Enzyme Classification

Nomenclature and Classification of Enzymes

Naming the Enzymes

Enzymes are generally named according to the substrate they complex with or the type of reaction they catalyze. The usual practice is to add suffix -ase to the name of the substrate involved. Thus, the enzyme cellulase, arginase and tyrosinase are named because their substrate cellulose, arginase and tyrosinase. But as the research progressed and more enzymes were discovered, they need for a standardized system of nomenclature arise. Therefore, in 1961, a systematic nomenclature for enzyme was recommended by a commission of International Union of Biochemistry. The enzymes were placed into six group according to the general type of reaction which they catalyze. Each enzyme was given a systematic name, accurately describing the reaction it catalyzes.

The common names of enzymes consist of:

The name of the substrate acted upon the enzyme.

The type of reaction catalyzes, and

Suffix -ase. For example.

Classes of Enzymes

The older system of classification is based on type of chemical reaction is catalyzed. However, numbering system of classification of enzymes have been introduced by commission on enzymes of the International Union of Biochemistry. It recognizes enzymes by numbers. Its understanding requires most professional biochemical and research publications and abstracts. The old system of classification is more convenient and introductory.

Major classes of enzymes based on the types of reactions they catalyze are as following:

Oxidoreductase (Oxidation Reduction Enzymes)

The enzymes that transfer H or O atoms, or electrons from one molecule to another, i.e. catalyze oxidation and reductase processes. The oxidoreductase is of two types:

Dehydrogenase, that involves transfer of hydrogen.

Dehydrogenase

AH₂ + B ———————————————– A + BH₂ – transfer of H₂

CH₃CHO + NADH₂ ———————————————– CH₃CH₂OH + NAD

Reductases, which transfers electrons or electron to oxygen.

Oxidase

AH₂ + O ———————————————– A + H₂O

For example:

Reduced cytochrome + ½ O₂ ———————————————– cytochrome + H₂O

Transferases

These are enzymes that catalyze transfer of a specific group from a donor molecule to an acceptor molecule. The group may be methyl-, acyl-, amino-, or phosphate. This group enzymes are very large and includes enzymes such as trans-glycosidases, trans-peptidases, transaminases, etc.

AB + C ———————————————– A + BC

The Transferases may be:

Transaminases, which transfer amino groups.

Phosphorylases, which add inorganic phosphates, for example

phosphorylases

Glycogen + Pi ———————————————– glucose 1 -phosphate

Hydrolases

The enzymes catalyze the addition of water across a specific bond of the substrate to form the products.

AB + H₂O ———————————————– AOH + BH

The Hydrolases include:

Proteinase, that hydrolyze protein by breaking peptide bonds.

Ribonucleases, which hydrolyze RNA.

Deoxy-ribonucleases, responsible for hydrolysis of DNA.

Lipases, which hydrolyzes fats (esters).

Lyases

These enzymes bring about non-hydrolytic addition or removal of groups from substrates. During the reactions, C-C, C-N, C-O or C-S bonds may be split. These include:

Decarboxylases, that removes CO₂.

Decarboxylase

CH₃COCOOH ——————– CH₃CHO + CO₂

Carboxylases, that add CO₂.

RuBP carboxylase

RUBP + H₂O + CO₂ ———————————————– 2GP

Isomerases

Theses enzymes are responsible for intermolecular rearrangement, i.e., one isomer is converted to another. For example, during respiration glucose 1-phosphate is converted into glucose 1-phosphate by phosphoglucomutase.

Ligases

These enzymes join two molecules by synthesis of new C-C, C-N, C-O or C-S bonds using energy from MP.

X + Y + ATP ———————————————– XY + ADP + Pi

These include:

Synthetases, that catalyze synthesis of tRNA.

Polymerases, which are responsible for linking monomers (sub-units) into a polymer such as DNA or RNA.