Significance of Affinity and Cooperativity in Oxygen Binding to Hemoglobin of Horse Fetal and Maternal Blood

Abstract

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 relationship between evolutionary and physiological variation in hemoglobin

“…It should be pointed out that the saturation at some other value of the partial pressure of oxygen, e.g., p 20 , might be optimized for oxygen unloading, and this value would be tunable both by changes in n and p 50 , or specifically, changes in L T0R4 and L 4 . The exact optimization would be hard to predict and will presumably include other fitness advantages, such as the transport of oxygen to the fetus (32)(33)(34). We still lack a full understanding of the importance for the phenotype of the evolutionary changes in the cooperativity observed in different mammals.…”