CH. XXVIH.] CLASSIFICATION OF ENZYMES 441 



The foregoing list is not by any means complete, but includes the 

 most important groups. The individual enzymes will be studied in 

 due course, but for the present we will take general considerations 

 only. 



Zymogens. These are the parent substances or precursors of the 

 enzymes. The granules seen in many secreting cells consist very 

 largely of zymogen, which in the act of secretion is converted into 

 the active enzyme. Thus, pepsin is formed from pepsinogen, trypsin 

 from trypsinogen, thrombin from thrombogen, and so forth. 



Activation of Enzymes. Co-enzymes. Many enzymes contained 

 in secretions are in a condition ready for action. In other cases 

 this is not so, and their action only occurs after they have been 

 rendered energetic by the presence or action of other substances, 

 termed activating agents or co-enzymes. 



The Specificity of Enzyme Action. In most cases the action of 

 an enzyme is extraordinarily limited ; thus there are three separate 

 enzymes to hydrolyse the three principal disaccharides, cane sugar, 

 lactose, and maltose, neither of which will act upon either of the 

 other two sugars in the list. Arginase splits arginine into orni thine 

 and urea, but will act upon no other substance. The "lock and 

 key " simile first introduced by Emil Fischer will aid us in under- 

 standing this specificity of action. Each lock must have its special 

 key: so the chemical configuration of an enzyme must be related 

 in some way to the configuration of the substrate to enable it to 

 enter and unlock its parts from one another. 



The Optimum Temperature of Enzyme Action. As the tempera- 

 ture rises the velocity of the action increases, until a temperature 

 is reached at which the activity is greatest. Most enzymes act 

 best at 40 C., but there are exceptions ; malt diastase, for instance, 

 acts best at 60 C. Beyond the optimum temperature a further rise 

 inhibits activity, until a temperature is reached when the enzyme 

 is destroyed. The fatal temperature as a rule is in the neighbour- 

 hood of 50 C. 



The effect of a rise of temperature is thus complex, and is of 

 a two-fold nature. In the first place, and between certain limits, 

 the law of Arrhenius is followed, that is, a rise of 10 doubles or 

 even trebles the velocity of the action of the enzyme, as it does 

 other chemical reactions. But as the temperature rises the velocity 

 of disintegration of the enzyme also rises. The optimum tempera- 

 ture is that at which the enzyme work is best done; this is a 

 temperature at which the accelerating effect is strong enough to finish 

 the reaction quickly, and the retarding effect due to enzyme destruc- 

 tion is not so great as to neutralise the accelerating effect. 



The Inexhaustibility of Enzymes. A small amount of enzyme 

 will act on an unlimited amount of substrate, provided sufficient 



