ACTION OF ENZYMES 57 



Accordingly, if the degree of reversibility is small, that is, if 

 the equilibrium point lies close up to the end point corresponding 

 to the substances being practically all converted into the forms 

 represented on the right-hand side of the equation of reaction, the 

 value of the second expression on the right-hand side of the equation 

 of velocity becomes very small compared to the value of the first 

 expression, and may be neglected. That is, the reaction may be 

 taken as running irreversibly, and this is what is usually done in 

 obtaining the equations for the velocity of most reactions which 

 concern us. 



This means that in those equations which run almost to com- 

 pletion, the value of the constant k 2 is small compared to k r Now 

 the constants are the reciprocals of the resistances to the re- 

 action running in the two opposed directions, and hence this 

 means that the resistance to such a reaction running from left to 

 right is small compared to the resistance for it running from right 

 to left. 



It must be pointed out, however, that in the later stages of 

 the reaction running from left to right, when a - x has become 

 very small compared to x, although 7^ is large compared to Jc 2 , the 

 second expression may cease to be negligible, and hence although 

 the equation obtained by neglect of the second expression, may 

 truly represent the course of the reaction throughout the greater 

 part of its length, there may be a difference between observed and 

 calculated results at the later stages of the experiment. 



The discrepancy will be less, the lower is the initial concentration 

 of the substratum, and, as we have seen in speaking of equilibrium 

 in dilute solutions of type 2, where one substance is resolved into 

 two others, the equilibrium point lies close up to complete resolution 

 into the two substances. Accordingly for this type of reaction in 

 dilute solution, which includes all the digestive processes, the second 

 expression can be allowed to drop without sensible error. 



In concentrated solutions, for this type of reaction, as the re- 

 action approaches completion, and the tendency to reversion begins 

 to become potent, the velocity of reaction must, however, fall off, 

 and the velocity constant calculated by neglecting the reversibility 

 expression (& 2 u? 2 ) must fall off in value, as it is actually found to 

 do by experiment. 1 



This important fact has been neglected by most experimenters, 

 and the drop in velocity has been attributed to combination between 

 the catalyst and the products of reaction. This explanation of the 

 effect of products of reaction in slowing the reaction is no doubt 



1 See p. 72. 



