Food Biochemistry and Food Processing (2 edition)

(Steven Felgate) #1

BLBS102-c08 BLBS102-Simpson March 21, 2012 12:8 Trim: 276mm X 219mm Printer Name: Yet to Come


8 Enzyme Activities 175

Figure 8.8.TheeffectofpHonthekcat/Kmvalue of an enzymatic
reaction that is not associated with the Henderson-Hasselbalch
equation.

the pKavalue can be determined from the point of intersection
of lines on the plot, especially for an enzymatic reaction that is
not associated with the Henderson-Hasselbalch equation pH=
pKa+log ([A−]/[HA]) (Fig. 8.8). Thus, the number of ionizing
groups involved in the reaction can be evaluated (Dixon and
Webb 1979, Tipton and Webb 1979).

Effects of Temperature

Temperature is one of the important factors affecting enzyme
activity. For a reaction to occur at room temperature without the
presence of an enzyme, small proportions of reactant molecules
must have sufficient energy levels to participate in the reaction
(Fig. 8.9A). When the temperature is raised above room tem-
perature, more reactant molecules gain enough energy to be
involved in the reaction (Fig. 8.9B). TheEAis not changed,
but the distribution of energy-sufficient reactants is shifted to a
higher average energy level. When an enzyme is participating in
the reaction, theEAis lowered significantly, and the proportion
of reactant molecules at an energy level above the activation
energy is also greatly increased (Fig. 8.9C). That means the
reaction will proceed at a much higher rate.
Most enzyme-catalyzed reactions are characterized by an in-
crease in the rate of reaction and increased thermal instability
of the enzyme with increasing temperature. Above the critical
temperature, the activity of the enzyme will be reduced signifi-
cantly; while within this critical temperature range, the enzyme
activity will remain at a relatively high level, and inactivation of
the enzyme will not occur. Since the rate of reaction increases
due to the increased temperature by lowering the activation en-
ergy EA, the relationship can be expressed by the Arrhenius
equation:k=Aexp(−EA/RT), whereAis a constant related
to collision probability of reactant molecules, R is the ideal gas
constant (1.987 cal/mol—deg),Tis the temperature in degrees
Kelvin (K=◦C+273.15), andkrepresents the specific rate
constant for any rate, that is,kcatorVmax. The equation can be

Figure 8.9.Plots of temperature effect on the energy levels of
reactant molecules involved in a reaction.(A)The first plot depicts
the distribution of energy levels of the reactant molecules at room
temperature without the presence of an enzyme.(B)The second
plot depicts that at the temperature higher than room temperature
but in the absence of an enzyme.(C)The third plot depicts that at
room temperature in the presence of an enzyme. The vertical line in
each plot indicates the required activation energy level for a reaction
to occur. The shaded portion of distribution in each plot indicates the
proportion of reactant molecules that have enough energy levels to
be involved in the reaction. Thex-axis represents the energy level of
reactant molecules, while they-axis represents the frequency of
reactant molecules at an energy level.

transformed into: lnK=lnA−EA/RT, where the plot of lnK
against 1/Tusually shows a linear relationship with a slope of
−EA/R, where the unit ofEAis cal/mol. The calculatedEA
of a reaction at a particular temperature is useful in predicting
the EAof the reaction at another temperature. And the plot is
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