14 



2. THE KINETICS OF ENZYME REACTIONS 



are recommended for a more detailed exposition of fundamental enzyme 

 kinetics. The mathematical approach to this subject is well presented by 

 Reiner (1959). 



THE DEPENDENCE OF ENZYME RATE ON THE SUBSTRATE 



CONCENTRATION 



The rate of an enzyme-catalyzed reaction does not increase linearly with 

 increase in substrate concentration, as would occur if the reaction were 

 nonenzymic, but increases progressively more slowly in a hyperbolic man- 

 ner to reach a level of maximal activity beyond which the rate is dependent 

 only on the enzyme concentration (Fig. 2-1). This deviation from nonenzy- 



(S )-► 



Fig. 2-1. Dependence of the rates of nonenzyme and 

 enzyme reactions on the concentrations of the reactants. 



mic kinetics in the case of the hydrolysis of sucrose by /?-fructofuranosidase 

 led Michaelis and Menten (1913) to assume that a complex was formed be- 

 tween the enzyme and the substrate, preliminary to the hydrolysis, and that 

 the over-all rate of the reaction was dependent on the rate of breakdown 

 of this complex into the products of the reaction, in this case glucose and 

 fructose. The formation of this complex was assumed to be essentially in- 

 stantaneous so that the concentration of the comi)lex was maintained at a 

 constant level dependent on the thei-modynamic equilibrium between en- 

 zyme, substrate, and complex. The rate expression on this basis can be 



