PROSTHETIC GROUPS, COENZYMES AND ENZYMES 



The formulas (4) and (4a) lead to some new considerations 

 on the nature of the Michaelis constant, K„, which had formerly 

 been very often interpreted as representing dissociation con- 

 stants of enzyme-substrate compounds. This is not at all the 

 case, if formulas (1) to (3) and therefore also (4) and {4a) are 

 valid. It was found, for example, that iT^ for DPNH is = 

 ks/ki; thus the ratio between the dissociation velocity constant 

 for DPN-ADH' and the association velocity constant of DPNH 

 + ADH.' 



The K„ for acetaldehyde in this system does not represent 

 a dissociation constant of an aldehyde-enzyme complex, but 

 should be = k^/k^ [H+].* 



Some other interesting formulas follow directly from 

 equations (1) to (3), when the whole system is considered to be 

 in the equilibrium state : 



kjctki K 

 and 



Z)ox k\kz ke 



■Dred k<iki kiK 



(6) 



The rather uncertain values of ki to Are, obtained in 1951, when 

 inserted in (5) gave a value of K that was of the right order of 

 magnitude (0.2 X 10~i^). Since this was the first time that 

 direct kinetic and equilibrium data had been correlated for an 

 enzyme system, it was of considerable interest to collect more 

 experimental data. It should be emphasized that formulas (5) 

 and (6) give excellent possibilities to check the validity of such 

 data, because K and D^jDj.^^ were determined independently 

 from equilibrium data. New determinations of ki to k^ have 



* It should be noticed that in previous pubUcations the values given 

 for ki include [H + 1 as a factor. We shall here use ^4 for the true reaction 

 velocity constant; thus 



^ = /t4 [DFNHADH'l [CH3GHO] [H+] 

 297 



