Ligand (biochemistry)

Receptor/Ligand binding affinity

The interaction of most ligands with their binding sites can be characterized in terms of a binding affinity. In general, high affinity ligand binding results from greater intermolecular force between the ligand and its receptor while low affinity ligand binding involves less intermolecular force between the ligand and its receptor. In general, high affinity binding involves a longer residence time for the ligand at its receptor binding site than is the case for low affinity binding. 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 of an associated enzyme.

  A ligand that can bind to a receptor, alter the function of the receptor and trigger a physiological response is called an Molar (nM = nanomolar).  

In the example shown to the left, ligand binding curves are shown for two ligands with different binding affinities. Ligand binding is often characterized in terms of the concentration of ligand at which half of the receptor binding sites are occupied (k_d). The ligand illustrated by the red curve has a higher binding affinity and smaller k_d than the ligand illustrated by the green curve. If these two ligands were present at the same time, more of the higher affinity ligand would be bound to the available receptor binding sites. This is how carbon monoxide poisoning.

Selective and non-selective

Selective ligands have a tendency to bind to a very limited types of receptors, while non-selective ligands bind to several types of receptors. This plays an important role in adverse effects, because they bind to several other receptors in addition to the one generating the desired effect.

See also

• Schild regression
• Allosteric regulation

References

• Ligand binding to hormone receptors in Endocrinology: An Integrated Approach by Stephen Nussey and Saffron Whitehead (2001) Published by BIOS Scientific Publishers Ltd. ISBN 1-85996-252-1.
• Molecular Recognition Processes in Molecular Biology of the Cell 3rd edition (1994) by Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts and James D. Watson. See Figure 3-9, Equilibrium ligand binding.