FRANK M. HUENNEKENS 



affinity, or binding, of the two components, remains obscure. 

 Other quantitative data, such as the variation in binding with 

 pH, ionic strength, and temperature, are noticeably lacking. 

 There remain only fragmentary, qualitative observations based 

 upon chemical modification of the coenzyme or protein. These 

 studies are similar in rationale to the search for the active center 

 of certain proteolytic enzymes, where the broad specificity of the 

 enzyme permits a variety of structurally similar substrates to be 

 used and the variation in reaction kinetics noted. Unfortu- 

 nately, the problem is somewhat analogous to the use of quantum 

 mechanical methods to relate absorption spectra to structure of 

 complex organic molecules; the correlation becomes more 

 exact in proportion to the a priori knowledge of both factors. 



Some very interesting observations have been made re- 

 cently by Racker, Velick, Boyer, and Chance on the binding of 

 DPN by 3-phosphoglyceraldehyde dehydrogenase (35). It 

 has been suggested that the nicotinamide portion of DPN is 

 bound to a glutathione residue of the protein, as shown below 



ENZYME 



Whether or not this tentative structure is correct, it is known that 

 combination of DPN and the enzyme gives rise to a new absorp- 

 tion band at 360 m/x. By contrast, reduced DPN forms a spec- 

 troscopic complex (Xn,ax ^t 325 mn) with horse liver alcohol 

 dehydrogenase or lactic dehydrogenase, but neither complex is 

 observed with glutamic dehydrogenase or yeast alcohol dehy- 

 drogenase. The question has been raised repeatedly whether 



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