Appendix 1.2A Equilibrium, Dissociation, and Affinity Constants

Confusingly, all of these terms are in current use to express the position of the equilibrium between a ligand and its receptors. The choice arises because the ratio of the rate constants k and k can be expressed either way up. In this chapter, we take KA to be k, kA and it is then strictly a dissociation equilibrium constant, often abbreviated to either dissociation constant or equilibrium constant. The inverse ratio, k+l k x, gives the association equilibrium constant, which is usually referred to as the affinity constant. 

One way to reduce the risk of confusion is to express ligand concentrations in terms of KA. This normalized concentration is defined as [A IKA and will be denoted here by the symbol eA. We can therefore write the Hill-Langmuir equation in three different though equivalent ways  

2 Appendix 1.2B Step-by-Step Derivation of the Hill-Langmuir Equation 

The Hill-Langmuir equation may be rearranged by cross-multiplying  

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