622 Discussion 



Weber, G. & Teale, F. J. W. (1959). Disc. Faraday Soc. 11, 134. 



YoNETANi, T., Takemori, S., Sekuzu, I. & Okunuki, K. (1958). Nature, Loud. 181, 1339. 



DISCUSSION 



Mechanism of Oxidative Phosphorylation 

 Slater: 



1. I do not agree with Chance that, under the conditions of the ATP-^^Pj exchange 

 reaction, reduction of the carrier would cause increased binding of both X and I (as 

 carrier ~I and X ~ 1). 



This exchange reaction is described, in the formulation of oxidative phosphorylation 

 used by Chance and ourselves, by the equation 



ATP + X + I ?^ ADP + Pi + X -- I (1) 



According to Chance's mechanism, reduced carrier combines with I to form an 

 energy-rich intermediate, (reduced carrier) ~ 1. Clearly, the reaction 



reduced carrier + I -^ (reduced carrier) ~' I (2) 



cannot occur spontaneously, but must be linked with an energy-generating or an 

 energy-transferring reaction. In the absence of oxygen, oxidation reactions cannot 

 provide the energy. The only possible source of this energy, in terms of Chance's 

 mechanism, is X ~ I, which can transfer its I to reduced carrier, thus 



reduced carrier -f- X '^ I ^ X -1- (reduced carrier) '->-' I (3) 



This reaction will cause no change in the amount of bound I, but will decrease the 

 binding of X. 



The exchange reaction (I) requires both X and X -^ I. Reaction (3) proceeding to 

 the right will cause a decrease in the concentration of X ~ I, and an increase in that 

 of X. Thus, on the basis of this mechanism, it is not possible to predict whether 

 reduction of carrier would lead to an increase in the rate of the exchange reaction. 



The rate-limiting step in the dinitrophenol-induced ATPase reaction is reaction 

 (1) proceeding to the right (the back reaction can be ignored, because the concentration 

 of X -— I is very small). It is true, as Chance says, that binding of I by carrier in a 

 low-energy compound (carrier • I), a reaction which could conceivably occur spon- 

 taneously, would decrease the rate of the ATPase reaction. Since, however, reduction 

 causes a decrease of the rate of the ATPase reaction, this explanation requires that 

 it is the reduced carrier that combines with I to form a low-enero-y compound 



reduced carrier 4- I ?^ (reduced carrier) .1 (4) 



A low-energy compound cannot itself yield ATP, but must first be converted to a 

 high-energy compound. In oxidative phosphorylation this presumably occurs by 

 oxidation yielding (oxidized carrier)~I. Thus, the explanation given by Chance 

 leads to the conclusion that the high-energy intermediate I compound is with oxidized 

 carrier, not with reduced carrier as required in his theory. 



However, there are other possible explanations of the inhibition of the ATPase, 

 for example that I is reduced. It is my view that the fact that reduction of the respira- 

 tory chain leads to an inhibition of the exchange reaction and of the ATPase provides 

 no support for the view that ^^I compounds are with reduced carrier. Nor does it 

 prove that the compound is with oxidized carrier. 



2. In my review (Slater, Rev. pure appl. Chein. 8, 221, 1958) I attempted to set 

 out as faithfully as possible Chance's mechanism of oxidative phosphorylation. 

 Chance and Wi\\iams,{Advanc. Enzymol. 17, 99, 1956) suggest that c2+ '-' I is formed 

 from c^+ and I during reduction of c^+ by b'^^ and that 'similar reactions may be 

 written for other couples'. The similar reaction leading to formation of DPN -^ I is 

 clearly the reduction of DPN by //-hydroxybutyrate, as'given in my Scheme C, which 

 still seems to me faithfully to summarize the mechanism suggested by Chance and 

 Williams (1956, he. cit.). In the following paragraphs of my review, I discuss in 

 detail the earlier suggestion of Chance, Williams, Higgins and Holmes (J. biol. Chein. 



