Structural Biochemistry/Protein function/Concerted Model
The Concerted Model, also known as MWC model or symmetry model, of hemoglobin is used to explain the cooperativity in oxygen binding as well as the transitions of proteins which made up of identical subunits. It focuses on the two states of the Hemoglobin; the T and R states. The T state of the hemoglobin is more tense as it is in the deoxyhemoglobin form while the R state of the hemoglobin is more relaxed as it is in the oxyhemglobin form. T state is constrained due to the subunit-subunit interactions while the R state is more flexible due to the ability of oxygen binding. The difference in deoxyhemoglobin and oxyhemoglobin is that the "deoxy" form doesn't have Oxygen and the "oxy" form is highly oxygen bounded. The binding of oxygen at one site increases the binding affinity in other active sites. Thus in the concerted model of the hemoglobin, it shows that the one oxygen binding to an active site will increase the probability of other oxygen binding to the other active sites in the hemoglobin. Overall, oxygen binding shifts the equilibrium toward the R state. This means that at high oxygen levels, the R form will be prevalent and at lower oxygen levels, the T form will be prevalent. Allosteric effectors of hemoglobin, such as 2,3-BPG, function by shifting the equilibrium towards or away from the T-state, depends on whether it's an inhibitor or a promoter. This model and the sequential model displays the extreme cases of R and T transitions. In a real system, properties from both models are needed to explain the behavior of hemoglobin.
Molecules can exist either in the T (tense) state, or R (relaxed) state. The T-state is the deoxy form of hemoglobin and the R-State is the fully oxygenated form. Equilibrium is shifted between both states. When the model has high oxygen affinity, it means that it is highly R state favored; and hemoglobin is T state favored when it has no bounded oxygen. In the concerted model, all oxygen binding sites on Hemoglobin in the T state must be filled before the molecule converts to the R state. This is also true in the conversion of the R state to the T state- all bounded oxygen must be released before a full conversion can take place.