17:3/ Enzyme Kinetics of Hydrolytic Reactions 



321 



is called sucrase and the products glucose and fructose. (Sucrase is a 

 glycosidase; it is often also called invertase because it inverts the optical 

 rotation due to sucrose.) The reverse arrow is omitted because at 

 equilibrium only the products of hydrolysis can usually be detected. 



Most hydrolase reactions have a negligible velocity in the absence of 

 any enzyme. If enzyme is added, the substrate is hydrolyzed at a 

 measurable rate. As time progresses, this rate decreases because the 

 substrate present decreases. One might be tempted to write the rate 

 equation 



The concentration of the water has been omitted because it is constant 

 in dilute solutions and may be included in k. 



Figure I. The concentration of 

 the substrate S of a hydrolytic 

 reaction as a function of time. 

 This is actually observed. 



Figure 2. The rate of disappear- 

 ance of S as computed from the 

 curve at left. 



However, a graph of the rate of the enzyme catalyzed reaction 

 plotted against [S] shows that at higher concentrations the term k in 

 Equation 1 is not constant. Similar studies show that this k is propor- 

 tional to the enzyme concentration at low concentrations but not at 

 high enzyme concentrations. This situation is illustrated in Figures 1, 

 2, 3, and 4. 



Michaelis and Menten pointed out that this failure to obey the 

 prediction of Equation 1 could easily be explained if the reaction 

 mechanism were oversimplified. A simple scheme which they proposed 

 is to assume that the enzyme and substrate formed an intermediate 



