Enzymes are proteins with high specificity, ie each catalyzes only one type of chemical reaction, or a small number of reactions. This is due to its shape, which presents on the surface molecular fittings, the active sites or centers where the substrates or reagents attach. An example of this is the ability of an enzyme to select a specific substrate from the myriad of molecules floating around its environment.
Therefore, substrates other than those that fit into the active sites of a specific enzyme cannot be part of that reaction, there must be another enzyme to catalyze its reaction. The specificity of enzymes for their substrates is similar to the specificity between a lock and its key.
