18 : 1/ Enzymes: Kinetics of Oxidations 



339 



The technical difficulties of measuring t lj2 ^ and p m are less than those of 

 measuring t pj2 . Accordingly, Equations 11 and 12 are convenient for 

 precise measurement of k 1 and k 3 . These values agree with those found 

 by using Equations 5 and 7 and those found by using Equations 9 and 10. 



Computed for 



"1 

 *2 



I0 6 M" 

 = 



sec 



k 3 a = 0.5 sec" 1 

 e = I0" 6 M 

 i = 4x|6" 6 M 



* 



Figure 5. The intermediate complex for the peroxidatic 

 reaction of catalase. The quantities shown above are used to 

 compute k 1 and k 3 . The curve illustrated is based on an 

 analog-computer study. Similar curves can be found experi- 

 mentally both for the enzyme-substrate complex of catalase 

 and for complex II of peroxidase. After B. Chance, "Velocity 

 Constants in Enzyme Reactions," Arch. Biochem. Biophys. 71 : 

 130 (1957). 



The rate constant k 2 is more difficult to determine precisely. An 

 upper limit for k 2 can be found by making suitable approximations from 

 the data at the end of the reaction. Also, as noted previously, knowing 

 k x and k 3 independently, one can obtain an estimate of k 2 from the 

 apparent Michaelis constant. Some typical values for the different 

 constants for bacterial catalase are 



X = H 2 2 



k 1 = 2 x 10' 



CH 3 OOH 



0.9 x 10 6 



C 2 H 5 OOH 



1.0 x 10 4 



M 



sec 



S = HCOONa 



£ 3 = 175 



CH3OH 



91 



C 2 H 5 OH 



13 



M 



-1 



sec 



-1 



An upper limit on the constant k 2 can be estimated as k 2 ^ 0.0002 sec' 



The other type of reaction discussed for catalase is the destruction of 

 hydrogen peroxide. This reaction is similar to the peroxidatic reactions, 



