Hemoglobin cooperative nature

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... 

The situation for Fe11 porphyrin is rather similar, with the existence of 5 = 0, 1 and 2 states. The iron in Fe(TPP)(THF)2 lies in the plane of the porphyrin.647 This result is of considerable relevance to model systems for hemoglobin and the question of the trigger for the conformational change associated with the cooperative uptake of dioxygen. In this example the presence of a symmetrical weak axial field is the crucial factor. While this particular situation probably does not occur in any natural hemoprotein, these results demonstrate conclusively that high-spin Fe11 will fit into the porphyrin plane. 

Fig. 3. Diagrammatic sketch showing the change in tertiary structure of a hemoglobin a chain on reaction with oxygen. Movement of the iron atom into the plane of the porphyrin ring causes a movement of helix F toward helix H, which expels tyrosine in position 140 from its pocket between the two helices. From (PIO), M. F. Perutz, Stereochemistry of cooperative effects in haemoglobin. Nature (London) 228, 726 (1970) with permission of the author and publisher.

M. F. Perutz Stereochemistry of cooperative effects of hemoglobin. Nature 228,726 (1970). 

Perutz, M.F., 1970, Stereochemistry of cooperative effects in hemoglobin. Nature, 228 726. 

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