Hemoglobin with concerted model

Figure 9 Two proposed models for allosteric interaction observed in hemoglobin, (a) A sequential model of allosteric interaction. When one O2 molecule binds to a subunit in the T-state, the subunit shifts to R-state and affects the affinity of neighboring subunits for the ligand binding, (b) An MWC two-state concerted model of allosteric interaction. When one O2 molecule binds to a subunit in all T-states, a concerted conformational change occurs to produce a protein with all R-state subimits. Square and circle represent T- and R-state subunits, respectively. Moreover, opened and closed diagrams represent free and O2 bound subunits, respectively...

What major differences exist between the sequential and concerted models for allostery in accounting for hemoglobin that is partially saturated with oxygen ... 

It should be emphasized that the concerted and sequential models are theoretical models, that is, the behavior of many allosteric proteins appears to be more complex than can be accounted for by either model. For example, the cooperative binding of Oz by hemoglobin (the most thoroughly researched allosteric protein) appears to exhibit features of both models. The binding of the first 02 initiates a concerted T — R transition that involves small changes in the conformation of each subunit (a feature of the sequential model). In addition, hemoglobin species with only one or two bound 02 have been observed. 

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