Determining the pharmacophore of a group of isosteres

[mycotheology]

Heres the question:
http://img442.imageshack.us/img442/484/sarr.png
Firstly does anyone know what Ki is? Ionisation constant? I'm guessing its proportional to the drugs potency. I could take a good guess by observing the differences between each isostere but I'm guessing there's some kind of method I should be able to use here.

 

[Ygggdrasil]

In biochemistry, K_I is the binding constant for an inhibitor. Put more precisely, in the binding interaction between an inhibitor I and enzyme E, which form enzyme-inhibitor complex EI:

E + I <--> EI

K_I is the equilibrium dissociation constant for the binding reaction:

K_I = \frac{[E]<i>}{[EI]}</i>

Therefore, more potent inhibitor have lower K_I values.