All metabolic pathways have to be regulated and controlled to stop the build-up of an end product that isn’t needed. The cell can control a metabolic pathway by the presence or absence of a particular enzyme. The cell can also regulate the rate of reaction of key enzymes.
Enzymes are able to bind to their substrate because they have an active site. An induced fit occurs where the active site of the enzyme is changed slightly to better fit the substrate after the substrate binds.
The active site has a high affinity to the substrate as well as being specific to the substrate. This orientates the reactants into the correct positions for the reaction to take place. The binding of the enzyme to its substrate also lowers the activation energy of the reaction (amount of energy needed to make a reaction happen).
If an enzyme is present, the amount of energy needed to make a product is lowered. As the products are made, they are no longer specific to the active site (they have a low affinity to the enzyme) and so are released.
Some metabolic reactions are reversible and the presence of a substrate or removal of a product will drive a sequence of reactions in a particular direction.