VALIDITY OF THE MICHAELIS-MENTEN THEORY 17 



The constant K as determined by a graphical procedure of this type may 

 be conveniently called the Michaelis constant and hereafter will be desig- 

 nated by K„^. The true dissociation constant of the ES complex will be de- 

 signated by K^ and will be called the " substrate constant." According to 

 the theory of Michaelis and Menten, £",„ = K„ but it is now known that 

 the experimentally determined ^,„ is not necessarily a dissociation constant 

 and hence the distinction between the two constants must be carefully 

 maintained. The differences in interpretation will be discussed in greater 

 detail later. 



Many enzyme reactions have been found to be adequately described by 

 this simple equation and the Michaelis constants are known for hundreds 

 of enzymes. Selected values are shown in Table 2-1 to illustrate the range 

 observed. A 150,000-fold difference in K^^ is exhibited even in this limited 

 table. The M ichaelis constant has the dimensions of concentration and is 

 actually the substrate concentration for which the rate is half the maximal 

 value. It is a constant only under defined experimental conditions and gen- 

 erally varies with temperature, pH. ionic strength, and other factors. The 

 values of A'j, the rate constant for the breakdown of the ES complex into 

 enzyme and products, vary similarly over a wide range, usually lying be- 

 tween 10~^ and 10* sec~i. 



It is, nevertheless, more and more apparent that many important enzyme 

 reactions do not conform to this theory and it has become increasingly 

 questionable if K can be generally interpreted as the dissociation constant 

 of the ES complex, even though the equation fits the experimental data. 

 Furthermore, the conformation of the Michaelis-Menten equation to the 

 data does not prove the validity of the mechanism postulated, inasmuch as 

 similar or identical equations may be derived from other assumptions. Since 

 much inhibition analysis involves the Michaelis-Menten formulation, it is 

 necessary that we now consider critically the applicability of this treatment 

 and the interpretation of the experimentally determined constants. 



VALIDITY OF THE MICHAELIS-MENTEN THEORY 



There are seven basic assumptions involved in the formulation and inter- 

 pretation of the Michaelis-Menten Eq. 2-7 and these must be evaluated so 

 that the range of applicability of the theory can be assessed. 



Assumption of an Enzyme-Substrate Complex 



The concept of intermediary complexes between enzyme and substrate 

 is of the utmost importance in enzyme kinetics and the understanding of 

 the mechanisms of inhibition. It should be mentioned that the idea of such 

 complexes did not originate with Michaelis and Menten but was suggested 

 by Henri (1902) and stated explicitly by Brown (1902) as follows: " Let 



