18:2/ Enzymes: Kinetics of Oxidations 341 



constants k x and k 3 . Only two of these equations are necessary, the 

 third providing an internal check on the validity of the equations in 

 (13). The foregoing analysis shows that the differential equations 

 chosen do agree with the data over wide ranges of concentration, ionic 

 strength, temperature, and so forth. At very high concentrations of 

 hydrogen peroxide, the equations in (13) apparently break down. 



For any given catalase, the values of k x and k 2 will be the same for 

 H 2 2 entering either the peroxidatic or catalatic reactions. The 

 constant k 3 is, in contrast, dependent on the hydrogen donor in the 

 peroxidatic reaction. For the bacterial catalase, the value of k 3 for the 

 catalatic reaction is about 1.7 x 10 7 M _1 sec _1 . In the catalatic 

 reaction, there is no spectrophotometric evidence for the existence of a 

 compound E-S-S. It seems almost inconceivable that a compound of 

 this type does not have at least a transitory existence. It is likewise 

 surprising that no complex of the form ESAH 2 has ever been detected. 

 These will be commented upon further in Chapter 22. 



The reactions of catalase have been discussed in comparative detail in 

 this section, and those of peroxidase are presented in the next section. 

 They have been emphasized because these reactions, although not 

 obeying Michaelis-Menten kinetics, are strong supporting evidence for 

 the existence of intermediate complexes during enzyme reactions. 

 Because it is possible to observe the concentrations of all the inter- 

 mediates and reactants, and to vary these concentrations, it is possible 

 to check that the reaction does obey the equations chosen. There is no 

 evidence that the hydrolase reaction does not also follow the equations 

 for the peroxidatic reactions of catalase. 



The spectra of catalase and peroxidase cannot be directly interpreted 

 at the present time, other than giving the quantitative amounts of the 

 various substances present. For additional information such as the 

 electronic state of the iron, or the existence of other types of intermediates, 

 one must turn to different lines of investigation. Some of these are 

 discussed in Chapter 22 and still others in the chapter on magnetic 

 measurements, 28. 



2. Peroxidase 



Peroxidases, like catalases, are heme compounds. They catalyze the 

 peroxidatic but not the catalatic reactions. Peroxidases are widely 

 distributed, being more abundant in plant cells than in animal cells. 

 Almost any ferriheme protein (that is, one in which the iron is in the 

 oxidized or ferric state) will act as a peroxidase. Even the ferric form 

 of hemoglobin (called methemoglobin), although unsuitable for oxygen 



