21:5/ Thermodynamics and Biology 399 



Under these conditions, the value of the peroxide concentration x is 

 about 



x2 = [Q 2 ][E][H 2 OY ^ 1Q _ 32 



or 



x = 10 ~ 16 moles/liter 



Likewise, p would be of the order of 10~ 16 moles/liter. 



Although this number is much larger than the hydrogen peroxide 

 concentration computed for a solution of oxygen in water, it is still 

 far less than can be detected at the present time. Conceivably, one 

 might find a compound AR 2 which would react with the intermediate 

 E-S 



AH 2 + ES^A + E + 2H 2 



so that equilibrium would favor the right-hand side more strongly than 

 in the reaction 



ES + S~E + 2 + 2H 2 



In this case, catalase could serve to oxidize AH 2 . At present, no such 

 compound is actually known. 



Even though it is not possible to carry out any direct experiments to 

 verify the value of k 4 , it is always assumed to actually exist. Many 

 other enzyme systems are simpler than catalase in that they can be 

 observed to catalyze reactions in either direction, depending on the 

 initial concentrations of the reacting molecular species. Catalase has 

 been discussed in detail because it illustrates an example of a reaction 

 to which the reasoning of thermodynamics can be applied to determine 

 an additional rate constant from equilibrium data. 



The interpretation of equilibrium data in terms of Gibbs' free energy 

 is a very important area in which thermodynamics can be used to 

 describe biologically significant events. Other related areas are found 

 in the following chapters. 



REFERENCES 



1. Glasstone, Samuel, Textbook of Physical Chemistry 2nd ed. (New York: 

 D. Van Nostrand Company, Inc., 1946). 



2. Glasstone, Samuel, K. J. Laidler, and Henry Eyring, Theory of Rate Pro- 

 cesses: The Kinetics of Chemical Reactions, Viscosity, Diffusion and Electrochemical 

 Phenomena (New York: McGraw-Hill Book Company, Inc., 1941). 



