82 EXPERIMENTAL OBSERVATIONS ON 



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 proceeding in the opposite direction, as, for example, in any 

 of the syntheses by enzymes mentioned on p. 31 et seq. Here the 

 reaction slows down also as it nears the equilibrium point, and we 

 might consider the slowing as due to the product of reaction, and 

 state that this combined with the enzyme and removed it from 

 the sphere of action. But for the reaction proceeding from left 

 to right, it is just this combination of enzyme and product of 

 reaction (now substratum) which is regarded as giving rise to 

 the action of the enzyme. It is evident, then, that our explanation 

 must be symmetrical on both sides of the equilibrium point, if 

 the expression may be allowed, and that it is better to regard 

 the relationship between enzyme and substratum on the one hand, 

 or cleavage product on the other, as favouring the reaction in a 

 determinate direction in each case, rather than as doing this in 

 one case, and simply inertly removing enzyme in the other. 



Returning to the consideration of a formula to suit the entire 

 course of the reaction, after the above discussion it becomes clear 

 that in the first place we must not remove the expression k 2 x 2 or 

 there will be experimental variation in the later stages increasing 

 as the equilibrium point is neared ; and, secondly, that into the 

 portion of each expression which represents the action of the enzyme 

 we must introduce a factor expressing that this action is not constant 

 throughout, but intensifies as the concentration of the substratum 

 diminishes, and here it must be remembered that for the expression 

 &! (a - x) the concentration of the substratum is a - x, and for the 

 second expression in the equation of velocity of reaction k 2 x 2 , the 

 concentration of the substratum is x. In other words, the influence 

 of the enzyme in either direction is not a constant but is some 

 function of the concentration of the substratum. As a simple 



approximation the factor introduced by Henri f 1 + e- J may be used, 



so that &j becomes \ M + e- j ; only it must be remembered that 



V &/ 

 such a correction equally applies to the reversed reaction, and hence 



