ROBERT A. ALBERTY 



EA + B . ^' ^ EC + D 

 EC . E + C 



ke 



a different rate equation is obtained. The steady-state treatment 

 of this mechanism yields the following equation for the initial 

 velocity of the forward reaction when terms involving (C) and 

 (D) are omitted. 



V 



1 + KJ{A) + ^b/(B) + ^ab/(A)(B) 



(11) 



The constants in equation (11) are given by Kj^ = kr,/ki, K^ = 

 ks/ks, Kj^^ = k2kr,/kiks, and V = k^iE)^, Thus the Michaelis 

 constant for A determined by using a high concentration of B 

 depends on the rate of dissociation of the EC complex and not at 

 all upon the rate of dissociation of A from EA. If the con- 

 centration of B is sufficiently low so that K^/(B) and K ji^-^/ (A) (B) 

 are much greater than unity, equation (11) reduces to the same 

 form as equation (9), but the two cases may be readily dis- 

 tinguished by experiments at higher concentrations of B. If the 

 third step in the above mechanism is rate-determining and the 

 equilibrium in the second step is not far to the right, the rate of 

 the over-all reaction will also depend upon the concentration of 

 D, and it may be extremely difficult to determine the initial 

 velocity if small concentrations of D have a large effect. 



The steady-state rate equations for a number of mechanisms 

 for coenzyme reactions have the form of equation (11), and so it 

 is not possible to distinguish between these mechanisms by use of 

 studies of the steady-state velocities alone. However, for the 

 mechanism 



E + A , EA , E' + C (12) 



E' + B , ED , E + D 



where E' might be an oxidized form of the enzyme, rate equation 

 (11) is obtained with iT^g = 0. 



572 



