ROBERT A. ALBERTY 



mum initial velocity have been obtained separately, and in some 

 studies, pains have not been taken to differentiate between the 

 reversible and irreversible effects of temperature change. 



The effect of temperature on enzymatic reactions has been 

 the subject of controversy. Several authors have claimed that 

 sharp breaks are obtained in plots of logarithm of rate versus 

 reciprocal absolute temperature. To prove that the breaks are 

 quite sharp requires accurate experiments at a large number of 

 temperatures. The interpretation of these data in terms of the 

 shift from one "pace-maker" reaction to another has been criti- 

 cized on physical chemical grounds by Morales (13a) and by 

 Kistiakowsky and Lumry (11). If there is a series of consecutive 

 steps havmg different activation energies, the experimental 

 activation energy becomes higher the lower the temperature, 

 and the abruptness of the change in slope will be dependent 

 upon the difference in activation energy of the two steps. If 

 this difference is of the order of 10 kcal. mole~^ the change in 

 slope will occur over approximately an 80 ° temperature range. 

 As pointed out by Kistiakowsky and Lumry changes in slope in 

 either direction and sharper changes in slope may be due to a 

 change of the enzyme from one form to another, these two forms 

 catalyzing the reaction with different activation energies. In 

 this case the temperature coefficient for the enzyme transfor- 

 mation will not contribute to the temperature coefficient 

 for the enzymatic reaction but will determine the tempera- 

 ture range over which the transition from one activation energy 

 to the other occurs. In order for the apparent activation energy 

 to change over a small range it is necessary that the energy of 

 isomerization of the protein be very great — of the order of 160 

 kcal. mole~^ — if the transition occurs over a temperature range 

 of 5°. 



Urease was one of the enzymes for which it had been claimed 

 that joined segments of straight lines are obtained by plotting 

 the logarithm of the rate against the reciprocal absolute tem- 

 perature with sharp breaks at "critical temperatures." Exten- 

 sive experimental work by Kistiakowsky and Lumry (11) showed 



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