22 : 3/ Thermodynamics of Enzyme Reactions 



411 



illustrated as shown in Figure 5 for the reaction of hydrogen peroxide 

 with catalase. As discussed in Chapter 18, there is one intermediate 

 complex formed; it is fairly stable and has a unique optical absorption 

 spectrum. The sketch in Figure 5 shows that there should be two 

 activated complexes for this reaction although neither has been observed 

 directly by any method. 



Figure 5. Absolute rate theory diagram for catalase. This 

 diagram distinguishes the activation energies and the free 

 energy changes due to the reaction. It also emphasizes the 

 difference between the intermediate complex E-S and the 

 activated complexes E-S* and (E-S)-S i . 



There are several difficulties in the application of absolute rate theory 

 to enzyme reactions. First, it is difficult to do experiments over a wide 

 enough temperature range to have significant values for AH* and A.S*. 

 Second, it has never been determined whether or not most enzyme 

 reactions are diffusion limited. Unless this is known, the application of 

 absolute rate theory is certainly questionable. As noted previously, it is 

 often quite difficult to arrive at an independent measure or even estimate 

 of AS*. Finally, no one has ever demonstrated any physical change 

 associated with the formation of the activated complex. 



Optical-spectrum absorption changes and magnetic moment changes 

 have been observed for several intermediate complexes in the reactions 

 of the heme proteins. These increase our faith in the existence of 

 intermediate complexes even for those enzymes for which no such change 

 has ever been demonstrated. However, the intermediate complexes 

 were postulated long before any were ever observed. It may be that 

 new techniques will some day make possible the direct verification (or 

 refutation) of the activated-complex hypothesis. 



