Food Chemistry

128 2 Enzymes

bitor; Km is unchanged whereas the values
of V are decreased such that V becomes
V/( 1 +[I]/Ki), i. e. non-competitive inhibition
can not be overcome by high concentrations of
substrate.
This also indicates that, in the presence of in-
hibitor, the amount of enzyme available for catal-
ysis is decreased.

2.5.2.2.3 Uncompetitive Inhibition

In this case the inhibitor reacts only with enzyme-
substrate complex:

(2.76)

Rearranging Equation 2.76 into an equation for
a straight line, the reaction rate becomes:

1

υ 0

=

Km

V

1

(A 0 )

+

1

V

(

1 +

( 1 )

Ki

)

(2.77)

The double reciprocal plot (Fig. 2.30c) shows
that in the presence of an uncompetitive inhibitor,
both the maximum velocity, V, and Km are
changed but not the ratio of Km/V. Hence the
slopes of the lines are equal and in the presence
of increasing amounts of inhibitor, the lines
plotted are parallel. Uncompetitive inhibition
is rarely found in single-substrate reactions. It
occurs more often in two-substrate reactions.
In conclusion, it can be stated that the three
types of reversible inhibition are kinetically
distinguishable by plots of reaction rate versus
substrate concentration using the procedure
developed byLineweaverandBurk(Fig. 2.30).

2.5.3 EffectofpHonEnzymeActivity

Each enzyme is catalytically active only in a nar-
row pH range and, as a rule, each has a pH opti-
mum which is often between pH 5.5and7.5(Ta-
ble 2.11).
The optimum pH is affected by the type and ionic
strength of the buffer used in the assay. The rea-

Fig. 2.30.Evaluation of inhibited enzyme-catalyzed re-

action according toLineweaverandBurk,[I 1 ]<(I 2 ).

aCompetitive inhibition,bnoncompetitive inhibition,

cuncompetitive inhibition

sons for the sensitivity of the enzyme to changes

in pH are two-fold:

a) sensitivity is associated with a change in pro-

tein structure leading to irreversible denatura-

tion,

b) the catalytic activity depends on the quantity

of electrostatic charges on the enzyme’s ac-

tive site generated by the prototropic groups

of the enzyme (cf. 2.4.2.4).

In addition the ionization of dissociable sub-

strates as affected by pH can be of importance to