Enzymes are classified into six categories according to the type of reaction catalyzed:

• Oxidoreductases: They catalyze oxidation/reduction reactions by transferring electron from the reductant to the oxidant. NADP or NAD+ are usually used as cofactors by this group of enzymes.
• Transferases: This class of enzymes transfer specific functional groups (e.g. a methyl or phosphate group) from one molecule to another.
• Hydrolases: They are responsible of catalyzing the hydrolysis of various chemical bonds, resulting in dividing a large molecule to smaller molecules.
• Lyases: This type of enzymes catalyzes the cleavage of various chemical bonds by means other than hydrolysis and oxidation, often forming a new double bond or a new ring structure. Lyases require only one substrate for the reaction in one direction, but two substrates for the reverse reaction, which is distinct from other enzymes.
• Isomerases: Isomerases catalyze isomerization changes within a single molecule, facilitating intramolecular rearrangements in which bonds are broken and formed. The product has the same molecular formula as the substrate but has different bond connectivity or spatial arrangement.
• Ligases: Ligases catalyze the joining of two molecules by forming a new covalent bond.