text: The interaction of ligands with their binding sites can be characterized in terms of a binding affinity. In general, high-affinity ligand binding results from greater attractive forces between the ligand and its receptor while low-affinity ligand binding involves less attractive force. In general, high-affinity binding results in a higher occupancy of the receptor by its ligand than is the case for low-affinity binding; the residence time (lifetime of the receptor-ligand complex) does not correlate. High-affinity binding of ligands to receptors is often physiologically important when some of the binding energy can be used to cause a conformational change in the receptor, resulting in altered behavior for example of an associated ion channel or enzyme.
  A ligand that can bind to and alter the function of the receptor that triggers a physiological response is called a receptor agonist. Ligands that bind to a receptor but fail to activate the physiological response are receptor antagonists.
  Agonist binding to a receptor can be characterized both in terms of how much physiological response can be triggered (that is, the efficacy) and in terms of the concentration of the agonist that is required to produce the physiological response (often measured as EC, the concentration required to produce the half-maximal response). High-affinity ligand binding implies that a relatively low concentration of a ligand is adequate to maximally occupy a ligand-binding site and trigger a physiological response. Receptor affinity is measured by an inhibition constant or K value, the concentration required to occupy 50% of the receptor.  Ligand affinities are most often measured indirectly as an IC value from a competition binding experiment where the concentration of a ligand required to displace 50% of a fixed concentration of reference ligand is determined. The K value can be estimated from IC through the Cheng Prusoff equation.  Ligand affinities can also be measured directly as a dissociation constant (K) using methods such as fluorescence quenching, isothermal titration calorimetry or surface plasmon resonance.
  Low-affinity binding (high K level) implies that a relatively high concentration of a ligand is required before the binding site is maximally occupied and the maximum physiological response to the ligand is achieved. In the example shown to the right, two different ligands bind to the same receptor binding site. Only one of the agonists shown can maximally stimulate the receptor and, thus, can be defined as a full agonist. An agonist that can only partially activate the physiological response is called a partial agonist. In this example, the concentration at which the full agonist (red curve) can half-maximally activate the receptor is about 5 x 10 Molar (nM = nanomolar).
  Binding affinity is most commonly determined using a radiolabeled ligand, known as a tagged ligand. Homologous competitive binding experiments involve binding competition between a tagged ligand and an untagged ligand.
  Real-time based methods, which are often label-free, such as surface plasmon resonance, dual-polarization interferometry and multi-parametric surface plasmon resonance (MP-SPR) can not only quantify the affinity from concentration based assays; but also from the kinetics of association and dissociation, and in the later cases, the conformational change induced upon binding. MP-SPR also enables measurements in high saline dissociation buffers thanks to a unique optical setup. Microscale thermophoresis (MST), an immobilization-free method was developed. This method allows the determination of the binding affinity without any limitation to the ligand's molecular weight.
  For the use of statistical mechanics in a quantitative study of the
  ligand-receptor binding affinity, see the comprehensive article
  on the configurational partition function.