Uncompetitive inhibition, despite having a prefix with the exact same meaning as noncompetitive inhibition, is a different category of reversible inhibition of enzymes. In the body, when an enzyme becomes overactive and the reactions it catalyzes, uncompetitive inhibition is one of three types that may kick in to stop it. Enzymes require substrates, or target molecules, to perform their duties. An inhibitor binds to the enzyme, no longer allowing it to function as a catalyst. In uncompetitive inhibition, this binding only occurs once the enzyme-substrate complex has been formed, not to the free enzyme itself. In pictures, with 'E' designating the enzyme, 'I' the inhibitor, and 'S' the active site of the substrate:

                                         \ S /         
 __    ____    ____                   ___ | | ___     ____
|  |___|   \  /    |       ____\     |   ||_||   \   /    |
|       E   \/     |       ____      |        E   \_/     |
|__________________|      /          |____________________|


                                       ||\
                                       ||
                                      \||

         \ S /    |I|    
      ___ | | ___ | |  ____
     |   ||_||   \\_//    |
     |        E   \_/     |
     |____________________|

Only once the enzyme substrate complex is formed can the inhibitor fit inside the enzyme (yes, I know that looks dirty, shush). Uncompetitive inhibitors are intended to slow down reactions with large concentrations. They are not the fastest, nor the most effective inhibitors, and their use in the body is rarer than competitive or noncompetitive inhibitors.

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