This KNF (Koshland, Nemethy, and Filmer) Sequential Model:model was developed to address concerns with the concerted model. One of the major problems with the concerted model is that it seemed unrealistic to expect all the subunits to change conformation together. Why shouldn't there be some difference in subunit conformation? The KNF model also fits the experimental data well.
Figure: EQUILIBRIA IN KNF MODEL