Food Chemistry

2.5 Kinetics of Enzyme-Catalyzed Reactions 127

Table 2.10.Examples of reversible enzyme inhibition

Enzyme EC- Sustrate Inhibitor Inhibi- Ki(mmol/l)

Number tion

typea

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

aC: competitive, NC: noncompetitive, and UC: uncompetitive.

In the presence of inhibitors, theMichaeliscon-
stant is apparently increased by the factor:

1 +

(I)

Ki

(2.73)

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 theMichaelis–
Menten equation (Equation 2.41). The Line-
weaver–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, Ki, can be calculated from
the corresponding intercepts with the abscissa in
Fig. 2.30a by calculating the value ofKmfrom
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 in-

active and the dissociation constants Kiand KEAi

are numerically equal, the following equation

is obtained by rearrangement of the equation

for a single-substrate reaction into its reciprocal

form:

1

υ 0

=

Km

V

(

1 +

( 1 )

Ki

)

1

(A 0 )

+

1

V

(

1 +

( 1 )

Ki

)

(2.75)

The double-reciprocal plot (Fig. 2.30b) shows

that, in the presence of a noncompetitive inhi-