- it gives an understanding of the possible ways by which metabolic activity may be
controlledin vivo; - it allows specific inhibitors to be synthesised and used as therapeutic agents to block
key metabolic pathways underlying clinical conditions (Section 18.1.2).
15.2.3 Effect of temperature and pH on enzyme reactions
Effect of temperature
The initial rate of an enzyme reaction varies with temperature according to the
Arrhenius equation:rate¼AeE=RT ð 15 : 14 Þ
whereAis a constant known as thepre-exponential factor, which is related to the
frequency with which molecules of the enzyme and substrate collide in the correct
orientation to produce the enzyme–substrate complex,Eis the activation energy
(J mol^1 ),Ris the gas constant (8.2 J mol^1 K^1 ), andTis the absolute temperature (K).
Thus a plot of the natural logarithm of the initial rate (or betterkcat) against the
reciprocal of the absolute temperature allows the value ofEto be determined.
Equation 15.14 explains the sensitivity of enzyme reactions to temperature as the
relationship between reaction rate and absolute temperature is exponential. The rate
of most enzyme reactions approximately doubles for every 10oC rise in temperature
(Q 10 value). At a temperature characteristic of the enzyme, and generally in the region
40 to 70C, the enzyme is denatured and enzyme activity is lost. The activity
displayed in this 40 to 70C temperature range depends partly upon the equilibration
time before the reaction is commenced. The so-calledoptimum temperature, at which
the enzyme appears to have maximum activity, therefore arises from a combination of
thermal stability, temperature coefficient and incubation time and for this reason is
not normally chosen for the study of enzyme activity. Enzyme assays are routinely
carried out at 30 or 37C (Section 15.3). Interestingly, recent work with enzymes from
mesophiles and thermophiles have indicated that some have a genuine temperature
optimum in that above a certain temperature the enzyme becomes reversibly less
active but not as a consequence of denaturation. The nature of the structural changes
responsible for such observations has yet to be determined.
Enzymes work by facilitating the formation of atransition state, which is a
transient intermediate in the formation of the product(s) from the substrate(s), that
has a lower energy barrier than that for the non-catalysed reaction. This results in a
decrease in the activation energy (Eact) for the reaction relative to that for the non-
enzyme-catalysed reaction (Fig. 15.7). A decrease in the energy barrier of as little as
5.7 kJ mol^1 , equivalent in energy terms to the strength of a hydrogen bond, will
result in a 10-fold increase in reaction rate. The energy barrier is, of course, lowered
equally for both the forward and reverse reactions, so that the position of equilibrium
is unchanged. As an extreme example of the efficiency of enzyme catalysis, the
enzyme catalase decomposes hydrogen peroxide 10^14 times faster than occurs in the
uncatalysed reaction! Figure 15.7 shows a simple energy profile for the conversion of596 Enzymes