414 Thermodynamics of Enzyme Reactions \Yl : 4 



normal enzyme appears quite plausible. Finally, the assumptions 

 necessary to derive the rate of decay of the activated form seem reason- 

 able for denaturation-type reactions. 



Steam has collected data for a large number of denaturation-type 

 reactions. These are summarized in the table on page 415. For 

 irreversible reactions, data are given on the values of A//* and AS* to 

 form the activated complex. For one reversible reaction, these and also 

 values of A// and AS" for the over-all equilibrium are listed. The lists 

 are impressive but do not in themselves support absolute rate theory. 

 Other evidence can be shown to make these numbers reasonable, and 

 thereby justify the application of absolute rate theory to these reactions. 



In order to compare these experimental results with theoretical 

 predictions, an additional assumption is needed. This is that the 

 activated form is similar to the final form of the molecule. For de- 

 naturation, this implies that chemical bonds are broken or at least 

 weakened during the formation of the activated complex. Thus, one 

 would expect that AS* should be positive for all denaturations. (Because 

 the standard state does not alter the value of AS* for monomolecular 

 reactions, it is possible to assign meaning to the sign of AS* independent 

 of the standard state.) It will be noted in the table that AS* is indeed 

 positive for all denaturations, as is AS , also. 



By various means, different authors have estimated values for A// 

 and AS associated with breaking the types of bonds found in proteins. 

 The estimates for A///bond vary from 4 to 8.5 kcal/mole, and for 

 AS/bond from 10 to 15 kcal/(mole°K). Taking the average of A///bond, 

 one may express the probable number of broken bonds represented by 

 the experimental values of A//* and AH . Similarly, the experimental 

 values of AS* and AS can be used to find a probable number of bonds 

 broken. These estimates are shown in the following table. The 

 differences are small in comparison to the range of theoretically calcu- 

 lated values for A///bond. Thus, this evidence strongly supports 

 absolute rate theory. 



Although the values of A//* vary by a very large amount from one 

 enzyme to the next, the calculated values of AG* are much more nearly 

 constant. This has been interpreted as supporting absolute rate theory. 

 However, the statement that AG* is almost constant is equivalent to 

 the statement that the denaturation rates are all within the same order 

 of magnitude. Perhaps those enzymes with a more rapid denaturation 

 rate cannot be studied, whereas those that are much less rapidly 

 denatured are not attractive for denaturation studies. 



In any case, it is clear that absolute rate theory is more useful than 

 collision theory in a discussion of denaturation of enzymes. One may 

 expect that this would be true of any monomolecular reaction in solution. 



