Body and drug basics
Binding affinity
Last updated
Binding affinity describes how tightly a molecule sticks to its receptor, and therefore how little of it needs to be around for a given share of receptors to be occupied. It is reported as a number from laboratory tests, and the convention is counter-intuitive: a smaller number generally means stronger binding. Affinity describes how well something attaches, and nothing about what follows from attaching.
Why it matters here
Affinity figures are among the most quoted numbers in peptide marketing, and they come from tests in dishes rather than from people. A tight-binding molecule that never reaches the tissue in question achieves nothing on the strength of its affinity alone.
What it is not
Affinity is not potency and not effectiveness. A molecule can bind extremely tightly and still switch the receptor on weakly or not at all, which is precisely what an antagonist does. Comparing affinity numbers taken from two different studies is also unreliable, because the value depends on the test conditions used to produce it.
Also called
- binding affinity
- binding-affinity
- affinity
- kd
- ki
- ic50
- ec50
- receptor affinity
Usually written Kd or Ki, and quoted alongside IC50 or EC50, which are related laboratory measures of the concentration needed to reach half of the maximum blocking or maximum response.
Compounds carrying this term
None on file yet. This term is in the controlled vocabulary but no compound record currently uses it — which is a statement about our coverage, not about the term.