2009_Book_FoodChemistry

128 2 Enzymes
bitor; K m is unchanged whereas the values
of V are decreased such that V becomes
V/(1 +[I]/K i ), i. e. non-competitive inhibition
can not be overcome by high concentrations of
substrate.
This also indicates that, in the presence of inhibitor,
the amount of enzyme available for catalysis
is decreased.
2.5.2.2.3 Uncompetitive Inhibition
In this case the inhibitor reacts only with enzymesubstrate
complex:
(2.76)
Rearranging Equation 2.76 into an equation for
a straight line, the reaction rate becomes:
1
= K m 1
υ 0 V (A 0 ) + 1 (
1 + (1) )
(2.77)
V K i
The double reciprocal plot (Fig. 2.30c) shows
that in the presence of an uncompetitive inhibitor,
both the maximum velocity, V, and K m are
changed but not the ratio of K m /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 by Lineweaver and Burk (Fig. 2.30).
2.5.3 Effect of pH on Enzyme Activity
Each enzyme is catalytically active only in a narrow
pH range and, as a rule, each has a pH optimum
which is often between pH 5.5and7.5 (Table
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 reaction
according to Lineweaver and Burk, [I 1 ] < (I 2 ).
a Competitive inhibition, b noncompetitive inhibition,
c uncompetitive inhibition
sons for the sensitivity of the enzyme to changes
in pH are two-fold:
a) sensitivity is associated with a change in protein
structure leading to irreversible denaturation,
b) the catalytic activity depends on the quantity
of electrostatic charges on the enzyme’s active
site generated by the prototropic groups
of the enzyme (cf. 2.4.2.4).
In addition the ionization of dissociable substrates
as affected by pH can be of importance to