Food Chemistry

94 2 Enzymes

Table 2.1.Examples of catalyst activity

Reaction Catalyst Activation krel(25◦C)
energy (kJ·mol−^1 )

1. H 2 O 2 →H 2 O+ 1 /2O 2 Absent 75 1. 0
   I⊕ 56. 5 ∼ 2. 1 · 103
   Catalase 26. 8 ∼ 3. 5 · 108


2. Casein+nH 2 O→ H 86 1. 0
   (n+1) Peptides Trypsin 50 ∼ 2. 1 · 106


3. Ethylbutyrate H 55 1. 0
   +H 2 O→butyric acid+ethanol Lipase 17. 6 ∼ 4. 2 · 106


4. Saccharose + H 2 O→ H 107 1. 0
   Glucose+Fructose Invertase 46 ∼ 5. 6 · 1010


5. Linoleic acid Absent 150–270 1. 0
   +O 2 →Linoleic acid Cu^2 + 30–50 ∼ 102
   hydroperoxide Lipoxygenase 16. 7 ∼ 107

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 lev-
els in the presence of chemical or enzymatic cat-
alysts are provided in Table 2.1. Changes in their
reaction rates are also given. In contrast to re-
actions 1 and 5 (Table 2.1) which proceed at
measurable rates even in the absence of cata-
lysts, hydrolysis reactions 2, 3 and 4 occur only
in the presence of protons as catalysts. However,
all reaction rates observed in the case of inor-
ganic catalysts are increased by a factor of at
least several orders of magnitude in the pres-
ence of suitable enzymes. Because of the pow-
erful activity of enzymes, their presence at levels
of 10−^8 to 10−^6 mol/l is sufficient forin vitroex-
periments. 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 (reac-

tion 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 SubstrateSpecificity.....................................

The substrate specificity of enzymes shows the

following differences. The occurrence of a dis-

tinct functional group in the substrate is the only

prerequisite for a few enzymes, such as some hy-

drolases. This is exemplified by nonspecific li-

pases (cf. Table 3.21) or peptidases (cf. 1.4.5.2.1)

which generally act on an ester or peptide cova-

lent 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-