ENZYMES 



271 



Since the enzyme-substrate complex is a very labile product and is 

 present for only a very short time, it is difficult to demonstrate its exist- 

 ence. However, in the case of catalase it has been possible to provide 

 evidence for the existence of such an intermediate compound with the 

 aid of very speedy, automatically recorded, electrophotometric measure- 

 ments. 



Factors affecting activity 



The speed with which a given reaction proceeds in the presence of 

 an enzyme is influenced by many variables, which include the following: 

 (1) concentration of substrate, (2) concentration of enzyme, (3) specific 

 activators such as coenzymes and metalHc ions, (4) temperature, (5) pH, 

 (6) oxidation-reduction potential, (7) ionic strength, and (8) products 

 of the reaction. 



When the influence of substrate concentration on the speed of an en- 

 zymatic reaction is studied, it is observed that the rate of the reaction 

 increases with substrate concentration up to a certain point beyond 

 which there is no increase in activity. This occurs because all the 

 enzyme eventually is converted into the intermediary compound by mass 

 action, and the limiting speed of the reaction then becomes that of the 

 decomposition of this complex. If under these conditions one doubles 

 the enzyme concentration, the rate of reaction will also double, since 

 twice as much enzyme-substrate complex will be available for decomposi- 

 tion. 



The rate of most enzyme-catalyzed reactions is increased about 1.2-4 

 fold by a 10° rise in temperature. This temperature effect is much lower 

 than that observed in the case of many uncatalyzed chemical reactions. 

 For this reason enzymatic reactions proceed at higher speeds at low 

 temperatures than the corresponding uncatalyzed ones. Most enzymes 

 are thermolabile and will lose activity when exposed to high tempera- 

 tures, e.g., 60°C., over prolonged periods of time. 



Acidity also has a profound effect on enzyme activity. Each enzyme 

 in the presence of a certain substrate has a characteristic pH at which 

 its activity is highest. Some enzymes, e.g., pepsin, require an acid 

 medium; others, e.g., trypsin, need alkaline conditions for maximum 

 activity. Most enzymes work best under conditions which are neither 

 strongly oxidizing nor reducing, and, in fact, are frequently inactivated 

 by strong oxidizing or reducing agents. 



The effectiveness of some enzymes is influenced by the ionic strength 

 (concentration of ions) of the solution in which they act; this is in addi- 

 tion to the specific effects of various anions and cations. In most cases, 

 enzymes are inhibited by the end products of the reactions which they 

 catalyze. 



