Industrial Biotransformations

38
2 The Enzyme Classification
Tab. 2.1 The main enzyme classes.
Enzyme class Catalyzed reaction
1. Oxidoreductases oxidation–reduction reactions
2. Transferases transfer of functional groups
3. Hydrolases hydrolysis reactions
4. Lyases group elimination (forming double bonds)
5. Isomerases isomerization reactions
6. Ligases bond formation coupled with a triphospate cleavage
As the systematic name may be very extensive and awkward to use, the Enzyme Commission
(EC) has also developed a numerical system based on the same criteria, which
can be used together with the recommended name to specify a particular enzyme.
According to this system, each enzyme is assigned a four-digit EC number (Table 2.2).
The first digit denotes the main class, which specifies the catalyzed reaction type. These
are divided into subclasses, according to the nature of the substrate, the type of the transferred
functional group or the nature of the specific bond involved in the catalyzed reaction.
These subclasses are designated by the second digit. The third digit reflects a
further division of the subclasses according to the substrate or co-substrate, giving rise to
the sub-subclasses. In the fourth digit a serial number is used to complete the enzyme
identification.
Tab. 2.2 Constitution of the four-digit EC number.
EC number: EC (i).(ii).(iii).(iv)
(i) the main class, denotes the type of catalyzed reaction
(ii) subclass, indicates the substrate type, the type of transferred functional group or the nature of
one specific bond involved in the catalyzed reaction
(iii) sub-subclass, expresses the nature of the substrate or co-substrate
(iv) an arbitrary serial number
As an example, aminoacylase (N-acyl-l-amino-acid amidohydrolase, as it is known
according to systematic nomenclature), an enzyme used in the industrial production of
l-methionine, has the classification number EC 3.5.1.14 (see the process on page 403).
The first number (i = 3) indicates that this enzyme belongs to the class of hydrolases. The
second number (ii = 5) shows that a carbon–nitrogen bond is hydrolyzed and the third
number (iii = 1) denotes that the substrate is a linear amide. The serial number (iv = 14)
is required for the full classification of the enzyme.