ENZYMES 169 



C. Influence of Substrate Concentration 



When the substrate concentration is varied, in general it is found that the 

 initial velocity changes with the concentration of substrate in a linear 

 manner. In other words the reaction is first order with respect to the 

 substrate. 



However this is no longer the case when the substrate concentration is 

 greater than a certain value : the initial velocity may then become indepen- 

 dent of the substrate concentration. In order to explain this, it is postulated 

 that the reaction takes place in two stages, a combination of the enzyme 

 with the substrate (E + S ^ X), and a decomposition of the enzyme- 

 substrate complex with regeneration of the enzyme (X ^ P + E). 



^^-^^ O O^O o OJD O y 



o%ooOo o 



qO o 



o 



Fig. 34 — Enzyme-substrate relations at a low, and at a high, concentration of substrate. 



When the amount of substrate is small, a little of the enzyme combines 

 with the substrate but many free enzyme molecules remain. The quantity 

 of the enzyme-substrate complex formed in unit time is proportional to the 

 concentration of the substrate, and the initial velocity, too, is proportional 

 to this concentration. But if, relative to the enzyme, there is plenty of 

 substrate, the enzyme will be present wholly in the form of the complex, 

 such that the addition of further molecules of substrate does not alter the 

 situation as far as the concentration of the enzyme-substrate complex is 

 concerned (Fig. 34). 



D. The Michaelis Constant ^ 



Let us represent by [E]q the total concentration of the enzyme and by [E] 

 the concentration of the free enzyme. [S] will represent the concentration 



