140 THE BIOCHEMISTRY OF B VITAMINS 



of slight changes in the local concentrations of these two substances at 

 the site of each enzyme molecule or because of variations in the redox 

 potential during the so-called aerobic-anaerobic phases of cellular activity. 

 One group of reactions in which these coenzymes participate include 

 those in which energy-rich phosphate compounds are formed by the 

 "oxidation" of an aldehyde; the reverse reactions, ones in which energy- 

 rich bonds are utilized for synthetic purposes, are the "reduction" of acids. 

 The "oxidation" of phosphoglyceraldehyde is a good example. The alde- 

 hyde forms a loosely bound addition product with phosphoric acid (a 

 compound analogous to an aldehyde hydrate) . On dehydrogenation this 

 inorganic phosphate is converted into a reactive acyl phosphate group 

 which can be transferred to adenosine diphosphate (ADP) , forming 

 adenosine triphosphate (ATP) . 



o o 



H H II o H H || 



ADP + HC— C— C + HO— P— OH + DPN DPN-2H + HC— C— C— OH + ATP 



I 1 H I J I 



o o o o o 



I H H I H 



HO— PO HO— PO 



i i 



H H 



11 11 



H 



O 



ADP + HC— C— C— O- 



i A H 



I H H I H H 



HO— PO HO— PO 



A i 



H H 



During catabolic phases of activity when glycogen and other organic 

 substrates are being oxidized for the purpose of supplying energy to cells, 

 this reaction (and other comparable ones) proceeds in the direction caus- 

 ing the dehydrogenation of the substrate and the creation of the reactive 

 phosphate bond. During anabolic phases, when part of the glycogen is 

 resynthesized by the organism, the reaction proceeds in the reverse direc- 

 tion and an energy-rich bond of adenosine triphosphate is utilized to 

 reduce, in effect, an acid to an aldehyde. This reaction represents a general 

 biological mechanism in which the phosphopyridine nucleotides mediate 

 the conversion of the latent chemical energy of reduced organic substrates 

 into the readily utilizable energy of the reactive phosphoric anhydride 

 compounds, or the reverse process in which available energy is conserved 

 by the formation of more highly reduced compounds. 



The numerous redox systems which have been shown to be enzymati- 

 cally coupled with the phosphopyridine "coenzymes can be classified 



