When early workers isolated certain enzymes, whimsical names were given. Some of these, such as Pepsin, Trypsin, Chymotrypsin, etc. are still used. Later, it was agreed to call the enzymes by adding the suffix "-ase" to the substrate. Thus, enzyme Lactase acts on the substrate lactose, and the products glucose and galactose are formed. Enzymes that hydrolyse starch (amylose) are termed as amylases; those that dehydrogenate the substrates are called dehydrogenases. These are known as the trivial names of the enzymes.
IUBMB System of Classification
International Union of Biochemistry and Molecular Biology (IUBMB) in 1964, (modified in 1972 and 1978), suggested the IUBMB system of nomenclature of enzymes. It is complex and cumbersome; but unambiguous. As per this system, the name starts with EC (enzyme class) followed by 4 digits.
First digit represents the class
Second digit stands for the subclass
Third digit is the sub-subclass or subgroup
Fourth digit gives the number of the particular enzyme in the list.
The enzymes are grouped into following six major classes (Box1).
Box1. Classification of Enzymes
• Class 1: Oxidoreductases
This group of enzymes will catalyse oxidation of one substrate with simultaneous reduction of another substrate or co-enzyme. This may be represented as
AH2 + B -------------→ A + BH2
for example,
Alcohol + NAD+ ----→ Aldehyde + NADH + H+
The enzyme is Alcohol dehydrogenase; IUB name is Alcohol-NAD-oxidoreductase; Code number is EC.1.1.1.1. Oxidoreductases may also oxidise substrates by adding oxygen, e.g. oxidases, oxygenases and dehydrogenases.
• Class 2: Transferases
This class of enzymes transfers one group (other than hydrogen) from the substrate to another substrate. This may be represented as
A-R + B → A + B-R , For example,
Hexose + ATP → Hexose-6-phosphate + ADP
The name of enzyme is Hexokinase and systematic name is ATP-Hexose--6-phosphate transferase.
• Class 3: Hydrolases
This class of enzymes can hydrolyse ester, ether, peptide or glycosidic bonds by adding water and then breaking the bond.
Acetyl choline + H2O --------→ Choline + acetate
The enzyme is Acetyl choline esterase or Acetyl choline hydrolase (systematic). All digestive enzymes are hydrolases.
• Class 4: Lyases
These enzymes can remove groups from substrates or break bonds by mechanisms other than hydrolysis. For example,
Fructose-1,6-bisphosphate → Glyceraldehyde-3-phosphate +dihydroxy acetone phosphate
The enzyme is Aldolase .
• Class 5: Isomerases
These enzymes can produce optical, geometric or positional isomers of substrates. Racemases, epimerases, cis-trans isomerases are examples.
Glyceraldehyde-3-phosphate → Di-hydroxy-acetone-phosphate
Enzyme is Triose phosphate isomerase.
• Class 6: Ligases
These enzymes link two substrates together, usually with the simultaneous hydrolysis of ATP, (Latin, Ligare = to bind). For example,
Acetyl CoA + CO2 + ATP → Malonyl CoA + ADP + Pi
Enzyme is Acetyl CoA carboxylase.
A summary of classification is given in Box 1. The differences between synthetase and synthase are shown in Box2.
Box2. Synthetase and Synthase are Different
