PHYSIOLOGY OF MICROORGANISMS 59 



which converts dextrose into alcohol and carbon dioxid. Extra- 

 cellular enzymes are usually digestive in their action. Different 

 kinds attack different substances. Microorganisms are known 

 which produce enzymes that will break down cellulose, starch, 

 sugars, fats, and proteins into simpler substances. The action 

 of an enzyme is said to be specific; a given enzyme will in gen- 

 eral change only one type of material. 



Enzymes are said to be organic catalysts (Gr. kata, down, 

 lyo, to dissolve), that is, % they bring about changes without 

 themselves becoming part of the final product. Many inorganic 

 catalysts, such as finely divided platinum, are known to the 

 chemist. Although catalysts do not form a part of the final 

 products, they certainly are a part of some of the intermediary 

 products in many cases, but become free again when the action has 

 been completed. Enzymes, then, are peculiar in that they are 

 not used up in the using. Theoretically the amount of change that 

 can be brought about by a given enzyme is limited only by the 

 time and conditions under which it must act. 



Most enzymes produce changes that are hydrolytic in nature, 

 that is, they bring about the incorporation of water into the organic 

 molecule with resultant disintegration. The digestion of gelatin 

 by the bacterial enzyme gelatinase, the conversion of starch into 

 maltose* by ptyalin and diastase, the digestion of proteins by 

 pepsin, the conversion of saccharose or cane-sugar into invert 

 sugar by invertase, and the clotting of milk by rennet are a few 

 examples of such hydrolytic changes. The following reaction 

 illustrates the hydrolytic cleavage of saccharose by the invertase 

 produced by yeast: 



C 12 H, 2 O U + H 2 O + Invertase !z=; C 6 H 12 O 6 4- C 6 H 12 O 6 + Invertase. 



Saccharose. Dextrose. Levulose. 



Other enzymes are active oxidizers. Changes of color in 

 dead or injured plant and animal tissues are sometimes due to 

 such oxidases. For example, potatoes turn black and apples 

 become brown when the cells are bruised. Some other enzymes 

 are said to be splitting. One of the best examples of these is the 

 zymase of yeast, which converts dextrose into alcohol and carbon 

 dioxid. 



