NUCLEOTIDES AND COENZYMES 



accurate, dimensioned pictures of the coenzymes. Actually, 

 some information along this line can sometimes be gained 

 from simple experiments. Thus, Bessey et al. (6) found that FAD 

 at pW 7 has only about 14% of the fluorescence of FMN but 

 that upon acidification of FAD the fluorescence rises to an equal 

 value. That this is not simply a splitting of the dinucleotide 

 in acid solution is shown by the fact that the fluorescence again 

 falls upon readjusting the pW to 7. Since admixture of an 

 equimolal amount of AMP with FMN does not depress the 

 fluorescence to this extent, it may be assumed that in FAD the 

 adenylic portion lies in close juxtaposition to, and interacts with, 

 the isoalloxazine moiety. This may be similar to the resonance 

 interactions observed between the parallel aromatic rings of the 

 para-cyclophanes (13): 



(CH2)„ (CH2)„ 



Assuming, then, that we may know the exact structure of a 

 coenzyme and something, at least, of the potential binding 

 groups on the protein (e.g., — OH, — NH2, — COOH, — SH, im- 

 idazole groups), can we draw any conclusions as to the type of 

 binding? Are there covalent bonds, electrostatic bonds, hy- 

 drogen bonds, van der Waals' forces, cage effects, or combina- 

 tions of these? We might also raise the question: Since the 

 active centers on the protein for both coenzyme and sub- 

 strate are probably small relative to the entire protein, what then 

 is the function of the bulk of the protein which is known to be 

 essential for enzyme catalysis? Unfortunately, kinetic studies 

 yield only Km values, whose exact interpretation in terms of 



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