Fractional saturation in case of n binding sites

[Andrey]

I have a confusion with terminology, which should be easy to resolve I guess..

 

 

What does term "fractional saturation" mean in the case when a receptor has more then 1 binding site? (let them be equivalent and independent).

 

By definition, it is the "fraction of the protein molecules that are saturated with ligand", so in case of one site it is just Y=[PL]/([P]+[PL]).

 

But when it comes to more binding sites, I am confused. Lets say there are 2 binding sites. Then, fractional saturation is either, following the definition, [PLL]/([P]+[PL]+[PLL]), or something like ([PL]+2[PLL])/2([P]+[PL]+[PLL]), meaning that it is a fraction of binding sites, saturated by the ligand.

 

If anyone can clarify this for me, I would be very grateful.

 

Thanks!