22 



2. THE KINETICS OF ENZYME REACTIONS 



[instead v = k-^^ (E) (S)] and such reactions would appear to be rare. Thus 

 A, B, and intermediates between them would represent the usually assumed 

 situations, but the other four possibilities with certain enzymes or under 

 particular experimental conditions should not be neglected. 



E+s 



ES \E-P 



It I > k-i > l<2 



l<l > l<2 > Ki 



E + P 



kp> k. > k 



E + P 



k.| > k| > k2 



k.| > k2> k, 



Fig. 2-4. Energy contours for simple enzyme 

 reactions with various relative values of the rate 

 constants. (From Laidler and Socquet, 1950.) 



Assumption That the Free Substrate Concentration Is Equivalent to the 



Substrate Added 



The substrate concentration is generally much higher than the enzyme 

 concentration under experimental conditions and Michaelis and Menten 

 therefore assumed that only the enzyme concentration, and not the substrate 

 concentration, was reduced by the formation of the ES complex. Enzyme 

 concentrations are probably within the range of 10~^— 10~'^ vaM in most 

 studies, so that even if the enzyme were saturated with substrate, there 

 would be no appreciable decrease in the substrate concentration, unless K„^ 

 is very low and a low concentration of substrate is used. However, within 

 the cell or in a limited region of the cell, such as the mitochondria, the con- 

 centrations of substrate and enzyme may be more comparable, especially 

 in closed systems of condensed enzyme units where the substrate is being 

 produced by another enzyme. When the formation of the ES complex re- 



