3.1 Drug-Receptor Interactions
Drug-receptor interactions are the molecular basis of drug action. Understanding binding kinetics and receptor properties is essential for rational drug design.
Receptor Types
G-Protein Coupled (GPCRs)
7-transmembrane domain. Signal via G-proteins. ~30% of drug targets. Seconds to minutes response.
Ion Channels
Ligand-gated or voltage-gated. Millisecond response. Nicotinic ACh, GABA-A, glutamate receptors.
Enzyme-Linked
Receptor tyrosine kinases. Insulin, growth factors. Minutes to hours response.
Nuclear Receptors
Transcription factors. Steroids, thyroid hormone. Hours to days response.
Binding Equilibrium
\( [D] + [R] \rightleftharpoons [DR] \)
\( K_d = \frac{[D][R]}{[DR]} = \frac{k_{off}}{k_{on}} \)
Kd = dissociation constant. Lower Kd = higher affinity. Concentration at 50% receptor occupancy.
Key Concepts
Affinity
Strength of drug-receptor binding. Measured by Kd.
Efficacy
Ability to activate receptor. Intrinsic activity (0-1).
Potency
Drug concentration for given effect. EC50 or ED50.
Selectivity
Preference for one receptor over others.