142 



A SYMPOSIUM ON RESPIRATORY ENZYMES 



The magnitude of Kr denotes the velocity of the reaction within 

 the complex, and the value of K^ gives a measure of the concentra- 

 tion of the complex. Thus in more dilute solutions of the substrate, 

 when the enzyme is not saturated, both Kr and K^ determine the 

 activity— the larger Kr and the smaller K^, the more active is the 

 enzyme. Unfortunately not enough data are present to permit of so 

 specific a comparison of enzymatic activities. An approximation, 

 however, can be made. If the concentration of the substrate, TPNHj 

 in this case, is very small as compared with K^, then equation 7 

 reduces to: 



2^ 

 (8) t; = —^(TPNHO(CR) total 



or u = K'(TPNHO(CR) total 



Under these conditions the velocity is proportional to the concen- 

 tration of the TPNH2 (lower left part of curve), and K\ which is 

 approximately equal to Kr/K^ is a measure of the activity— the 

 larger the velocity constant Kr and the smaller the dissociation con- 

 stant Ki, the more active is the enzyme. Neither Kr nor K^ are 

 aflFected by the concentration of the substrate or enzyme, whereas 

 the turnover number may be. 



In more concentrated solution, i.e., when the enzyme is saturated 

 with substrate, only Kr determines the activity (u = Kr(CR)totai)- 

 Thus the significance of the dissociation constant is apparent only 

 when the substrate is present in low concentration— a low dissocia- 

 tion constant enhances the activity. On the basis of this criterion of 

 activity, a comparison between the activities of the various flavo- 

 proteins for low concentrations of substrate is given in Table 3. 



Table 3.— Specific reaction velocities at 25° C, 



