492 NIACIN 



a number of systems involving hydrogen transfer, the apoenzymes are 

 generally specific, a different apoenzyme being required for each substrate 

 system. The apoenzymes contribute more to specificity than the prosthetic 

 group. In other words, these two coenzymes are capable of acting with a 

 number of apoenzymes, alternating from one to another as needed, hence 

 the term "mobile" coenzymes.^^ Some of the apoenzyme-coenzyme com- 

 plexes are capable of catalyzing reactions other than that of their specific 

 substrates, but the reaction rates are generally low. There are some excep- 

 tions to this, however, in which a given coenzyme-apoenzyme system can 

 catalyze the oxidation of a number of substrates with efficiency .^^ 



3. The Coenzyme-Enzyme Complex (Holoenzyme) 



DPN and TPN are usually readily dissociable from their apoenzymes. 

 Reduced DPN and TPN appear to have a lower affinity for their apo- 

 enzymes than the oxidized forms. This may be due to the fact that quater- 

 nary nitrogen of the pyridine ring is changed to a tertiary amine in the 

 reduced coenzyme. The elimination of this strong basic group increases 

 the acidic properties of the coenzyme and may increase its dissociability 

 from the apoenzyme, hence leaving the coenzyme free to migrate to an- 

 other enzyme system where it can donate its hydrogens to another sub- 

 strate or hydrogen acceptor. 



Some exceptions to the ready dissociability of coenzymes from their 

 apoenzymes have been recorded. Cori et alP have been able to crystallize 

 phosphoglyceraldehyde dehydrogenase in combination with DPN. Others 

 have reported a very firm attachment of coenzymes in the particulate 

 matter of cells which catalyze reactions in the Krebs cycle .^^ 



4. Point of Hydrogenation 



The point of reversible attachment of hydrogen to the pyridine nucleus 

 is not entirely certain. It is clear from the studies of Karrer and associ- 

 ates^^- ^^"^^ with N^-substituted model compounds that hydrogen is attached 

 to one of the carbons adjacent to the ring nitrogen. Available evidence, 

 especially from the studies of Knox and Grossman,^* • ^^ indicates the prob- 

 able point as the 6 position. 



" J. K. Parnas, Nature 151, 577 (1943). 



" A. Meister, /. Biol. Chem. 184, 117 (1950). 



" C. F. Cori, S. F. Velick, and G. T. Cori, Biochim. et Biophys. Acta 4, 160 (1950). 



'4 F. M. Huennekens and D. E. Green, Arch. Biochem. 27, 418, 428 (1950). 



'6 W. E. Knox and W. I. Grossman, J. Am. Chem. Soc. 70, 2172 (1948). 



