Oxygen sigmoid, binding

Hemoglobin (Hb) behaves differently. Its sigmoidal binding curve can be fitted by an association-constant expression with a greater-than-first-power dependence on the oxygen concentration ... 

Figure 7.12 T-to-R transition. The observed binding curve for hemoglobin can be seen as a combination of the binding curves that would be observed if all molecules remained in the T state or if a l of the molecules were in the R state. The sigmoidal curve is observed because molecules convert from the T state into the R state as oxygen molecules bind.

We can determine quantitatively the physiological significance of the sigmoid nature of the hemoglobin oxygen-binding curve, or, in other words, the biological importance of cooperativity. The equation... 

Fig. 3. Oxygen transport by perfluorocarbons versus hemoglobin (Hb) (a) In the case of PFCs, O2 dissolution is characterized by loose, nondirectional van der Waals interactions. Oxygen solubility follows Henry s law, that is, is directly proportional to the gas s partial pressure (curve c). (b) In the case of Hb, a strong, localized chemical bond is established with the iron atom of a heme. Successive binding of four O2 molecules to the four hemes of Hb is cooperative, and saturation occurs when all four iron atoms are bound. Hence, the sigmoid shape of the O2 uptake curve, which levels off when the partial pressure of O2 on earth is attained (curve d) [20].

Thus, for hemoglobin (Hb) the oxygen binding curves are sigmoidal as is shown in the above figure. The fact that n exceeds the unit can be ascribed physically to the fact that attachment of O2 to one heme group... 

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