An enzyme that catalyses the key reactions in the formation of prostaglandins from arachidonic acid, linolenic acid, and eicosa-5,8,11,14,17-pentaenoic acid. Nonsteroidal anti-inflammatory drugs, such as aspirin and ibuprofen, inhibit the cyclo-oxygenase enzymes COX1 and COX2, thus blocking the synthesis of prostaglandins and related compounds, such as prostacyclins and thromboxane A2, and relieving the symptoms of fever, inflammation, and pain associated with them. Inhibition of COX1 by aspirin is the basis for its antithrombotic (blood-thinning) properties.