2009_Book_FoodChemistry

126 2 Enzymes
and concentration of inhibitors, i. e. compounds
which decrease the rate of catalysis, and activators,
which have the opposite effect. Metal ions
and compounds which are active as prosthetic
groups or which provide stabilization of the enzyme’s
conformation or of the enzyme-substrate
complex (cf. 2.3.2 and 2.3.3) are activators. The
effect of inhibitors will be discussed in more detail
in this section.
Inhibitors are found among food constituents.
Proteins which specifically inhibit the activity
of certain peptidases (cf. 16.2.3), amylases or
β-fructofuranosidase are examples. Furthermore,
food contains substances which nonselectively
inhibit a wide spectrum of enzymes. Phenolic
constituents of food (cf. 18.1.2.5) and mustard oil
(cf. 17.1.2.6.5) belong to this group. In addition,
food might be contaminated with pesticides,
heavy metal ions and other chemicals from
a polluted environment (cf. Chapter 9) which can
become inhibitors under some circumstances.
These possibilities should be taken into account
when enzymatic food analysis is performed.
Food is usually heat treated (cf. 2.5.4) to suppress
undesired enzymatic reactions. As a rule, no inhibitors
are used in food processing. An exception
is the addition of, for example, SO 2 to inhibit
the activity of phenolase (cf. 8.12.6).
Much data concerning the mechanism of action
of enzyme inhibitors have been compiled in recent
biochemical research. These data cover the
elucidation of the effect of inhibitors on funtional
groups of an enzyme, their effect on the active
site and the clarification of the general mechanism
involved in an enzymecatalyzed reaction
(cf. 2.4.1.1).
Based on kinetic considerations, inhibitors are
divided into two groups: inhibitors bound irreversibly
to enzyme and those bound reversibly.
2.5.2.1 Irreversible Inhibition
In an irreversible inhibition the inhibitor binds
mostly covalently to the enzyme; the EI complex
formed does not dissociate:
(2.68)
The rate of inhibition depends on the reaction
rate constant k 1 in Equation 2.68, the enzyme
concentration, [E], and the inhibitor concentration,
[I]. Thus, irreversible inhibition is a function
of reaction time. The reaction cannot be reversed
by diluting the reaction medium. These criteria
serve to distinguish irreversible from reversible
inhibition.
Examples of irreversible inhibition are the reactions
of SH-groups of an enzyme with iodoacetic
acid:
(2.69)
and other reactions with the inhibitors described
in section 2.4.1.1.
2.5.2.2 Reversible Inhibition
Reversible inhibition is characterized by an equilibrium
between enzyme and inhibitor:
(2.70)
The equilibrium constant or dissociation constant
of the enzyme-inhibitor complex, K i , also known
as the inhibitor constant, is a measure of the extent
of inhibition. The lower the value of K i ,the
higher the affinity of the inhibitor for the enzyme.
Kinetically, three kinds of reversible inhibition
can be distinguished: competitive,
non-competitive and uncompetitive inhibition
(examples in Table 2.10). Other possible cases,
such as allosteric inhibition and partial competitive
or partial non-competitive inhibition, are
omitted in this treatise.
2.5.2.2.1 Competitive Inhibition
Here the inhibitor binds to the active site of the
free enzyme, thus preventing the substrate from
binding. Hence, there is competition between
substrate and inhibitor:
(2.71)
According to the steady-state theory for a singlesubstrate
reaction, we have:
V(A 0 υ 0 = ( )
K m 1 + (I)
K
+(A
i
0 )
(2.72)