136 



LIGHT AND LIFE 



0.6 



0.8 



1.2 



lig. 19. The absorption spectrum of the same solutions of FMN as in Fig. 18, 

 but measured at longer wavelengths in a 10 cm cell. From Beincrt (2). 



measurement of the second order rate constant tor complex forma- 

 tion, A'l =: 10.8 X 10^ A/-1 sec-i, and the equiHbrium constant, 

 K =z 0.7 X 10-8 ^ -phe first order rate constant for dissociation is 

 accordingly 13.5 X H)-^ sec-^. Potentiometric titrations by Vestling 

 (36) indicate a standard oxidation reduction potential, E\ = 0.190 v 

 for free FMN, —0.123 v for the complex. If the positive potential 

 shift is attributed to increased interaction of the protein with the 

 reduced form of the nucleotide, then the dissociation constant of 

 the protein and FMNHg is 6 X lO-ii M. 



Ordinarily one would expect the rate constant for complex forma- 

 tion between enzyme and coenzyme to be too rapid to measure by 

 simple direct methods. However, the OYE complex probably does 

 not dissociate significantly under functional conditions. It is likely 

 that when the complex is resolved chemically, conformational changes 

 Dccur in the protein, the reversals of which are rate-limiting in the 

 recombination reaction. 



OYE catalyzes the oxidation of TPNH by various acceptors, but 

 its natural acceptoi is unknown. When Ehrenberg and Ludwig (7) 

 incubated the tn/vnic w ith excess TPNH in air the peculiar spectral 



