Term information
The dissociation constant is commonly used to describe the affinity between a ligand (L) (such as a drug) and a protein (P) i.e. how tightly a ligand binds to a particular protein. Ligand-protein affinities are influenced by non-covalent intermolecular interactions between the two molecules such as hydrogen bonding, electrostatic interactions, hydrophobic and Van der Waals forces. They can also be affected by high concentrations of other macromolecules, which causes macromolecular crowding.[1][2] The formation of a ligand-protein complex (C) can be described by a two-state process \mathrm{C} \rightleftharpoons \mathrm{P} + \mathrm{L} the corresponding dissociation constant is defined K_{d} = \frac{\left[ \mathrm{P} \right] \left[ \mathrm{L} \right]}{\left[ \mathrm{C} \right]} where [P], [L] and [C] represent molar concentrations of the protein, ligand and complex, respectively. source: http://en.wikipedia.org/wiki/Affinity_%28pharmacology%29#Protein-ligand_binding