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.