Food Biochemistry and Food Processing (2 edition)

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110 Part 2: Biotechnology and Ezymology

with the name(s) of the substrate(s), provides a basis for de-

termining the systematic name and EC number for naming

individual enzymes.

Common and Systematic Names


 The common name (recommended, trivial, or working

name) follows immediately after the EC number.


 While the common name is normally that used in the liter-

ature, 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 of Classification and Numbering of

Enzymes

The first EC, 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 following meaning:

1. The first number shows to which of the six divisions
   (classes) the enzyme belongs.


2. The second figure indicates the subclass.


3. The third figure gives the sub-subclass.


4. The fourth figure is the serial number of the enzyme in its
   sub-subclass.

The main classes are as follows:

 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, cycloiso-

merases).


 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 classifica-

tion is based on the order “donor:acceptor oxidoreductase.” The

common name often takes the form “substrate dehydrogenase,”

wherever this is possible. If the reaction is known to occur in the

opposite direction, this may be indicated by a common name of

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

1.1.1.9 isd-xylose reductase). “Oxidase” is used only in cases

where O 2 is an acceptor. Classification is difficult in some cases

because of the lack of specificity toward the acceptor.

Class 2: Transferases

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

group or a glycosyl group), from one compound (gener-

ally regarded as donor) to another compound (generally re-

garded as acceptor). The classification is based on the scheme

“donor:acceptor group transferase.” The common names are nor-

mally formed as “acceptor grouptransferase.” In many cases, the

donor is a cofactor (coenzyme) carrying the group to be trans-

ferred. The aminotransferases constitute 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 because they have a very wide

specificity, and it is not easy to decide if two preparations de-

scribed by different authors are the same, or if they should be

listed under different entries.

While the systematic name always includes “hydrolase,” the

common name is, in most cases, formed by the name of the

substrate with the suffix-ase. It is understood that the name of

this substrate with the suffix means a hydrolytic enzyme. The

peptidases, subclass 3.4, are classified in a different manner from

other enzymes in this class.

Class 4: Lyases

Lyases are enzymes cleaving C–C, C–O, C–N, and other bonds

by means other than hydrolysis or oxidation. They differ from

other enzymes in that two substrates are involved in one reac-

tion direction, but only one in the other direction. When acting

on the single substrate, a molecule is eliminated, leaving an

unsaturated residue. The systematic name is formed according

to “substrate group-lyase.” In common names, expressions like

decarboxylase, aldolase, and so on are used. “Dehydratase” is

used for those enzymes eliminating water. In cases where the

reverse reaction is the more important, or the only one to be

demonstrated, “synthase” may be used in the name.

Class 5: Isomerase

These enzymes catalyze changes within one molecule.

Class 6: Ligases

Ligases are enzymes catalyzing the joining of two molecules

with concomitant hydrolysis of the diphosphate bond in ATP or

a similar triphosphate. The bonds formed are often high-energy

bonds. “Ligase” is commonly used for the common name, but

in a few cases, “synthase” or “carboxylase” is used. Use of the

term “synthetase” is discouraged.

General Rules and Guidelines for

Classification and Nomenclature of Enzymes

Table 6.1 shows the classification of enzymes by class, subclass,

and sub-subclass, as suggested by the Nomenclature Committee