VELOCITY OF REACTION 193 



of the reverse reaction, that such slowing is caused by one only of 

 these products in each case; but rather the contrary, for the enzyme 

 in whichever direction the reaction is going will probably act upon 

 one of the cleavage products only, and dependency upon the 

 relative concentrations, either attach it to the other cleavage 

 product or detach it from it. But while there is nothing in the 

 formation of chemical compounds between the ferment and either 

 the substratum or one of its cleavage products to negative the 

 view that the retardation caused by the products is anything else 

 than the expression of a tendency to reversion, it must be pointed 

 out that the formation of such chemical compounds is an hypothesis 

 invented ad hoc to explain the retardation, and that there is no 

 experiment as proof of the existence of such compounds. 



That the enzyme enters into some relationship with the sub- 

 stratum, as a result of which the velocity of reaction is established 

 or increased, is certain; and it is equally certain that the enzyme 

 also enters into some relationship at a later stage in the reaction 

 with one of the products of the reaction, as a result of which the 

 reaction is slowed, or when a position is considered beyond the 

 equilibrium point, as a result of which the action is made to 

 proceed in the opposite direction. But it is by no means certain 

 that this relationship is that of a chemical compound in the 

 ordinary sense of the word; no such compounds have been 

 isolated, there is no exact relationship pointing to any chemical 

 combination between enzyme and substratum, and the amount 

 of enzyme compared to that of the substratum which it can act 

 upon at the same instant or in an exceedingly short time interval 

 is such as to preclude in all probability the existence of a chemical 

 compound in the ordinary sense of the term. 



It is hence most probable that the influence of the enzyme 

 as an energy transformer is one of a physical character, and at 

 any rate the formation of chemical compounds must at present 

 be taken as unproven. Accordingly it is much safer to make use 

 of a point of view which leaves the question open, and to regard 

 the retardation due to products of reaction as the sign of the 

 tendency to reversal or influence of the products tending to react 

 in the opposite direction, rather than being due to removal of 

 enzyme by combination with such products in a reaction which is 

 regarded as irreversible. 



The matter becomes clearer when we consider the reaction as 



13 



