7.3 Formation of prostaglandin
Figure 14 models the way that the enzyme cyclooxygenase (COX) catalyses the formation of prostaglandin from arachidonic acid. Note how important the shape of the arachidonic acid molecule is. It needs to be just right to match the shape of the active site on the COX molecule and, as you have seen, of the 16 possible geometrical isomers, only one will fit. The one with all the double bonds cis has all the atoms in the right place to fit into the cavity containing the active site of COX and is the right shape to cyclise and form prostaglandin.