ENZYMES 275 



and then donating them to another; in other words, it serves as an acetyl 

 carrier. This is illustrated in the following scheme: 



apoenzyme 1 



acetyl X -\- Co A >■ X + acetyl Co A 



acetyl Co A -t Y ^^"^"'^"^'i . acetyl Y + Co A 



The apoenzymes 1 and 2 are specific for X and Y; for example, different 

 apoenzymcs are required for the formation of citric acid than for acetyla- 

 tion of choline. 



3. Pijridino Coenzymes. Several coenzymes have been found necessary 

 for the numerous dehydrogenation reactions which constantly occur in 

 all living cells. Warburg and co-workers demonstrated the need for one 

 such substance for the enzymatic dehydrogenation of glucose-6-phosphate 

 to phosphogluconic acid. The coenzyme was isolated in pure form and 

 shown to contain three molecules of phosphoric acid, two of pentose, 

 one of adenine, and one of nicotinamide (later identified as the pellagra- 

 curing vitamin) . This substance was called "coenzyme II," but now is 

 preferably designated as triphosphopyridine nucleotide (TPN, Fig. 10-6) . 

 It has been shown to be a component, for example, of the dehydrogenases 

 that act on glucose-6-phosphate and on isocitrate, and for the enzyme 

 system that converts malate to pyruvate and carbon dioxide (reaction 

 15, Fig. 13-4). 



Another coenzyme in this group is called cozymase, coenzyme I, or 

 preferably diphosphopyridine nucleotide (DPN) . It has exactly the same 

 chemical structure as TPN except that it contains only two phosphate 

 groups, as is indicated by the name. The extra phosphate group in 

 TPN is the one on the second carbon of the ribose residue in the 

 adenylic acid half of the molecule. Among dehydrogenases which require 

 DPN are those involved in the oxidation of D-glyceraldehyde-3-phosphate, 

 lactate, ethanol, malate, L-a-glycerophosphate, and glucose. 



DPN and TPN are called "pyridino" coenzymes because of the pyridine 

 ring in the nicotinamide component. It is also the pyridine ring which 

 undergoes chemical reaction when the coenzymes function in oxidation- 

 reduction reactions. The exact nature of this important change, which 

 is the same for both DPN and TPN, may be understood by studying 

 the structural formulas given in Fig. 10-6. In the oxidized form the 

 pyridine nitrogen has a valence of five and exists as the basic ion of a 

 quaternary ammonium salt. This positive charge is neutralized by one 

 of the negatively (acidic) charged phosphate groups of the molecule. 

 In Fig. 10-6 these groupings are starred and appear as follows: 



^ ^CH I 



+N* -0— P— 



' 1 



♦O 



