APPROACHES TO THE ANALYSIS OF SPECIFIC MEMBRANE TRANSPORT 597 



The effect of the anisotropic situation of the hydroiytic enzyme on the 

 dissociation equiHbrium represented by K' can consequently be expressed 

 as follows, using {^h) and (6), 



[AH] X [BOH] [H+], 



^ = [AB] = ^[H.O].oM,x p^ (,) 



This equation shows that the poise of the dissociation equilibrium repre- 

 sented by K' is proportional to the ratio of the hydrogen ion electro- 

 chemical activity in phase I to that in phase II. The electrochemical 

 activity of the hydrogen ion in the two phases may, of course, differ either 

 because of a difference of hydrogen ion chemical activity or because of a 

 membrane potential, a potential of about 60 mV being equivalent to a 

 hydrogen ion chemical activity ratio of 10. The membrane potential or 

 hydrogen ion chemical activity difference across the membrane could be 

 generated by a photoelectric effect or by a metabolic oxidoreduction 

 involving a flow of electrons across the membrane. Equation (7) shows 

 that the work done in creating the asymmetry of [H^] across the mem- 

 brane can be coupled to synthesis of x\B by dehydration of AH and BOH, 

 the hvdrogen ions of the water that is eliminated travelling to phase I and 

 the hydroxyl ions travelling to phase II. Synthesis of AB is, of course, 

 promoted by lowering the chemical activity of the hydrogen ion or by a 

 negative potential in phase I relative to phase II. 



It will be helpful to consider the reaction catalyzed by glucose-6- 

 phosphatase as a relevant and quantitative example of the above principle 

 of chemiosmotic coupling. The equilibrium constant. A.'', for the hydrolysis 

 of glucose-6-phosphate to glucose and inorganic phosphate is approxi- 

 mately 250 in homogeneous aqueous solution at pH 7 [47], and the 

 concentration of glucose-6-phosphate in equihbrium with io~^ M glucose 

 and ID"'- M phosphate would be only 4 x lO"" M. If glucose-6-phosphatase 

 were located in the anisotropic membrane complex as described above, a 

 pH difference of only 3 units between phases I and II, or a potential 

 difference of 60 mV and a pH difference of 2 units, would lower the 

 dissociation constant by a factor of 1000 and would raise the concentration 

 of glucose-6-phosphate in equilibrium with iq-'- m glucose and 10 - M 

 phosphate to 4x10-^ m — a concentration high enough to enter the 

 phosphohexose isomerase reaction of the glycolytic pathway at near the 

 maximum rate. This fact is all the more interesting since the glucose-6- 

 phosphatase of Escherichia coli (as discussed above) and of liver cells [48] 

 appears to be appropriately situated in a membrane complex. I need 

 hardly point out that a similar, but greater asymmetry of electrochemical 

 hydrogen ion activity to that considered in the above example, could be 

 responsible for converting the ATPases of the particulate systems of 

 photosynthetic and oxidative phosphorylation into the x\TP-synthesizing 



