22 : 6/ Thermodynamics of Enzyme Reactions 417 



about 1.5 A. In other words, it is necessary for the iron atoms in the 

 two heme groups to come directly into contact with one another. 



In the diffusion-independent region, absolute rate theory may be 

 applied to both the catalase and the peroxidase reaction. The accom- 

 panying table summarizes the data. It can be seen that low values for 

 A//* do not imply diffusion control. 



TABLE II 

 Absolute Rate Theory Parameters for Catalase and Peroxidase 



Rate 

 Enzyme Constant A//* AS* re 1 M 



Catalase* k x kcal/mole - 25 entropy units/mole 



Catalase* k 3 5 -7 



Peroxidase* k 5 8 —5 



* Cf. Chapter 18 for definition of rate constants. 



The values of AS* were computed relative to a standard state of 1 mole 

 per liter concentration of all reactants in water-like solutions. If no 

 changes occurred in bond structure in forming the activated complex, 

 one would predict that 7 entropy units would be lost owing to fewer 

 particles present. Thus, the second and third reactions in the table are, 

 within experimental error, in agreement with this number. The values 

 for AS* suggest that the activated complex does not alter or restrict 

 the freedom of the reactants, except to the extent that they may be 

 regarded as one instead of two molecules. 5 



The value of — AS* for k x is much higher. An attempt to estimate a 

 maximum for — AS* indicates that if all the freedom of the peroxide were 

 lost, AS* should be —28 entropy units/mole. Thus, the intermediate 

 value observed is in accord with the concept that most of the entropy 

 of the peroxide is lost. 



6. Summary 



In this chapter, thermodynamics and kinetic theory have been applied to 

 a discussion of enzyme rate constants. Two types of theories were 

 discussed, collision theory and absolute rate theory. The first is always 

 valid but is difficult to apply to enzyme reactions because it introduces 

 the product of two factors, neither of which can be measured or com- 

 puted directly. However, collision theory does predict correctly that 



5 An alternative interpretation is that bound water must be released during the 

 formation of the activated complex. As this would increase the entropy, there 

 would then have to be a loss of freedom on the part of the reactants. 



