Protein , association binding equilibria

A protein, MW 35,000, at a concentration of 0.5 mg/mL, is dialyzed against a ligand at an initial concentration of 4 x 10 5M. At equilibrium, the ligand concentration in the protein compartment is 2.3 x 10 5M, and it is 1.7 x 10"5M in the initial compartment, which contains no protein. Calculate the association constant for the interaction. Assume a single protein-ligand binding site. 

While a number of proteins have been crystallized in this manner, the majority of studies have focused on a robust system comprising the tetrameric protein streptavidin and the vitamin biotin. The choice of this system is primcirily motivated by the strong bond between biotin and streptavidin (having an association equilibrium constant, Ka Tbe binding properties were recently... 

Thermodynamically it would be expected that a ligand may not have identical affinity for both receptor conformations. This was an assumption in early formulations of conformational selection. For example, differential affinity for protein conformations was proposed for oxygen binding to hemoglobin [17] and for choline derivatives and nicotinic receptors [18]. Furthermore, assume that these conformations exist in an equilibrium defined by an allosteric constant L (defined as [Ra]/[R-i]) and that a ligand [A] has affinity for both conformations defined by equilibrium association constants Ka and aKa, respectively, for the inactive and active states ... 

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