PRODUCTION OF ENZYMES 113 



mineral salt, and in the cases mentioned a calcium salt) to 

 form an insoluble compound, the curd or coagulum. 



Another variety is the "activating" enzymes or "kinases" 

 such as the enterokinase of the intestine. The action here 

 is a splitting of the zymogen or mother substance, or form in 

 which the enzyme is built up by the cell so as to liberate the 

 active enzyme. 



Of a character quite distinct from the splitting enzymes 

 are 



II. The zymases. Their action seems to be to cause a 

 "shifting or rearrangement of the carbon atoms" so that 

 new compounds are formed which are not assumed to have 

 been constituents of the original molecule. Most commonly 

 there is a closer combination of the carbon and oxygen 

 atoms, frequently even the formation of CO2 so that con- 

 siderable energy is thus liberated. Examples are the zymase 

 or alcoholase of yeast which converts sugar into alcohol and 

 carbon dioxide; CeH^Oe = 2C2H6O + 2CO2: also urease, 

 which causes the change of urea into ammonia and carbon 

 dioxide. Another common zymase is the lactacidase in lactic 

 acid fermentation. 



III. Oxidizing enzymes also play an important part in 

 many of the activities of higher plants and animals. Among 

 the bacteria this action is illustrated by the formation of 

 nitrites, nitrates and sulphates and the oxidation of alcohol 

 to acetic acid as already described. 



IV. Rediwing enzymes occur in many of the denitrifying 

 bacteria and in those which liberate H2S from sulphates. A 

 very widely distributed reducing enzyme is " catalase" which 

 decomposes hydrogen peroxide. 



As previously stated, most of the physiological activities of 

 bacteria are due to the enzymes that they produce. It is 

 evident that for action to occur on substances which do 

 not diffuse into the bacterial cell — starches, cellulose, com- 

 plex proteins, gelatin — the enzymes must pass out of the 

 bacterium and consequently may be found in the surround- 

 ing medium. Substances like sugars, peptones, alcohol, 

 which are readily diffusible, may be acted on by enzymes 

 retained within the cell body. In the former case the 

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