proton-pump inhibitor
Your stomach lining has tiny pumps that push acid into the stomach, the very last step in making it sour enough to digest food. A proton-pump inhibitor disables those pumps directly, so even when the body shouts 'make acid', the final machine stays switched off, calming heartburn and letting ulcers heal.
The target is the gastric H+/K+-ATPase, the proton pump that exchanges hydrogen ions for potassium at the surface of acid-secreting cells. PPIs such as omeprazole are themselves inactive prodrugs; they accumulate in the highly acidic space beside the pump, where the acid converts them into a reactive sulfenamide.
That reactive form then bonds covalently to a cysteine on the pump, locking it irreversibly. Because acid secretion only resumes when the cell makes brand-new pump protein, a short-lived drug can suppress acid for a full day or more, which is the elegant trick of the class.
An honest caveat: long-term acid suppression is not free, with possible effects on the absorption of vitamin B12, magnesium, and calcium and changes to the gut's microbial balance, so prolonged use is weighed against need.
Omeprazole's short plasma half-life belies a long effect, because once it covalently disables the pump, acid stays low until the cell synthesizes fresh enzyme.
Why a short half-life can still give all-day acid control.
PPIs are acid-activated covalent inhibitors: their inactive prodrug form is essential, because it would be destroyed if it became reactive anywhere but next to the pump.