CARBOHYDRATES 61 



are set free when the action is accomplished. They are thus 

 made available for transforming a new portion of material, and 

 a given weight of enzyme may transform many hundred times 

 its weight of substrate. One might think that under these cir- 

 cumstances the reaction would go on indefinitely, but as a matter 

 of practical experience this is not the case. Various factors 

 contribute to this result. In the first place, there evidently is 

 a gradual destruction of the enzyme. Then also, many reactions 

 of biological importance are reversible, and an accumulation of 

 end products will slow up the process. These products them- 

 selves may affect the speed of the reaction either way. If they 

 tend to speed up the process, the effect is called auto catalysis. 

 If they tend to inhibit the action, the effect is called negative 

 auto catalysis. Then also, there may be a slight amount of per- 

 manent combination between the enzyme and the substrate, thus 

 diminishing the amount of the enzyme. All of these factors 

 described above tend to slow down the reaction except auto- 

 catalysis. Enzymes as a rule behave as do other catalysts, but 

 they are produced only by living cells, are always colloidal 

 which influences their behavior, and are more easily destroyed 

 than are inorganic catalysts. The velocity of enzyme action is 

 that of a unimolecular action (only one substance involved in 

 the reaction). 



There are some facts which do not quite harmonize with the 

 classification of enzyme action as truly catalytic. A catalyst is 

 supposed to have no effect upon the character of a reaction, or 

 its final point of equilibrium. The destruction of glucose by 

 enzyme action may yield, however, under the influence of dif- 

 ferent enzymes, widely differing products, e. g., C0 2 and alco- 

 hol, lactic acid, or butyric acid and hydrogen. Apparently the 

 enzymes concerned not only catalyze, but also direct the course 

 of the action. 



It was formerly believed that the enzyme of yeast which 

 breaks down sugar into alcohol and carbon dioxide acted only 

 within the living cell, and that the vital activities of the cell 

 were intimately connected with its action. Organisms of the 



