HUGO THEORELL 



THE CHEMICAL NATURE OF THE FMN-BINDING GROUPS IN 

 THE PROTEIN OF O.Y.E. 



The strong influence of anions on the stability of the FMN- 

 protein bonds directly suggests that the bonds between the 

 phosphoric acid residue of FMN and the protein are saltlike 

 themselves. The effect of />H and anions on the FMN-apopro- 

 tein system indicates that FMN is linked to the protein in very 

 much the same manner as an anion to a weakly basic ion ex- 

 changer. 



There are several reasons to believe from the kinetic ex- 

 periments that primary amino groups in the protein serve as 

 binding sites for the phosphoric acid residue. 



7. Above pH 9 a sharp drop of ki occurs, corresponding to 

 the expected discharge of — NH3 groups in this region. 



2. Lowering the temperature by 10° shifted the ki curve 

 upwards on the pH scale by approximately the amount expected 

 for amino groups (0.3 pH unit). 



3. 0.4 M NaCl shifted this part of the ki curve in the 

 opposite direction, as expected from the known influence of the 

 ionic strength on the pK. of primary amino groups. 



When we move from neutrality toward acid reaction the 

 positive charge of the amino groups keeps constant, but the 

 second negative charge of 



O— 

 / 



— p=o 



\ 



o— 



begins to disappear by the addition of a proton (/?K'~6). The 

 observed effects of temperature change and ionic strength were 

 the same on the k\ curve and on the titration curve of FMN. 

 The decrease in ki toward the acid side thus is attributed to the 

 change of FMN from the divalent to the monovalent state. 



It is uncertain how the peculiar change in k^ between /?H 8 

 and 9 should be interpreted; the protein could change shape, 



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