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.
AH2 + B ———————————————– A + BH2 – transfer of H2
CH3CHO + NADH2 ———————————————– CH3CH2OH + NAD
Reductases, which transfers electrons or electron to oxygen.
AH2 + O ———————————————– A + H2O
Reduced cytochrome + ½ O2 ———————————————– cytochrome + H2O
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
Glycogen + Pi ———————————————– glucose 1 -phosphate
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).
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.
CH3COCOOH ——————– CH3CHO + CO2
Carboxylases, that add CO2.
RUBP + H2O + CO2 ———————————————– 2GP
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.
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
Synthetases, that catalyze synthesis of tRNA.
Polymerases, which are responsible for linking monomers (sub-units) into a polymer such as DNA or RNA.