A mathematical model is proposed which explains some biphasic saturations of Binding Proteins by their substrates through an effect of a competitive inhibition. The inhibitor can be the substrate itself especially when the retention phenomenon is occuring. This model has been verified with two periplasmic Binding Proteins of Escherichia coli: the Glutamine Binding Protein and the Leucine-Isoleucine-Valine Binding Protein. A significant connection is found between experimental results and the hypothesis.