2009_Book_FoodChemistry

2.2 General Remarks, Isolation and Nomenclature 97
rity can be tedious and is open to criticism. Today,
electrophoretic methods of high separation
efficiency or HPLC are primarily used.
The behavior of the enzyme during chromatographic
separation is an additional proof of purity.
A purified enzyme is characterized by a symmetrical
elution peak in which the positions of the
protein absorbance and enzyme activity coincide
and the specific activity (expressed as units per
amount of protein) remains unchanged during repeated
elutions.
During a purification procedure, the enzyme activities
are recorded as shown in Table 2.3. They
provide data which show the extent of purification
achieved after each separation step and show the
enzyme yield. Such a compilation of data readily
reveals the undesired separation steps associated
with loss of activity and suggests modifications or
adoption of other steps.
2.2.5 Multiple Forms of Enzymes
Chromatographic or electrophoretic separations
of an enzyme can occasionally result in separation
of the enzyme into “isoenzymes”, i. e. forms
of the enzyme which catalyze the same reaction
although they differ in their protein structure. The
occurrence of multiple enzyme forms can be the
result of the following:
a) Different compartments of the cell produce
genetically independent enzymes with the
same substrate and reaction specificity, but
which differ in their primary structure. An
example is glutamate-oxalacetate transaminase
occurring in mitochondria and also
in muscle tissue sarcoplasm. This is the
indicator enzyme used to differentiate fresh
from frozen meat (cf. 12.10.1.2).
b) Protomers associate to form polymers of differing
size. An example is the glutamate dehydrogenase
occurring in tissue as an equilibrium
mixture of molecular weights M r =
2.5 · 10 5 − 10 6 .
c) Different protomers combine in various
amounts to form the enzyme. For example,
lactate dehydrogenase is structured from
a larger number of subunits with the reaction
specificity given in Fig. 2.2. It consists of
five forms (A 4 ,A 3 B, A 2 B 2 ,AB 3 and B 4 ), all
derived from two protomers, A and B.
2.2.6 Nomenclature
The Nomenclature Commitee of the “International
Union of Biochemistry and Molecular
Biology” (IUBMB) adopted rules last amended
in 1992 for the systematic classification and
designation of enzymes based on reaction specificity.
All enzymes are classified into six major
classes according to the nature of the chemical
reaction catalyzed:
1. Oxidoreductases.
2. Transferases.
3. Hydrolases.
4. Lyases (cleave C−C, C−O, C−N, and
other groups by elimination, leaving double
bonds, or conversely adding groups to double
bonds).
5. Isomerases (involved in the catalysis of isomerizations
within one molecule).
6. Ligases (involved in the biosynthesis of
a compound with the simultaneous hydrolysis
of a pyrophosphate bond in ATP or
a similar triphosphate).
Each class is then subdivided into subclasses
which more specifically denote the type of
reaction, e. g. by naming the electron donor of an
oxidation-reduction reaction or by naming the
functional group carried over by a transferase or
cleaved by a hydrolase enzyme.
Each subclass is further divided into subsubclasses.
For example, sub-subclasses of
oxidoreductases are denoted by naming the
acceptor which accepts the electron from its
respective donor.
Each enzyme is classified by adopting this system.
An example will be analyzed. The enzyme
ascorbic acid oxidase catalyzes the following reaction:
L − Ascorbic acid + 1 2 O 2 ⇋
L − Dehydroascorbic acid + H 2 O (2.2)
Hence, its systematic name is L-ascorbate: oxygen
oxidoreductase, and its systematic number
is E.C. 1.1.10.3.3 (cf. Formula 2.3). The systematic
names are often quite long. Therefore,
the short, trivial names along with the systematic
numbers are often convenient for enzyme designation.
Since enzymes of different biological origin
often differ in their properties, the source and,
when known, the subcellular fraction used for iso-