2009_Book_FoodChemistry

2.2 General Remarks, Isolation and Nomenclature 95
Table 2.2. Substrate specificity of a legume
α-glucosidase
Substrate Relative Substrate Relative
activity
activity
(%) (%)
Maltose 100 Cellobiose 0
Isomaltose 4.0 Saccharose 0
Maltotrisose 41.5 Phenyl-α-
Panose 3.5 glucoside 3.1
Amylose 30.9 Phenyl-α-
Amylopectin 4.4 maltoside 29.7
Fig. 2.2. Examples of reaction specificity of some enzymes
amples in Table 2.2 and 3.24). In the latter cases
a reliable assessment of specificity is possible
only when the enzyme is available in purified
form, i. e. all other accompanying enzymes, as
impurities, are completely removed.
An enzyme’s substrate specificity for stereoisomers
is remarkable. When a chiral center is
present in the substrate in addition to the group
to be activated, only one enantiomer will be
converted to the product. Another example is the
specificity for diastereoisomers, e. g. for cis-trans
geometric isomers.
Enzymes with high substrate specificity are of
special interest for enzymatic food analysis. They
can be used for the selective analysis of individual
food constituents, thus avoiding the time consuming
separation techniques required for chemical
analyses, which can result in losses.
2.2.2.2 Reaction Specificity
The substrate is specifically activated by the
enzyme so that, among the several thermodynamically
permissible reactions, only one occurs.
This is illustrated by the following example:
L(+)-lactic acid is recognized as a substrate by
four enzymes, as shown in Fig. 2.2, although
only lactate-2-monooxygenase decarboxylates
the acid oxidatively to acetic acid. Lactate dehydrogenase
and lactate-malate transhydrogenase
form a common reaction product, pyruvate, but
by different reaction pathways (Fig. 2.2). This
may suggest that reaction specificity should
be ascribed to the different cosubstrates, such
as NAD + or oxalacetate. But this is not the case
since a change in cosubstrates stops the reaction.
Obviously, the enzyme’s reaction specificity as
well as the substrate specificity are predetermined
by the structure and chemical properties of the
protein moiety of the enzyme.
Of the four enzymes considered, only the lactate
racemase reacts with either of the enantiomers of
lactic acid, yielding a racemic mixture.
Therefore, enzyme reaction specificity rather
than substrate specificity is considered as a basis
for enzyme classification and nomenclature
(cf. 2.2.6).
2.2.3 Structure
Enzymes are globular proteins with greatly differing
particle sizes (cf. Table 1.26). As outlined
in section 1.4.2, the protein structure is determined
by its amino acid sequences and by its conformation,
both secondary and tertiary, derived
from this sequence. Larger enzyme molecules often
consist of two or more peptide chains (subunits
or protomers, cf. Table 1.26) arranged into
a specified quaternary structure (cf. 1.4.2.3). Section
2.4.1 will show that the three dimensional
shape of the enzyme molecule is actually responsible
for its specificity and its effective role as
a catalyst. On the other hand, the protein nature
of the enzyme restricts its activity to a relatively
narrow pH range (for pH optima, cf. 2.5.3) and
heat treatment leads readily to loss of activity by
denaturation (cf. 1.4.2.4 and 2.5.4.4).
Some enzymes are complexes consisting of a protein
moiety bound firmly to a nonprotein component
which is involved in catalysis, e. g. a “prosthetic”
group (cf. 2.3.2). The activities of other
enzymes require the presence of a cosubstrate
which is reversibly bound to the protein moiety
(cf. 2.3.1).