Food Biochemistry and Food Processing (2 edition)

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