596 PETER MITCHELL 



in which the square brackets stand for electrochemical activities and K' is 

 a thermodynamic " constant ", independent of the properties of the enzyme 

 and independent of pH, other things being equal. In writing equation (2), 

 I have followed the custom of omitting the activity of the water in the 

 system as this is a constant in homogeneous aqueous physiological media, 

 and has a value corresponding to 55-5 M water. In an inhomogeneous 

 (pseudo-equilibrium) system, however, the activity of the water at the 

 site (e) of the hydrolytic process may not correspond to that of physio- 

 logical aqueous media, and in this case it must be included as follows. 



[AH], X [BOH], 

 [AB], X [H^O], 



[AH], X [BOH], 



= K (3«) 



= AlH^O], ' (3^.) 



It will be seen that the usual hydrolysis "constant", K\ is a variable 

 which is proportional to [HoO],, the electrochemical activity of the water 

 at the active centre of the enzyme. 



It is a well-known fact that enzyme reactions — such as the catalysis of 

 the transfer of the phosphoryl group (or phosphorylium ion) by phospho- 

 kinases — can be etfectively anhydrous even though the enzyme molecules 

 are surrounded by water. It would not, therefore, be unrealistic to assume 

 that the active centre of a hydrolytic enzyme, situated anisotropically in a 

 membrane complex, could be inaccessible to water, but could be accessible 

 to hydrogen (but not hydroxyl) ions from one side (phase I) and could be 

 accessible to hydroxyl (but not hydrogen) ions from the other side (phase 

 II). For the sake of simplicity, we will not make any assumptions about 

 the anisotropy of the enzyme with respect to the accessibility of AB, AH, 

 and BOH to its active centre. The electrochemical activity of the water at 

 the active centre, e, of the enzyme would be given by the dissociation 

 constant of the water, K^^,, defined as follows, 



[H,.0]. ^ [»-]-[""''■■ (4) 



and smce, [OH ],i = A„, — • (5) 



[W Jii 



equation (4) can be written, 



.„ OT _ [Hiix[H,0]i„,„ 



[HP]^ - ^^^^ (6) 



