How Drugs Work: Receptors and Enzymes
Most drugs act through one of two mechanisms: they either bind to a receptor or they inhibit an enzyme. Understanding this distinction makes the rest of pharmacology more predictable.
Receptor agonists and antagonists
A receptor is a protein on or in a cell that, when activated, triggers a response. An agonist binds and activates it; an antagonist binds and blocks it. Beta-2 agonists like salbutamol open airways by activating beta-2 receptors; beta-blockers like atenolol slow the heart by blocking them.
The key question for any receptor drug: does it turn the receptor on (agonist) or off (antagonist)? That single distinction explains most of the drug's effects.
Enzyme inhibitors
Some drugs work by blocking an enzyme rather than a receptor. ACE inhibitors like lisinopril block the enzyme that converts angiotensin I to II, lowering blood pressure. Statins block HMG-CoA reductase, reducing cholesterol synthesis. The pattern is: identify the enzyme, block it, and the downstream effect follows.
Enzyme inhibitors are often named with '-inhibitor' in the class, but the key is knowing which enzyme and what the blockage achieves.
Practical example
ACE inhibitors vs beta-blockers
A patient with hypertension could receive either an ACE inhibitor (enzyme block) or a beta-blocker (receptor block). Both lower blood pressure but through entirely different mechanisms. Knowing which mechanism explains why their side effects differ: ACE inhibitors cause dry cough; beta-blockers cause fatigue and bradycardia.
Key takeaways
- Drugs act mainly through receptors (agonist/antagonist) or enzymes (inhibitor).
- Agonists activate, antagonists block — that distinction explains most effects.
- Enzyme inhibitors block a specific enzyme; the downstream effect follows.
- Knowing the mechanism predicts the side effects and interactions.
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