Keys and Locks
Cells carry receptors that react when the right molecule fits. Move the body’s own substance and a made-up drug, and watch the fight for the locks and how the response changes.
Blocker: fits but gives no signal
Body substance: 33%
Drug C: 0%
Empty: 67%
Body’s own substance
Kd: Body substance = 2, Drug C = 0.5. Kd measures how hard it is to stick: smaller means it sticks more easily (higher affinity).
Calculation: share filled = (amount / Kd) / (1 + sum of every amount / Kd); response = sum of share x signal strength
“Key and lock” is an analogy. Real receptors are thought to be softer: they change shape a little as a molecule fits. This figure covers only the competing case.
How to play
Press Auto to watch the scenes run through the body’s own substance and Drugs A, B and C. Stop at any time and move the sliders yourself. Pick Drug A (agonist), B (partial agonist) or C (blocker). Every name and number is made up.
Learn more
A receptor is a protein or similar structure on a cell; when a particular molecule (a ligand) binds, the cell responds. A drug that binds without triggering a response and gets in the way of others is an antagonist; one that triggers a response is an agonist. How readily a molecule sticks (affinity) and how strongly it triggers a response afterwards are separate properties. The receptor idea is said to have been developed in the early 1900s by different researchers working independently. “Key and lock” was an analogy proposed for enzymes in the 1890s; later, the “induced fit” idea suggested that shapes adjust a little as binding happens.