182 



A CONCEPTUAL INTRODUCTION TO BIOENERGETICS 



Plots of E vs a H + and of E vs pH are shown in Figure 7-5. It can be seen that 

 at the physiological pH of 7, E ml on the \HE scale is —0.42 v. 



0.0 



0,4 2 



-0.82 



-m 7 



(all reduced) 



100% 

 (all oxidized) 



% oxidized 

 (b) 



Figure 7-5. Reversible Potential of an Oxidation-Reduction Reaction: (a) as a function of 

 pH,onthe normal hydrogen electrode (NHE) scale; (b) as a function of per cent oxidation. 

 Definition of E m7 : potential (on the NHE scale) when pH = 7 and when the redox system 

 is 50 per cent oxidized. 



As a further clarification and as a summary, Figure 7-6 shows schema- 

 tically the relation between the NHE scale of £"°'s (pH = 0), to which AE 

 values have been traditionally related through —AE = nEE, and the physio- 

 logical scale, E m7 (pH = 7). The latter is now commonly used as a relative 

 measure of free energy changes in biological reactions. The values in Table 

 7-5 have been measured simply by putting a platinum wire into a mixture of 

 equal concentrations of sodium succinate and sodium fumarate at pH 7, 

 containing an enzyme and a mediator (discussed later), and measuring its 

 voltage against a hydrogen electrode in the same solution. Such measured 

 values can be used to predict the direction of reaction, or as a basis for com- 

 parison, but not for the determination of AE, because the effective con- 

 centrations (activities) are not known. It is well to be clear on this limitation 

 of the £" -, listing;. 



Difficulty often arises in this subject because of notation. Different 

 authors use different subscripts and superscripts. In this book we have de- 

 fined, and use, only E, E°, and E m7 . One should be aware of the variations 

 which one may find. Further, one should understand clearly that the values 

 given in the table for intermediary processes of oxidation are midpoint 

 values; that although these redox systems are generally poised at their most 

 stable point (Figure 7-5), a tight control must be kept by the living system at 

 all times on the concentration of oxidized and reduced states of each system; 



