132 Albert L. Lehninger, et al. 



These findings on the ATP exchanges may be considered in 

 the context of a mechanism of energy coupHng in oxidative 

 phosphorylation which we postulated earlier, as follows 

 (Lehninger, et al., 1958; Cooper and Lehninger, 1957&; 

 Lehninger, 1955): 



(1) CARRIERrbd. + X + carrier;^. <± CARRIERqx. -- X 



+ carrier;^^. 



(2) CARRIERox. -- X + Pj ;^ CARRIERqx. + P -- X 



(3) P .^ X + ADP <± ATP + X 



In this formulation, X is postulated to be an enzyme protein 

 interacting with a specific carrier (i.e. CARRIERj^^ed.) during 

 its oxidation to produce a "high-energy" compound with the 

 oxidized form of the carrier (CARRIERqx. '^ X). This 

 "charged" carrier is now postulated to undergo a phos- 

 phorolytic reaction, presumably by nucleophilic attack of 

 phosphate at the " high energy" bond to form P -^ X (a phos- 

 pho-enzyme). The P ^-^ X so generated donates its P to ADP. 

 In principle the reaction pattern was first postulated by Lip- 

 mann (1946) and has since been elaborated by many others. 

 As written, it is a skeleton mechanism ; it is not excluded that 

 intermediate group -transfer reactions may occur between 

 reactions (1) and (2) and between (2) and (3), such as those 

 postulated by Chance and Williams (1956). The formulation 

 of reactions (1), (2) and (3) involves the oxidized form of the 

 carrier as the high-energy or charged form, for which we give 

 evidence elsewhere (Wadkins and Lehninger, 1957, 1958). 

 On the other hand. Chance has favoured a thermodynamically 

 equivalent formulation which utilizes the reduced form as the 

 high-energy form (Chance and Williams, 1956; Chance and 

 Hollunger, 1957). 



This reaction scheme readily accounts for the findings on the 

 ATP exchanges. It is seen that this formulation provides for 

 incorporation of ^^Pj into ATP in the absence of net electron 

 transport at maximum rates when the carriers are oxidized. 

 This is made possible by reversible phosphorolysis of the high- 



