Enzymes speed up the rates of reactions within the body. An enzyme acts by binding to a substrate, the substance that will be acted on by the enzyme. When the substrate binds to the enzyme, the reaction takes place.
The enzyme has a region called the active site, and this is what the substrate binds to. The enzyme and the substrate do not react with one another; rather, the enzyme brings down the amount of energy needed for the reaction to occur, the activation energy. Negative feedback will cease the reaction. With negative feedback, the end product of the reaction will fit into another site of the enzyme and inhibit its activity. The induced-fit model of enzyme action states that when a substrate binds to an enzyme, the enzyme causes the substrate to change its shape, speeding up the reaction even more.
Because enzymes are proteins, they can work only under certain conditions of temperature and pH. If conditions are not optimal for the enzyme, it becomes ineffective. Extreme conditions of temperature and pH denature enzymes, changing their shapes so much that they are rendered useless.
The actions of enzymes can be inhibited in two ways. In competitive inhibition, the inhibitor molecule binds to the active site, not allowing the substrate to bind. With non-competitive inhibition, the inhibitor molecule binds to another location on the enzyme, changing the structure of the active site so that no substrate can bind.