optimized with respect to the effectiveness of the Bohr shift occurring at the O2 release site, when it is measured by the change in O2 saturation per unit change in P50. With relatively low cooperativity (n=2.69) of horse Hb under physiological conditions, the effectiveness of the Bohr shift for fetal blood at O2 uptake site and maternal blood at O2 release site is high. These facts imply that the position and the cooperativity of horse Hb OEC are optimized to receive maximal benefit from the double Bohr shift. Before exercise, the position of the OEC for adult mares is nearly optimized for the effectiveness of the Bohr shift occurring at the O2 release site, whereas, at maximal exercise, the position of the OEC tends to become advantageous for O2 transport efficiency.

The cooperativity of human fetal and adult hemoglobins is optimized: a consideration based on the effectiveness of the Bohr shift.

The physiological significance of the cooperativity of human hemoglobin (Hb) is considered from the viewpoint of the effectiveness of the Bohr shift at the sites of O(2) release and uptake across the placental membrane. The effects of the Bohr shift was examined by changing the O(2) saturation of Hb (S(pO2)) per unit change in P(50), -dS(PO2)/d P(50), where P(50) is partial pressure of O(2) at half saturation. The Bohr shift at the sites of O(2) uptake and release was found to be highly effective in both fetal and maternal bloods at physiological degree of cooperativity (Hill's coefficient, n=2.65). From the results obtained in this paper, it is concluded that the positions of OECs of fetal and maternal Hbs are regulated to receive a maximal benefit from the Bohr shift, and that a relatively low n value of human tetrameric Hb is adequate for the O(2) and CO(2) exchange across the placental membrane.