Two questions, never one
Last time we left a ligand sitting in a binding site with two questions hanging over it. Here we answer them, because keeping them apart is the single most clarifying idea in pharmacodynamics. The first question — *how tightly does it bind?* — is affinity. The second — *once bound, how strongly does it activate the receptor?* — is efficacy. They are independent. A molecule can be a champion gripper and a terrible activator, or vice versa.
Measuring the grip: Kd
Affinity gets a number called the dissociation constant (Kd). It is the concentration of drug at which half the receptors are occupied at equilibrium. The trick that trips up everyone at first is that Kd runs *backwards*: a low Kd means high affinity. If only a tiny concentration is needed to fill half the receptors, the drug must be clinging very tightly.
Drug A: Kd = 1 nM -> needs 1 nM to occupy half the receptors -> high affinity Drug B: Kd = 1000 nM -> needs 1000 nM for the same -> low affinity Lower Kd = tighter grip = higher affinity Rule: at a concentration equal to Kd, receptor occupancy = 50%
Measuring the voice: intrinsic activity
Efficacy is captured by intrinsic activity, a scale from 0 to 1. A full agonist sits at 1 — bound receptors are switched fully on, and at high enough doses it reaches the maximum possible response, the Emax. A pure antagonist sits at 0 — it binds beautifully but produces no signal at all, like a key that fits the lock yet refuses to turn. In between lives the partial agonist, with intrinsic activity somewhere around 0.2 to 0.8.
This is why the affinity/efficacy split matters clinically. An antagonist needs *high affinity but zero efficacy* — grip the receptor hard so nothing else can, but stay silent. A useful agonist needs *both* a decent grip and a real voice. Separating the two lets you read any drug as a pair of dials.