In this course you have found out that:
The sensation of pain is caused by the release of a chemical (prostaglandin) that stimulates the nerve endings and sends an electrical message to the brain.
Pain can be reduced if the formation of prostaglandin can be inhibited.
Prostaglandin is formed, from arachidonic acid, in a cavity in the active site of the enzyme cyclooxygenase (COX).
Geometrical isomerism can be important in controlling the shape of molecules.
The specific shape of the arachidonic acid molecule is caused by four carbon–carbon double bonds in its carbon chain which limit the rotation allowed within the molecule. Only one of the isomers, in which all four double bonds are cis, has the atoms in the correct place for prostaglandin formation.
Aspirin can release an acetyl group, which bonds to the active site of COX and prevents arachidonic acid from entering the cavity, so inhibiting the formation of prostaglandin.