Enzymes are proteins that act as biological catalysts. The function of an enzyme is to increase the rate of a biological reaction. When an enzyme is in contact with a compatible reactant called a substrate, a reaction can occur.
An enzyme is actually a complex of several factors that together make up the molecule. The apoenzyme is the polypeptide part of the enzyme. It has an inactive form that is known as a proenzyme or a zymogen. When the proenzyme is inactive, it may contain more amino acids than it would in its active form. Non-protein structures on the enzyme are known as co-factors.