CARBON DIOXIDE 



one another, although the equiHbrium constant is preponderantly in 

 favor of dissociation. The active group of the coenzyme is pyridine. 

 Two such prosthetic groups are known at present, viz-, diphospho- 

 pyridine nucleotide (abbreviated DPN, also known as cozymase and 

 coenzyme I), and triphosphopyridine nucleotide (abbreviated TPN, 

 also known as Warburg's coenzyme and coenzyme II). The coenzymes 

 are dinucleotides, each containing two bases, adenine and nicotinamide, 

 two molecules of ribose, and either two (DPN) or three (TPN) mole- 

 cules of phosphoric acid. The structural formula of DPN is shown in 

 scheme I. The point of attachment of the third phosphoric acid 

 residue in TPN is as yet unknown. Most of the pyridine nucleotide 

 dehydrogenases have DPN as the prosthetic group; only two are defi- 



H2C— O— P— O— P— O — CHj 



O- OH 



Structural formula of diphosphopyridine nucleotide 



CH 

 HC C— CONH2 



HC CH 



V 



R— O— P— OR' 



CH 



/\ 

 HC C— CONH2 



II I 



HC CHj 



V 



I H+O- 



1 I 



R— O— P— OR' 



O O 



Reversible reduction of diphosphopyridine nucleotide 



169 



