Why does an enzyme only catalyse one reaction?

Enzymes are proteins, which have a specific 3D tertiary structure, with a specifically shaped active site. The active site can only bind one substrate to form an enzyme-substrate complex, so can therefore only catalyse one reaction.For some extra detail in an extended answer, it may be worth mentioning that the reason active sites can bind only particular substrates is due to both conformation, but also the charges of the amino acids present in the active site. The charge of the active site can either attract or repel substrates. In terms of conformation, it is important to note the difference between the lock and key model (enzyme and substrate fit exactly geometrically into each other) or induced fit mechanism (active site shape slightly changes to accommodate substrate).