344 Enzymes: Kinetics of Oxidations / 1 8 : 3 



As with catalase, the conditions as x or a approaches complete utiliza- 

 tion are necessary to determine k 2 and A; 4 . These are difficult in that 

 the auto-oxidation of the first complex becomes more important as the 

 hydrogen-donor concentration decreases. Also similar to the catalase 

 reaction is the approximation 



' h = TT— ~ ( 24 ) 



Pmhl2\ a 



The value for the kinetic rate constant k 1 depends on whether S is 

 hydrogen peroxide, methyl hydrogen peroxide, or ethyl hydrogen 

 peroxide. The constants k 3 and k 5 are different for different hydrogen 

 donors. All three vary for peroxidases from different sources. A 

 typical set of values is 



k x = 0.9 x 10 7 M _1 sec _1 l f E = horse-radish peroxidase 



k 3 = 2 x 10 7 M" 1 sec" 1 I for I S = H 2 2 

 k 5 = 2.4 x 10 5 M" 1 sec" 1 J [^H = HN0 2 



The values for k 3 are comparably large even when AH is the protein 

 cytochrome c; this is particularly impressive when one considers the 

 size of the molecules involved. 



Studies of the kinetics of peroxidases agree with the reaction mechan- 

 ism presented. This supports the reality of intermediate complexes of 

 the type discussed in Michaelis-Menten kinetics. It also shows that the 

 reactions may be far more complex than those postulated in the previous 

 chapter. 



3. Biological Oxidations 



The previous two sections dealt with the enzymatically catalyzed 

 oxidation of reduced compounds, using a peroxide such as HOOH as 

 the oxygen donor. These reactions are convenient to study; they help 

 to verify the physical existence of transient, enzyme-substrate complexes. 

 However, both peroxidase and catalase are believed to be unusual 

 respiratory enzymes in that the most frequent intracellular oxygen 

 donor is the molecule 2 . The pathway from the reduced compound to 

 the molecular oxygen is often a long one involving many catalyzed steps. 

 (See, for example, the biological oxidation of glucose diagrammed in 

 Chapter 8, Figure 8.) Only the last step actually involves molecular 

 oxygen, but many steps along the way are spoken of as oxidations. 



The oxidations within biological systems may be divided in many 

 fashions into different types. One depends on whether the oxidizing 

 substance (which itself becomes reduced) is 2 , H 2 2 , or some other 

 compound. A second type of division separates those oxidations 

 incorporating oxygen into the molecule from those involving the removal 

 of hydrogen. These may be illustrated by the following two reactions 

 of ethyl alcohol 



