Food Biochemistry and Food Processing

136 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking

processing. Readers should visit the IUBMB en-
zyme nomenclature website for the most up-to-date
details on enzyme classification and nomenclature.

CLASSIFICATION AND
NOMENCLATURE OF ENZYMES

GENERALPRINCIPLES

• First principle. Names purporting to be names of
  enzymes, especially those ending in -aseshould
  be used only for single enzymes, that is, single
  catalytic entities. They should not be applied to
  systems containing more than one enzyme.


• Second principle. Enzymes are classified and
  named according to the reaction they catalyze.


• Third principle. Enzymes are divided into groups
  on the basis of the type of reactions catalyzed,
  and this, together with the name(s) of the
  substrate(s), provides a basis for determining the
  systematic name and EC number for naming
  individual enzymes.

COMMON ANDSYSTEMATICNAMES

• The common name (recommended, trivial, or
  working name) follows immediately after the EC
  number.


• While the common name is normally that used in
  the literature, the systematic name, which is
  formed in accordance with definite rules, is more
  precise chemically. It should be possible to
  determine the reaction catalyzed from the
  systematic name alone.

SCHEME OFCLASSIFICATION ANDNUMBERING
OFENZYMES

The first Enzyme Commission, in its report in 1961,
devised a system for the classification of enzymes
that also serves as a basis for assigning EC numbers
to them. These code numbers (prefixed by EC),
which are now widely in use, contain four elements
separated by periods (e.g., 1.1.1.1), with the follow-
ing meaning:

• The first number shows to which of the six
  divisions (classes) the enzyme belongs,


• The second figure indicates the subclass,


• The third figure gives the sub-subclass, and
  
  
  • The fourth figure is the serial number of the
    enzyme in its sub-subclass.
  
  
  
  

The main classes are

• Class 1.Oxidoreductases (dehydrogenases,
  reductases, or oxidases),


• Class 2.Transferases,


• Class 3.Hydrolases,


• Class 4.Lyases,


• Class 5.Isomerases (racemases, epimerases,
  cis-trans-isomerases, isomerases, tautomerases,
  mutases, cycloisomerases), and


• Class 6.Ligases (synthases).

Class 1. Oxidoreductases

Enzymes catalyzing oxidoreductions belong to this

class. The reactions are of the form AH 2 
B A 


BH 2 or AH 2 
B
A 
BH 
H
. The substrate

oxidized is regarded as the hydrogen or electron

donor. All reactions within a particular sub-subclass

are written in the same direction. The classification

is based on the order “donor:acceptor oxidoreduc-

tase.” The common name often takes the form “sub-

trate dehydrogenase,” wherever this is possible. If

the reaction is known to occur in the opposite direc-

tion, this may be indicated by a common name of

the form “acceptor reductase” (e.g., the common

name of EC 1.1.1.9 is D-xylose reductase). “Oxi-

dase” is used only in cases where O 2 is an acceptor.

Classification is difficult in some cases because of

the lack of specificity towards the acceptor.

Class 2. Transferases

Transferases are enzymes transferring a group (e.g.,

the methyl group or a glycosyl group), from one

compound (generally regarded as donor) to another

compound (generally regarded as acceptor). The

classification is based on the scheme “donor:acceptor

grouptransferase.” The common names are normally

formed as “acceptor grouptransferase.” In many cas-

es, the donor is a cofactor (coenzyme) carrying the

group to be transferred. The aminotransferases con-

stitute a special case (subclass 2.6): the reaction also

involves an oxidoreduction.

Class 3. Hydrolases

These enzymes catalyze the hydrolysis of various

bonds. Some of these enzymes pose problems be-