410 Thermodynamics of Enzyme Reactions /22 : 3 



Equation 9 contains the prediction that the slope of the Arrhenius 

 plot should be A//* (or AZs* as noted earlier). All the terms in Equation 

 9 may be found experimentally except AS*. Accordingly, it might be 

 possible to test absolute rate theory by determining AS* by some other 

 means. If this is not possible, it is hard to see any real advantage to 

 an arbitrary correction factor AS* as opposed to the arbitrary product 

 aZ of the Arrhenius collision theory. 



For those familiar with quantum mechanics, a rigorous derivation of 

 reaction rate theory can be found in the text by Kimball and Eyring. 

 They show that under certain conditions the rate constant can be 

 approximated by an expression of the type found in Equation 9. But 

 even collision theory can be interpreted to give an expression of this 

 type, as was previously noted. The really outstanding positive implica- 

 tion of absolute rate theory is that the complex E-S* does have a 

 physical existence, and that AS* does have a real significance as an 

 entropy change. These implications are extremely difficult to check in 

 most enzyme reactions. 



Equation 9 applies equally well to monomolecular reactions, bimolec- 

 ular reactions, and polymolecular reactions, provided that one uses 

 dimensionless concentration ratios in defining k. For any reaction 

 except a monomolecular one, both k and AS* have meaning only if they 

 are specified relative to some standard state. 



The term AS* could in theory reveal a great deal about what is 

 happening at the molecular level during the reaction. Various factors 

 may contribute to AS*. For monomolecular reactions, AS* represents 

 a change in bond structure; a positive value means denaturation or 

 loosening of bonds and a negative value the opposite. In polymolec- 

 ular reactions, AS* includes changes in bond structure as well as 

 changes associated with decreasing the number of free particles in 

 forming the activated complex. The values of AS* are per mole so 

 that the bond structure changes are independent of the standard state. 

 However, the value of AS* due to the change in the number of particles 

 will depend on the standard state. (If charged molecules react, di- 

 electric effects will also contribute to AS*.) In some reactions, such as 

 that between hemoglobin and oxygen, it is possible by changing the 

 standard states used to alter the sign of AS*. It is meaningless then, 

 concerning a polymolecular reaction, to consider just the sign of AS*; 

 it is necessary to consider how this value compares with that obtained 

 by assuming that no changes occur in the bond structure, except along 

 the reaction coordinate. 



The activated complex A ■ B* of absolute rate theory should not be 

 confused with the intermediate complexes of Chapters 17 and 18; the 

 two types of complexes are very different. The distinction can be 



