Aspirin acetylates a specific serine residue

COX-1 is constitutively expressed in most tissues, and is responsible for the physiological production of prostaglandins.

COX-2 is induced by cytokines, mitogens, and endotoxins in inflammatory cells and is responsible for the elevated production of prostaglandins during inflammation.

Both isoforms are covalently modified, and hence inactivated, by reaction with aspirin (acetylsalicylic acid).

As shown, aspirin acetylates a specific serine residue, which in turn blocks access of the fatty acid substrate to the cyclooxygenase active site.