2009_Book_FoodChemistry

94 2 Enzymes
Table 2.1. Examples of catalyst activity
Reaction Catalyst Activation k rel (25 ◦ C)
energy (kJ · mol −1 )
1. H 2 O 2 → H 2 O+1/2 O 2 Absent 75 1.0
I ⊕ 56.5 ∼ 2.1 · 10 3
Catalase 26.8 ∼ 3.5 · 10 8
2. Casein+n H 2 O → H ⊖ 86 1.0
(n+1) Peptides Trypsin 50 ∼ 2.1 · 10 6
3. Ethylbutyrate H ⊖ 55 1.0
+H 2 O → butyric acid+ethanol Lipase 17.6 ∼ 4.2 · 10 6
4. Saccharose + H 2 O → H ⊖ 107 1.0
Glucose+Fructose Invertase 46 ∼ 5.6 · 10 10
5. Linoleic acid Absent 150–270 1.0
+O 2 → Linoleic acid Cu 2+ 30–50 ∼ 10 2
hydroperoxide Lipoxygenase 16.7 ∼ 10 7
species A contain enough energy to combine
with the catalyst and, thus, to attain the “activated
state” and to form or break the covalent bond
that is necessary to give the intermediary product
which is then released as product P along with
free, unchanged catalyst. The reaction rate
constants, k +1 and k −1 , are therefore increased
in the presence of a catalyst. However, the
equilibirum constant of the reaction, i. e. the
ratio k 1+ /k −1 = K, is not altered.
Activation energy levels for several reactions and
the corresponding decreases of these energy levels
in the presence of chemical or enzymatic catalysts
are provided in Table 2.1. Changes in their
reaction rates are also given. In contrast to reactions
1 and 5 (Table 2.1) which proceed at
measurable rates even in the absence of catalysts,
hydrolysis reactions 2, 3 and 4 occur only
in the presence of protons as catalysts. However,
all reaction rates observed in the case of inorganic
catalysts are increased by a factor of at
least several orders of magnitude in the presence
of suitable enzymes. Because of the powerful
activity of enzymes, their presence at levels
of 10 −8 to 10 −6 mol/l is sufficient for in vitro experiments.
However, the enzyme concentrations
found in living cells are often substantially higher.
2.2.2 Specificity
In addition to an enzyme’s ability to substantially
increase reaction rates, there is a unique enzyme
property related to its high specificity for both the
compound to be converted (substrate specificity)
and for the type of reaction to be catalysed (reaction
specificity).
The activities of allosteric enzymes (cf. 2.5.1.3)
are affected by specific regulators or effectors.
Thus, the activities of such enzymes show an
additional regulatory specificity.
2.2.2.1 Substrate Specificity
The substrate specificity of enzymes shows the
following differences. The occurrence of a distinct
functional group in the substrate is the only
prerequisite for a few enzymes, such as some hydrolases.
This is exemplified by nonspecific lipases
(cf. Table 3.21) or peptidases (cf. 1.4.5.2.1)
which generally act on an ester or peptide covalent
bond.
More restricted specificity is found in other
enzymes, the activities of which require that the
substrate molecule contains a distinct structural
feature in addition to the reactive functional
group. Examples are the proteinases trypsin and
chymotrypsin which cleave only ester or peptide
bonds with the carbonyl group derived from
lysyl or arginyl (trypsin) or tyrosyl, phenylalanyl
or tryptophanyl residues (chymotrypsin). Many
enzymes activate only one single substrate or
preferentially catalyze the conversion of one
substrate while other substrates are converted
into products with a lower reaction rate (cf. ex-