ENZYMES 201 



This specific action may be explained on the basis of the lock 

 and key hypothesis, which was first proposed by Pasteur to ex- 

 plain why an enzyme would attack a dextro sugar and not a levo 

 one. If we assume that the compound contains some special 

 grouping or configuration of molecules which the enzyme may 

 "fit," then only enzymes with the corresponding configuration 

 will be able to "unlock" the compound and decompose it (Fig. 8). 



Classification of Enzymes. — Enzymes are classified according 

 to the work they do and are named with endings in ase. They 

 are thus classified as follows : 



I. Hydrolases. II. Oxidases. 



a. esterases. a. oxidases proper. 



b. carbohydrases. b. peroxidases. 



c. glucosidases. c. catalases. 



d. proteases. d. reductases. 



e. urease. III. Fermentases. 



IV. Carboxylases. 

 V. Coagulases. 



The hydrolases are concerned in the reactions of hydrolysis, 

 which accompany digestion, and will be discussed in Chapter 

 XXII. The oxidases and fermentases are concerned primarily 

 with respiration and the release of energy. They will be discussed 

 more fully in Chapters XXIV and XXV. 



The carboxylases split organic acids into carbon dioxide and 

 the remaining aldehyde, ketone, or amino group. Thus pyruvic 

 acid is split up into carbon dioxide and acetaldehyde, according 

 to the equation : 



CH 3 CO.COOH->CH 3 COH+C0 2 . 



The coagulases, which bring about the coagulation of various 

 classes of materials, have been mentioned before. Rennin is 

 the enzyme of milk which causes coagulation or curdling and is 

 of much importance in cheese-making. Pectase, which coagulates 

 soluble pectin bodies to a jellylike mass in the presence of calcium, 

 is in this group of enzymes. Vitellase (Lagrange, 1926) from bac- 

 teria and molds permits them to coagulate egg yolk and, there- 

 fore, belongs in this category. 



Variety in Plants.— The number of enzymes which a plant may 

 contain is surprisingly large. From the leaves of the common 

 beet have been taken invertase, diastase, and maltase; from the 

 stem, invertase, diastase, inulase, and emulsin; and from the root, 



