CATALYSIS AND ENZYME ACTION 63 



Proteoclastic enzymes can be divided into three groups 

 according to their behaviour at different concentrations of 

 hydrogen ions. First there is the group of pepsin-like enzymes 

 which act best in a decidedly acid medium (C H about io~ 2 ) 

 and are destroyed by weak alkali. Next there is the group of 

 trypsin-like enzymes which act best in an alkaline medium 

 (C H about io~ 9 ) ; finally there is the group of erepsin-like 

 enzymes which act best near the neutral point (C H about io~~ 7 ) . 

 All enzymes are destroyed by marked acidity or alkalinity. 

 Associated with these relations to the concentration of 

 hydrogen ions are differences in the substances attacked and 

 the products produced by their activity. The latter we shall 

 defer to a later chapter (p. 120). 



Proteoclastic enzymes are not the only ones affected by 

 varying reaction. Other enzymes show an optimum activity 

 at definite concentrations of hydrogen ions near the neutral 

 point but the range of their action is not so extended. 



The influence of acidity and alkalinity on enzymes has been 

 attributed to a reaction with the enzyme. Presuming that the 

 enzyme is an acid, a base, or an amphoteric substance we can 

 trace out varying relations between the enzyme and the 

 concentration of hydrogen ions. Suppose that the enzyme is 

 active only in the form of an ion, then at some point near the 

 neutral point, the isoelectric point of an amphoteric substance, 

 the minimum amount of ions will be present and the activity 

 least.* 



Trypsin, for instance, has been stated to behave like a 

 dibasic acid. 



To make the matter clearer we can refer to the dissociation 

 of the known dibasic acid, carbonic acid. This according to 

 the law of mass action occurs in two stages as shown by the 

 following equations : 



K y C n2C o 3 =CH-C H co~3~ and K^CTICO^ C H .C C o~ 3 ~ in which 

 K,= 3-o4Xio~ 7 and K a = 1-291 Xio~ n .t 



We see that the following molecular and ionic species occur, 

 hydrogen ion, undissociated carbonic acid, bicarbonate ion, 

 and carbonate ion, and in the following table are given the 

 amounts of these substances when the total amount of carbonic 

 acid is equal to unity. 



* S. P. L. S0renson, Ergeb. d. Physiol., 1912, vol. 12, p. 451. 



f J. Walker and W. Cormak, Journ. Chem. Soc., 1900, Trans, p. 5. 



