APPROACHES TO THE ANALYSIS OF SPECIFIC MEMBRANE TRANSPORT 595 



membrane or other complex with the "corresponding" constants deter- 

 mined in homogeneous sohition for the soluble enzymes or catalytic 

 carriers extracted from the complex. The train of thought that I propose 

 to follow wall incidentally suggest a new way in which the function of the 

 particulate systems that couple oxidoreductive or photon-activated electron 

 transport to phosphorylation could be dependent upon their structure. 

 But I do not propose to do more than touch upon this incidental suggestion 

 today as it would lead us outside the context of this session of the 

 Symposium. 



SOME EFFECTS OF ANISOTROPY UPON AN ENZYME-CATALYZED 

 HYDROLYTIC REACTION 



For the sake of simplicity, let us consider the effect of one variable, pH, 

 on an enzyme catalyzed reaction of the hydrolase type represented by the 

 equation, 



AB + H.3O =^ AH + BOH (1) 



Kinetic considerations 



When the enzyme, E, is dissolved in a homogeneous aqueous medium, 

 the usual constants, such as the maximum velocity or the Michaelis 

 constant, will vary with the pH of the medium, and the kinetic constants 

 of this variation will be characteristic of the enzyme because the catalytic 

 activity depends upon the degree of ionization of acidic and basic groups 

 in different parts of the protein molecule. If, however, the enzyme is 

 situated in a membrane separating two aqueous phases which are poised at 

 different hydrogen ion activities, the degree of ionization of the acidic and 

 basic groups on the two sides of the enzyme will be different, and as the 

 state of the enzyme molecules will not be defined by a single pH value, we 

 cannot properly define the kinetic constants relating the characteristics of 

 the enzyme activity to the pH. It follows from the generalization of this 

 kind of consideration, that the kinetic constants of enzymes or catalytic 

 carriers as usually defined in homogeneous systems are not strictly com- 

 parable to the "corresponding" constants for the same catalysts when 

 present in natural membranes or other anisotropic complexes. 



Thermodyuiunic considerations : (Jlieniiosniotic couplinif 



In the homegeneous system represented by equation (i), the enzyme 

 will catalyze the equilibration of the reaction according to the equation, 



[AH] X [BOH] 



[K^ — = ^ ^'^ 



