SPECIFICITY OF ENZYMES 343 



after having completed its work, unless, of course, the products 

 of the reaction have any effect on it. 



In the group of carbohydrates the action of the enzymes 

 is usually regarded as being more or less specific, each disac- 

 charide being hydrolysed only by its own enzyme, e.g. cane 

 sugar by invertase, milk sugar by lactase, and malt sugar by 

 maltase. 



That this specific activity is in some way connected with 

 the molecular structure of the substances would appear from 

 the researches of Fischer on the action of enzymes upon artifi- 

 cial glucosides. Fischer, by the action of methyl alcohol and 

 hydrochloric acid on glucose, obtained two stereoisomeric 

 methyl glucosides known respectively as the a and ^ variety. 

 Now these two substances differ from each other only by the 

 arrangement in space of the groups attached to the terminal 

 carbon atom, and it is found that while the a modification is 

 readily converted by maltase into glucose and methyl alcohol, 

 the ^ modification is not affected by maltase at all, but is, on 

 the other hand, hydrolysed by emulsin, which has no action 

 on the a compound. 



It would appear from this that the structure of the mole- 

 cule which is to be decomposed is the determining factor. 



Incidentally it may be mentioned that the fact that emulsin 

 and maltase are complementary in their action upon a and ^ 

 methyl glucosides, enables one to classify a glucoside as be- 

 longing to the a type if it is attacked by maltase and not by 

 emulsin, or to the /3 type if it is attacked by emulsin and not 

 by maltase. 



Several other examples of this selective action on the part 

 of enzymes for different optical isomers have been described 

 by Fischer and Abderhalden, who found that whereas d-alanyl- 

 d-alanine, d-alanyl-1-leucine were split up by enzymes, their 

 stereoisomers d-alanyl-1-alanine and 1-alanyl-d-alanine were not. 



This peculiar dependence upon structure has led Fischer 

 to suggest that the relationship which exists between the sub- 

 stance to be decomposed and its enzyme is similar to that 

 existing between a lock and its key ; or, in other words, unless 

 the molecular structures of the two substances fit each other no 

 interaction can take place. 



These facts, of course, give strong support to the theory of 



