Receptor binding: the basics

Affinity

Affinity describes how tightly a molecule associates with a receptor, usually expressed as a dissociation constant. A lower value means tighter binding. It is measured in a defined system, and values from different systems are not directly comparable.

Selectivity

Selectivity is affinity for one target relative to others. Few molecules bind only one thing; selectivity is a ratio, not an absolute. A peptide described as selective is selective by some margin, and the margin is the informative part.

What binding does not tell you

Binding is not activation. A molecule can occupy a receptor without triggering it, or trigger it partially. Nor does binding data describe what happens in a whole organism, where distribution, metabolism and clearance all intervene.

This is why binding affinity, potency in a cell assay, and effect in an animal model are three different measurements that frequently rank compounds differently.