18 



Enzymes: Kinetics of 

 Oxidations 



I. Catalase 



The respiratory enzymes catalyze the oxidation of many different 

 substrates. Some are directly oxidized in one or two steps, whereas 

 others follow a long pathway with many steps. The respiratory enzymes 

 discussed in this chapter have been selected because of the reversible 

 changes of their absorption spectra which occur during a catalyzed 

 reaction. The changes in the absorption spectra of these enzymes can 

 be related to the concentrations of intermediate compounds similar to 

 E-S, postulated in the last chapter. Quantitative time studies of the 

 absorption spectra during reactions have led to an understanding of 

 the mechanism of action of these enzymes. In this chapter, the mechan- 

 ism is emphasized, deferring most of the details of absorption spectro- 

 photometry to Chapter 26. The physical basis of molecular absorption 

 spectra is discussed in Chapter 27 because these spectra are important 

 in many other phases of biophysics, as well as in measuring the kinetics 

 of enzyme-catalyzed oxidations. 



The physiological role of the first respiratory enzyme discussed here, 



332 



