FERMENTS, OR ENZYMES 73 



reaction is, however, the same at whatever rate it proceeds, and the 

 enzyme does not appear in the final products. Many enzymes such as 

 diastase can be found unaltered in amount after they have completed 

 their action. This is determined by adding a fresh supply of substrate 

 (that is, of material to be acted on), when the enzymic action proceeds 

 again in the usual way. The same is no doubt true for all enzymes, 

 though as yet it can actually be proved for only a few of them. Enzymes, 

 therefore, may be defined as catalysts produced by living organisms. 



The Properties of Enzymes 



Although enzymes are examples of catalysts, they exhibit many proper- 

 ties that appear to differ from those of inorganic catalysts. It will, 

 therefore, be advisable in considering each quality to compare it in 

 catalysts and enzymes, for by this method a much clearer conception of 

 the nature of enzyme action can be gained (Bayliss 19 ). Those properties 

 that are strictly peculiar to enzymes we shall consider later. 



1. Most enzymes are remarkably specific in their action, whereas inor- 

 ganic catalysts are very much less so. Thus, in the case of the enzymes 

 which bring about inversion of disaccharides, this specificity is clearly 

 shown. There is a special enzyme for each of the three disaccharides 

 maltose, lactose and cane sugar and one of these can not replace 

 another. 



Still more strikingly is this specificity of enzyme action demonstrated 

 in the fact that certain enzymes, such as zymase (expressed from yeast), 

 will act only on bodies having a certain configuration, that is, having 

 their side chains arranged in a certain way. Thus, there are two varie- 

 ties of dextrose (a and /?), which differ from each other solely in the 

 fact that the side chains are arranged in different positions with rela- 

 tion to the central chain of carbon atoms. This form of isomerism is 

 called stereoisomerism because the two bodies rotate the plane of polar- 

 ized light to an equal degree in opposite directions. Zymase acts on one 

 of these but not on the other, and there are innumerable examples of the 

 same kind. Indeed, of all bodies that exist in two stereoisomers only 

 one is found in living cells and it is on this variety alone that the enzymes 

 in animals can act. A similar specificity exists between certain drugs and 

 their pharmacological action. 



Specificity of action is explained by supposing that a union occurs 

 between the substrate and the enzyme, and for this union to take 

 place the enzyme must possess a configuration which corresponds accu- 

 rately with that of the substrate. The process has been compared to a 

 lock and key; the key must be shaped to fit the lock, or it can not 

 operate. The specificity does not, however, in itself disprove the close 



