EFFECTS OF TEMPERATURE: MULTIENZYME SYSTEMS 



769 



The behavior of some of the multienzyme systems treated in Chapter 7 

 with respect to their responses to variation of the temperature will now be 

 discussed in general terms. 



(A) MonoUnear chains. In the simple irreversible system: 



El Ej 



A -> B -> C 



(15-38) 



the rate of formation of C will always depend on the rate of reaction 1 if 

 the system is in a steady state, and on the rate of reaction 2 if it is not 

 in a steady state. Therefore, as long as the system remains in a steady state 

 with a change in the temperature, the over-all rate will vary in the same 

 manner as Uj, the concentration of the intermediate B adjusting to the new 

 conditions. This concentration of B depends on K^ and Ko. the Michaelis 

 constants for E^ and Eg, and on the ratio VJV^ (Eq. 7-3). A rise in (B) 

 will occur if there is an increase in F1/F2, an increase in K^ or a decrease 

 in K^ brought about by the temperature change, which is one way of 

 stating that (B) will rise when reaction 1 is accelerated relatively more 

 than reaction 2. The change in (B) will thus depend on the values of all 

 the activation enthalpies associated with each rate constant for each en- 

 zyme reaction. Thus with regard to V^Vo- the change with temperature 

 is given bv: 





exp 



zl^2* - AH* 

 R 



(15-39) 



where J//^* and J/Zg* are the enthalpies of activation for the breakdown 

 of the ES complexes for reaction 1 and reaction 2 respectively. The differ- 

 ence in the activation enthalpies thus determines the direction and the de- 

 gree of change in F^/Fo and this is a major factor in the alteration of (B) 

 following a variation of the temperature. 



If an increase in the temperature speeds reaction 1 sufficiently, relative 

 to the rate of reaction 2. the system may pass out of a steady state. This 

 may occur more readily if a limit is imposed on the degree to which (B) 

 may rise. If the system leaves the steady state, the temperature dependence 

 will be related more to reaction 2 since it is now controlling the rate of 

 formation of C. It is upon this basis that changes in the temperature char- 

 acteristics have usually been interpreted and this problem will be discussed 

 in greater detail later. However, if the disappearance of A is being measured, 

 such a shift in the temperature dependence will not be observed. 



Care must be taken in those cases where diffusion of the substrate into 

 the multienzyme compartment might be limiting the enzyme rates inasmuch 

 as the observed responses to temperature change may be related more to 

 the diffusion process than to the enzyme reactions. It has often been assumed 



