Many biological reactions are catalyzed by enzymes. As you should remember, enzymes are proteins that speed up the rate of biochemical reactions. However, some reactions may not be needed at a certain time. Hence it may be necessary to slow them down or prevent them all together. This is achieved using molecules called inhibitors.
There are two categories of inhibitors. There are reversible and irreversible inhibitors. Irreversible inhibitors binds to the enzyme tightly via covalent or noncovalent bonds with little change of dissociation. For example, aspirin binds irreversibly to cyclooxygenase to prevent the production of prostaglandins that are involved in pain and other inflammation symptoms during injury.
Reversible inhibitors binds weakly to the enzymes and therefore can be easily dissociated under certain conditions. They can be categorized into three main types, i.e., competitive, uncompetitive, and noncompetitive inhibition.
In competitive inhibition, the inhibitors resemble the substrate that binds to the active site. Therefore they compete to associate with the enzyme. The inhibitor is more likely to bind with the enzyme because it has a higher affinity for the enzyme. Inhibition can be overcome by increasing the substrate. Despite higher affinity, having more substrate presents a greater mathematical chance that they will bind with the enzyme instead of the inhibitor.
In uncompetitive inhibition, the inhibitor binds to a site on the enzyme other than the active site, called the allosteric site. This results in the formation of an enzyme-substrate inhibitor (ESI) complex which renders the enzyme inactive. (Note that the inhibitor binds only to the ES complex). Increasing the substrate concentration will not overcome inhibition since the substrate does not compete for the allosteric site.
In noncompetitive inhibition, the inhibitor binds equally well to both the enzyme and the enzyme-substrate complex.