Principles of Biochemistry (2+1)

8.1.4 Classification

Enzymes are classified into six major classes according to the reactions they catalyze.

Class 1. Oxidoreductases: These are enzymes which catalyze the reduction or oxidation of a molecule. The oxidation is the reverse of reduction and that an enzyme has to catalyze the forward and reverse reactions to the same degree and hence named "oxidoreductase."

A_reduced+ B _oxidised ↔ A_oxidised + B _reduced

e.g. Malate dehydrogenase

Malate + NAD ↔ Oxaloacetate + NADH⁺ + H⁺

Class 2. Transferases: These enzymes catalyze the transfer of a group of atoms from one molecule to another. The common names are formed as acceptor group transferases. e.g. Hexokinase involves the transfer of a phosphate between ATP and a sugar molecule.

A.R+ B= A+ B.

e.g. Hexokinase involves the transfer of a phosphate between ATP and a sugar molecule.

Hexokinase

ATP+ D-Hexose ↔ ADP + D-Hexose-6-phosphate

Class3. Hydrolases: These enzymes catalyse the hydrolysis of various bonds (and their reverse reactions). The hydrolysis of an ester would be an example of such a reaction.

β-Galactosidase

e.g. β-Galactoside + H₂O ↔ An alcohol + β-Galactose

Class 4. Isomerases: These enzymes catalyze the conversion of a molecule into an isomer.

e.g cis-trans interconversion of maleate and fumarate

Class 5. Lyases: Reactions which add a small molecule such as water or ammonia to a double bond (and the reverse, elimination, reactions) are catalyzed by lyases.

X Y

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C—C ↔ X—Y + C = C

Fumerase

e.g Fumerate + H₂O ↔ L Malate

Class 6 Ligases: These enzymes catalyze reactions which make bonds to join together (ligate) smaller molecules to make larger ones.

Acetyl-CoA carboxylase

e.g. ATP + Acetyl-CoA + CO₂ ↔ ADP + Pi + Malonyl-CoA

Each of these classes has more specific subclasses and each subclass has sub-sub classes as well. The key to using this classification scheme is to look at the reaction the enzyme catalyzes, decide which type of reaction it is, and apply the appropriate name.

Specific enzyme names are systematically derived by specifying the substrate (the molecule being acted upon -- the reactant), the type of reaction, and appending the suffix ase.

Each enzyme is assigned a four-digit classification number and a systematic name, which identifies the reaction catalyzed. An example is the naming of the enzymes catalyzing the reaction.

e.g. E.C.2.7.1.1 Hexokinase.

ATP + D-glucose → ADP + D-glucose 6-phosphate

The formal systematic name of this enzyme is ATP: glucose phosphotransferase, which indicates that it catalyzes the transfer of a phosphate group from ATP to glucose. It is placed in Class 2 and its classification number is 2.7.1.1, where the first digit stands for the class 2 name transferase, the third digit for the sub-subclass (phosphotransferases with a hydroxy1 group as acceptor), and the fourth digit for D-glucose as the phosphate group acceptor. The trivial name is hexokinase.