394 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



(5) Enzymes, like catalysts, act more rapidly at high temperatures, 

 but there is a limit to the increase in the rate produced by enzymes. 



They are unstable catalysts; they are usually destroyed at a 

 temperature of 56 to 60 or 65. At o their action is nil, or consider- 

 ably less than at room temperature ; at body temperature and up 

 to 45 their catalytic action is at the optimum ; at higher temperatures 

 it is more rapid, but the enzyme rapidly undergoes destruction so that 

 the result of the action is generally less than at 37 to 45. 



(6) Enzymes are very sensitive to the presence of salts, acids and 

 alkalies. Many enzymes will not act unless salt is present. Some, 

 like pepsin, act only in the presence of very dilute acid (*iN) ; others, 

 like trypsin, act best in the presence of dilute alkali ('iN). Most en- 

 zymes act best in a very faintly alkaline medium. The action of all 

 enzymes is stopped by acid or alkali exceeding *lN. 



(7) Some enzymes require the presence of particular salts or other 

 substances for their action, i.e. require a co-enzyme. E.g. phosphates 

 are essential in the fermentation of sugar to alcohol and carbon dioxide, 

 the fat-hydrolysing enzymes require the presence of bile salts, oxidis- 

 ing enzymes require the presence of iron or manganese salts. 



(8) Some enzymes in their action are inhibited by other enzymes or 

 anti-enzymes. 



(9) Many enzymes require liberation from a precursor before they 

 ackproenzymes, e.g. trypsin and its precursor trypsinogen. 



(A full account of the action of enzymes is given by Prof. Bayliss 

 in the " Nature of Enzyme Action". Only the general principles of 

 the action of enzymes can be mentioned here.) 



Nomenclature. 



Enzymes are designated by the suffix -ase, the first part of the word 

 being that of the name of the substance upon which the enzyme acts. 

 The substance upon which the enzyme acts is known as the substrate 

 or hydrolyte. Most enzymes act by hydrolysis and are hydrolytic. 

 Those which act upon the carbohydrates are sometimes termed sucro- 

 clastic (sugar-splitting), upon fats, lipolytic or lipoclastic, upon pro- 

 teins, proteolytic or proteoclastic. Other enzymes act by oxidation of 

 the substrate and are termed oxidases. Another group of enzymes 

 acts upon amino groupings forming hydroxy or keto groups and en- 

 zymes can also remove carbon dioxide from carboxylic acid groups. 

 They may therefore be classified as follows : 



