342 Enzymes: Kinetics of Oxidations / 1 8 : 2. 



transport, 1 will act as a peroxidase. However, the enzymes named 

 peroxidases have much more rapid reaction rates. 



All peroxidases can use the respiratory enzyme, cytochrome c, as a 

 hydrogen donor. Cytochrome c is part of the cytochrome chain which 

 couples many oxidation chains to molecular oxygen (see Section 4). 

 Thus, it seems possible that the peroxidases might function in normal 

 respiration to use the enzymatically produced peroxides as oxidants. 

 In spite of the relatively large concentrations of peroxidases in mam- 

 malian white blood cells, yeast, and the cells of several higher plants, 

 no definite information is available concerning the physiological role of 

 the peroxidases. They are presented here as a further, slightly more 

 complicated, system to which the type of reasoning developed by 

 Michaelis and Menten can be directly applied and the deductions tested 

 spectrophotometrically. 



Peroxidases are more complicated than catalases in that there are 

 more complexes formed between the substrate and the enzyme. 

 Whereas only one complex is enzymatically active in the case of catalase, 

 two are active in peroxidase reactions. Symbolically, one may represent 

 the reactions as in the case of a single hydrogen donor such as reduced 

 cytochrome c by the following 



e-p-p' x ki p 



E + S^E-Sr 



P a k 3 p' 



E-S I + AH^±E-S n + A 



/>' a ki 



E-S u + AH ^ E + Products + A 



In the case of a dual hydrogen donor such as ascorbic acid, the 

 equations may be written 



E + S^E-S 

 ES + AH 2 ^ES n + (AH)* 

 E-S n + AH 2 -^ E + (AH)* + Products 



2 (AH)*^AH 2 + A 



Evidence for this reaction scheme is based on electron spin resonance 

 data. This method is discussed in Chapter 28. 



The discussion here is restricted to the case of single hydrogen donors 

 present in excess in the external medium. Furthermore, k 6 is assumed 



1 Both oxyhemoglobin and reduced hemoglobin are ferrous forms. 



