SPECIFIC NATURE OF CATALYTIC ACTION 223 



Thus, as has been beautifully shown by the researches of 

 E. Fischer, an enzyme may act upon one stereo- isomer and not 

 upon the other, the action being thrown out by the changes in 

 position of a single group. 



To use Fischer's striking analogy, the ferment and its sub- 

 stratum must fit like key and lock, or the reaction does not occur. 



A similar selective action is seen in the case of the organisms 

 which induce fermentation, as was known before the date of 

 Fischer's researches upon the enzymes, as a result of the investi- 

 gations of Pasteur, who showed that only dextrorotatory racemic 

 acid was attacked, and was able to separate the laevorotatory 

 stereo-isomer by such means. It is only those sugars with six 

 or nine carbon atoms that are fermentable, and of these only certain 

 of the stereo- isomers belonging to the " d " series. This fact 

 has been used by Fischer for the purpose of separating the "I" 

 stereo- isomers from the inactive mixture of "d" and "I" sugars 

 obtained by synthetic procedures. 



It can accordingly be predicted, when the constitution and 

 stereo-chemical relations of a body are known from its derivation, 

 whether it will be attacked by a given enzyme or not. 



It is worthy of note that it is not whether a sugar is artificial 

 or natural which determines the attack, but whether it possesses 

 a certain molecular configuration the identity of which must extend 

 even into stereochemical exactness. Hence it follows that the 

 specific action does not mean that an enzyme can attack one sub- 

 stance only ; it may attack many hundreds if only they all possess 

 a given molecular and stereo-identical grouping, and it is this 

 relationship upon which the specific action is based. Also the 

 extent of action depends on the group attacked. Thus both 

 invertase from yeast, and emulsin from bitter almonds, attack 

 the glucoside, amygdalin; but invertase only detaches a molecule 

 of glucose, leaving the remainder untouched, while emulsin, attack- 

 ing a different grouping, breaks the amygdalin up into benzaldehyde, 

 hydrocyanic acid, and glucose. Other natural glucosides, not 

 possessing the particular grouping attackable by invertase, are 

 resistant to that enzyme, and are attacked only by emulsin. 



An interesting fact in the case of the cell, showing physiologi- 

 cal adaptation to environment and nutrition, is that the action 

 depends upon the food-supply; the cells forming in all probability 

 enzymes to suit the configuration of the molecules at their dis- 



