Enzymes are proteins that speed up the chemical reactions associated with substrate molecules. When an enzyme attaches to a substrate it weakens its chemical bonds. The area on an enzyme where substrates attach is known as the active site. Once a substrate attaches to the active site of an enzyme and undergoes a chemical reaction, products are formed.
Enzymes have specific shapes for them to fit specific substrates that they are associated with. The active site for an enzyme is like a glove that specifically fits only certain substrates.