2009_Book_FoodChemistry

2.5 Kinetics of Enzyme-Catalyzed Reactions 127
Table 2.10. Examples of reversible enzyme inhibition
Enzyme EC- Sustrate Inhibitor Inhibi- K i (mmol/l)
Number
tion
type a
Glucose 1.1.1.47 Glucose/NAD Glucose-6- C 4.4 · 10 −5
dehydrogenase
phosphate
Glucose-6-
phosphate
dehydrogenase 1.1.1.49 Glucose- Phosphate C 1 · 10 −1
6-phosphate/
NADP
Succinate 1.3.99.1 Succinate Fumarate C 1.9 · 10 −3
dehydrogenase
Creatine kinase 2.7.3.2 Creatine/ATP ADP NC 2 · 10 −3
Glucokinase 2.7.1.2 Glucose/ATP D-Mannose C 1.4 · 10 −2
2-Deoxyglucose C 1.6 · 10 −2
D-Galactose C 6.7 · 10 −1
Fructose- 3.1.3.11 D-Fructose-1, AMP NC 1.1 · 10 −4
biphosphatase
6-biphosphate
α-Glucosidase 3.2.1.20 p-Nitrophenyl-α- Saccharose C 3.7 · 10 −2
D-glucopyranoside Turanose C 1.1 · 10 −2
Cytochrome 1.9.3.1 Ferrocytochrome c Azide UC
c oxidase
a C: competitive, NC: noncompetitive, and UC: uncompetitive.
In the presence of inhibitors, the Michaelis constant
is apparently increased by the factor:
1 + (I)
(2.73)
K i
Such an effect can be useful in the case of
enzymatic substrate determinations (cf. 2.6.1.3).
When inhibitor activity is absent, i. e. [I] =0,
Equation 2.72 is transformed into the Michaelis–
Menten equation (Equation 2.41). The Lineweaver–Burk
plot (Fig. 2.30a) shows that the
intercept 1/V with the ordinate is the same in the
presence and in the absence of the inhibitor, i. e.
the value of V is not affected although the slopes
of the lines differ. This shows that the inhibitor
can be fully dislodged by the substrate from the
active site of the enzyme when the substrate is
present in high concentration. In other words,
inhibition can be overcome at high substrate
concentrations (see application in Fig. 2.49). The
inhibitor constant, K i , can be calculated from
the corresponding intercepts with the abscissa in
Fig. 2.30a by calculating the value of K m from
the abscissa intercept when [I]=0.
2.5.2.2.2 Non-Competitive Inhibition
The non-competitive inhibitor is not bound to the
active site of the enzyme but to some other site.
Therefore, the inhibitor can react equally with
free enzyme or with enzyme-substrate complex.
Thus, three processes occur in parallel:
(2.74)
Postulating that EAI and EI are catalytically inactive
and the dissociation constants K i and K EAi
are numerically equal, the following equation
is obtained by rearrangement of the equation
for a single-substrate reaction into its reciprocal
form:
1
= K m
υ 0 V
(
1 + (1) ) 1
K i (A 0 ) + 1 (
1 + (1) )
V K i
(2.75)
The double-reciprocal plot (Fig. 2.30b) shows
that, in the presence of a noncompetitive inhi-