Food Chemistry

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 .5Phenyl-α-

Panose 3 .5glucoside 3. 1

Amylose 30 .9Phenyl-α-

Amylopectin 4. 4 maltoside 29. 7

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 stereoiso-
mers 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 consum-
ing separation techniques required for chemical
analyses, which can result in losses.

2.2.2.2 ReactionSpecificity

The substrate is specifically activated by the

enzyme so that, among the several thermody-
namically 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 dehy-
drogenase 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

Fig. 2.2.Examples of reaction specificity of some en-

zymes

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 dif-

fering particle sizes (cf. Table 1.26). As outlined

in section 1.4.2, the protein structure is deter-

mined by its amino acid sequences and by its con-

formation, both secondary and tertiary, derived

from this sequence. Larger enzyme molecules of-

ten consist of two or more peptide chains (sub-

units or protomers, cf. Table 1.26) arranged into

a specified quaternary structure (cf. 1.4.2.3). Sec-

tion 2.4.1 will show that the three dimensional

shape of the enzyme molecule is actually respon-

sible 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 pro-

tein moiety bound firmly to a nonprotein compo-

nent which is involved in catalysis, e. g. a “pros-

thetic” 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).