CHAPTER IV 

 CATALYSIS AND ENZYME ACTION 



reactions brought about by living cells are 

 I .characterised by the ease with which apparently inert 

 substances are altered. Reactions which usually require 

 strong acids or alkalies and prolonged heating are produced 

 rapidly by extracts obtained from living tissues. Hence we 

 must devote further attention to the means by which the rate 

 of chemical reactions can be affected. 



In studying the rate of chemical change we saw that the 

 rate of change depended upon the concentration of the reacting 

 substances and on the velocity constant, k. Thus the 



equation, ~ x = Ci-C 2 , where k = the velocity constant and 



G! and C 2 the concentrations of the reacting substances, 

 indicates that if the concentration of the reacting substances 

 is the same, a change in the velocity constant, k, will cause a 

 change in the rate of reaction. Such an alteration in rate is 

 known as a catalytic action and the change may be either an 

 increase (positive catalysis) or a decrease (negative catalysis) 

 in the rate of reaction. 



A catalyst does not add to or subtract energy from the 

 reacting system, thus as the equilibrium constant K of a 

 reversible reaction is not altered* the rate of the reverse 

 change must be equally accelerated. This is shown by the 



equations on p. 37 where K = and if K remains the same 



K 



and k' is altered k" must be altered to the same extent. 



In the above the assumption is made that ah 1 reactions that 

 can be brought about by catalytic activity are proceeding in 

 the absence of a catalyst, but, in some cases, infinitely slowly. 

 This assumption is not universally granted as some workers 

 believe that the enzyme is capable of a trigger-like action in 

 releasing a chemical change which will not occur until the 



* The catalyst may unite with one of the reacting substances and 

 by altering the active mass cause a slight change in the equilibrium 

 conditions. 



57 



