Like a wrench that stays the same after unscrewing a lot of bolts, so does an enzyme keep working after it finishes a chemical reaction. This reaction helps increase the efficiency of enzymes, because they can be used over and over again. Enzymes are really specific, each enzyme only works with particular kinds of molecules, called substrates. Enzymes can also speed up the same chemical reaction going backwards. Enzymes may break a molecule into two pieces, but will put it back together only if it’s provided with the two pieces.
There are basically two theories to suggest how enzymes and substrates fit together. These are the lock and key mechanism and the induced fit mechanism. The lock and key mechanism says that the structure of the substrate fits exactly to the structure of the active site. The induced fit mechanism says that the active site of the enzyme can change itself a little to help the substrate fit nicely.