ENZYME KINETICS 



The initial rate of hydrolysis of urea by urease has been 

 investigated over a 5000-fold range of urea concentration by 

 Kistiakowsky and co-workers. It was found that a four-param- 

 eter equation was required to represent these data, one of the 

 parameters allowing for inhibition at high substrate concentra- 

 tions. The type of three-parameter rate equation which was 

 found to represent the data at the lower concentrations could be 

 derived by assuming two types of independent active sites, 

 differing in their Michaelis-Menten rate parameters, or pairs of 

 identical sites which interact so that the kinetic parameters of a 

 site are altered when the neighbor site undergoes combination 

 with urea. 



MECHANISMS OF COENZYME REACTIONS 



When two substrates are involved in an enzymatic reaction 

 the complexity of the mechanism may be increased considerably. 

 The simplest way in which the Michaelis-Menten mechanism 

 may be elaborated to include a second substrate is 



E + A , *' - EA (8) 



EA + B — ^ E 4- C -f D 



The steady-state treatment of this mechanism yields the rate 

 equation 



1 + [^2 + ^3(B)]A'i(A) ^'^ 



Thus the maximum initial velocity will be directly proportional 

 to (B) and the Michaelis constant of A will be a linear function 

 of (B). Chance has found that this rate equation and mechanism 

 represent the data for peroxidase under conditions where only 

 one enzyme-substrate complex must be considered. 



If one of the products is only slowly dissociated from the 

 enzyme as indicated in the following mechanism 



E -t- A , ^' ' EA 

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