COMPONENTS OF THE ENERGY-COUPLING MECHANISM 45 



that the swelling and contraction are reflections of the action of " mechano- 

 enzyme" systems similar to the actomyosin of muscle, in which inter- 

 mediate enzymes of the energy-coupling mechanism may act as " mechano- 

 enzymes" and undergo change of shape or charge distribution. If the 

 ATP ADP exchange enzyme can exist in phosphorylated form, this might 

 difi^er in configuration or in geometrical arrangement from the unphos- 

 phorylated form and account for changes in the geometry or properties of 

 the membrane [26, 34]. There is in fact a striking resemblance between the 

 ATP-ase activity of the actomyosin system and that of the phosphory- 

 lation mechanism. 



Mechanisms of membrane changes 



Y 



I Electron transport »- carrier~X 



Carrier ~X + P, < " carrier + P~X 



P~X + (E 



—(E) + X 

 .P 

 'ADP 



^. ADP ^=^ (!i'^=^ di' 

 \- k ^ADP i ^ 



ATP 



■■mechanoproteins" 



n ATP + 



Membrane 

 protein 



Protein 

 phospho 

 kinase 



Fig. 6. Two possible mechanisms for alteration of membrane state through 

 ATP-driven changes of shape or conformation of protein molecules. In the first, 

 the mechano-enzyme may be an intermediate enzyme of energy coupling, such as 

 \Ej whose shape may change as a function of binding of P or ADP or ATP. In the 

 second, an independent membrane protein (possibly the "phosphoprotein " of 

 mitochondria) may be activated by ATP to yield mechanical changes. 



Figure 6 indicates two possible ways in which contraction might be 

 visualized. In the upper half is shown a representation in which an inter- 

 mediate enzyme of the energy-coupling sequence is the "mechano- 

 enzyme" activating the contractile changes. It is postulated to change 

 shape or charge distribution when it is phosphorylated. 



On the other hand, it is possible that the contractile protein is not a 

 member of the coupling sequence itself but perhaps is in equilibrium with 

 it. We have suggested that the "phosphoprotein" of mitochondria is a 

 possible candidate, since earlier work with Friedkin had shown that the 



