368 THE FIRST ORGANISMS 



by the action of a specific dehydrogenase. In the lactic acid 

 bacillus, on the other hand, where there is no carboxylase, 

 pyruvic acid is reduced directly to lactic acid and is not 

 decarboxylated to any considerable extent. Thus the highly 

 specific action of enzymes is a very important factor in the 

 organisation of protoplasm. Less specific catalysts would not 

 have this capacity to determine the direction in which any 

 particular organic substance in the protoplasm would under- 

 go chemical change. 



The mechanism of enzymic reactions has now been studied 

 from various points of view by many authors but, so far, the 

 problem cannot be considered to have been solved. In its 

 most general form, the participation of enzymes in metabol- 

 ism may be presented as follows: The substance which is 

 undergoing the reaction in question (the substrate) first 

 forms a very short-lived intermediate compound with the 

 enzymic protein. This requires a certain correspondence of 

 structure between the enzyme and the substrate. If this is 

 absent no catalysis whatsoever can take place. When this 

 correspondence exists the reaction between the enzyme and 

 the substrate requires considerably less energy of activation 

 and therefore takes place very fast at ordinary temperatures. 

 However, owing to the specific properties of the enzyme 

 molecule, the intermediate, enzyme-substrate, compound is 

 very unstable. It very soon undergoes a further alteration, in 

 the course of which the substrate is changed in the appro- 

 priate way and the enzyme is regenerated and can once more 

 form an intermediate compound with a fresh portion of the 

 substrate. 



Reactions whereby the substrate is transformed without 

 the help of an enzyme usually require a high energy of 

 activation and therefore take place so slowly that they cannot 

 play a decisive part in metabolism, which is rapid. When the 

 enzyme is present, the high energy barrier seems to be broken 

 down and the route via the intermediate compound seems 

 to be considerably easier and faster. 



Thus, in order that any chemical ingredient may actually 

 take part in metabolism, it must first interact with a protein 

 to form a definite intermediate compound. If not, its chemi- 

 cal potentialities will be realised so slowly as to be of no 



