Relationships between the slope of the oxygen equilibrium curve and the cooperativity of hemoglobin as analyzed using a normalized oxygen pressure scale.

The oxygen equilibrium curve (OEC) for hemoglobin, which is usually expressed as a S vs. P plot, was expressed by normalizing P by Pdmax, where S is oxygen saturation, P is partial pressure of oxygen, and Pdmax is P at which the slope of the usual OEC is maximized. The maximal slope of normalized OEC gives Pdmax.S'max, where S'max is the maximal slope of the usual OEC, a measure for the oxygen transport efficiency of hemoglobin. Here, the term "efficiency" is used in the sense that the oxygen release from hemoglobin becomes more sensitive to oxygen pressure changes as S'max becomes larger. An analysis using 38 sets of published oxygen equilibrium data for human adult hemoglobin under various experimental conditions showed that (a) expressing OEC by means of S vs. P/Pdmax or S vs. log P is advantageous for analyzing the slope of OEC compared to usual S vs. P plot and (b) while the OEC differs depending on experimental conditions, S'max varies in close linear correlation to nmax (the maximal slope of the Hill plot which measures oxygen binding cooperativity), and Pdmax.S'max is almost equal to nmax/4. Thus, the parameter expressing oxygen transport efficiency is closely related to the parameter expressing oxygen binding cooperativity.