140 



GENERAL BIOCHEMISTRY 



CONH., 



o^ 



^H / II I \ H-^OH 



H^^^-^ CHoOPOPOCH.; ^^-^H 



"Ml " 

 _o o 



DPN + 



H3O + 



H2O + 2[H1 



CONH2 



NHc 



O O O 



-H/ II I \H 



H^^^^-^CHoOPOPOCH., -^-^H 



I II " 



_o o 



DPNH 



The reduced form at the right has gained two electrons and one 

 proton by this reaction and is called reduced diphosphopyridine 

 nucleotide or DPNH. Note that the upper left group corresponds to 

 nicotinamide, one of the B vitamins. The purine is adenine, the 

 sugar D-ribose, and the combination with phosphate is adenylic acid 

 (see page 122). [H] represents a suitable source of hydrogen atoms. 

 The above reaction occurs in connection with a number of enzymatic 

 jjrocesses, for example: 



-OOCGH2CHOHCOO- + DPN+ + H2O <— ^ 



malate 



-OOCCH2CCOO- + DPNH + H3O+ 

 O 



oxalacetate 



The enzyme specific for this reaction is called malic dehydrogenase, 

 and the double-pointed arrow signifies reversibility. The reaction 

 goes in either direction, depending upon the concentrations of the 

 materials present. As is now customary, malate and oxalacetate are 

 written as the ions rather than as the free acids malic and oxalacetic, 

 since these ions are the forms natural to the cells employing this 

 reaction. 



Another coenzyme, called adenosine triphosphate or ATP, par- 



