CHEMICAL PROPERTIES OF ENZYMES 39 



and of five volumes edited by Nord and Werkman (1941-1945) 

 is recommended. 



Classification of enzymes. Several plans have been proposed 

 as bases for the classification of enzymes. The simplest of these 

 is to group them into extracellular or digestive enzymes and intra- 

 cellular or respiratory enzymes. The two classifications are also 

 sometimes designated as exoenzymes and endoenzymes, respec- 

 tively. Exoenzymes occur in secretions which pass to the ex- 

 terior of the living cell through the protoplasmic membrane and 

 cell wall. Ptyalin, the amylolytic enzyme in saliva, pepsin, the 

 proteolytic enzyme in the gastric juice, and sucrase, the inverting 

 enzyme of yeast, are extracellular, and much of our knowledge of 

 enzymes has been gained by study of their activities. Little or no 

 energy that is available to the cell is liberated by these enzymes. 

 The endoenzymes act inside the living cell and are not excreted 

 into its environment. Such enzymes are incapable of diffusing 

 through the cell membrane. Some of them can react when re- 

 moved from living cells, whereas others produce their character- 

 istic reactions only in vivo. In contrast to exoenzymes, they 

 liberate large quantities of energy to provide for the metabolic 

 activities of the cell. In Myxomycetes and certain animals that 

 ingest their food, digestion is intracellular instead of extracellular, 

 as is generally the situation among fungi. 



Enzymes may also be classified according to the type of chemi- 

 cal changes produced, that is, whether they are oxidative, hydro- 

 lytic, reductive, or synthetic, or on the basis of the type of 

 chemical decomposed, for example, whether it is carbohydrate, 

 protein, fat, glucoside, or pigment. In general, the name of each 

 specific enzyme is formed from the name of the substrate by 

 substituting ase for the last syllable. The list on page 40 includes 

 a few of the better-known enzymes occurring in fungi and the 

 end-products of the enzymic reactions. 



Chemical properties of enzymes. Concerning the chemical 

 nature of enzymes there are two schools of thought. One is 

 typified by the researches of Willstatter, Oppenheimer, and Wald- 

 schmidt-Leitz, and the other by those of Sherman, Northrop, 

 Sumner, and others. 



The first of these theories is that enzymes contain a special 

 reactive or prosthetic group which possesses a specific affinity 

 or ability to combine with definite groupings in the substrate. 



