Competitive inhibitors compete with the substrate for the enzyme's active site, while noncompetitive inhibitors interact with the enzyme at a location other than the active site. Both types of inhibitors can prevent a specific chemical reaction from occurring.
Enzymes are proteins found in living things that increase the rate of chemical reactions. They interact with a reactant called a substrate. The enzyme and the substrate fit together like a lock and key. The location where enzymes and substrates fit together is called the active site. Without enzymes many chemical reactions in the body would not occur or would occur too slowly to be of benefit.
Inhibitors are substances which alter or interfere with the action of the enzyme to slow it down or stop it. Competitive inhibitors mimic the structure of the substrate. They can fit into the active site, prohibiting the substrate from interacting with the enzyme. This can either slow down or prevent a reaction from occurring depending upon the concentrations of the inhibitor and substrate.
Noncompetitive inhibitors do not fit into the active site. Instead they react with the enzyme and change its shape. Once the change in shape occurs, the substrate can no longer fit into the active site and the reaction cannot occur.