There are two key ways by which enzymes can be inhibited. Competitive inhibition - this involves a molecule other than the substrate binding to the active site of the enzyme (the location where the substrate would usually bind). Therefore, the active site is blocked preventing the substrate from being able to bind. Noncompetitive inhbition - this involves a molecule that binds to a site on the enzyme other than the active site, called the allosteric site. The binding of the molecule to the allosteric site causes conformational changes to the enzyme's active site. Therefore, the substrate can no longer fit with the active site of the enzyme and therefore cannot bind.