146 - The Cell 



COOH 



I 

 -C— H 



I 

 -C- 



H- 



H— C— H 



•I 

 COOH 



succinic acid 



succinic 

 dehydrogenase 



COOH 



I 

 H-C 



II 

 H-C 



I 



COOH 



fumaric acid 



FAD 

 or 



FMN 



oxidized 



2e~ 



■2^ 



+ 



FAD- H. 



FMN- H, 



reduced 



Fig. 8-3. Another group of dehydrogenases, here 

 represented by succinic dehydrogenase, acts only 

 upon substrates possessing the molecular configura- 



I 

 tion H — C — H This group performs most of its trans- 



I 

 H— C— H 



I 

 actions with FAD or FMN (p. 147). 



cleotide (FMN). In each of these molecules 

 one of the B vitamins (p. 348) is represented: 

 niacin (in DPN and TPN) and riboflavin (in 

 FAD and FMN). 



In essence, the acceptor nucleotides serve 

 as cofactors, acting in conjunction with the 

 dehydrogenase enzymes of the cell. Each ful- 

 fills its role by alternately undergoing reduc- 

 tion and oxidation in a continually repeating 

 fashion. The electrons (and hydrogen) re- 

 ceived from a substrate are continually passed 

 on from one acceptor to another of lower po- 

 tential, and the energy liberated by each 

 transaction is conserved by the cell in the 

 form of newly formed high-energy phosphate 

 bonds (Fig. 8-4). 



The dehydrogenases are quite specific — not 

 only in reference to (he molecular configura- 

 tion of the substrate, but also in regard to 



the acceptor nucleotide to which a transfer of 

 electrons and hydrogen can be made. Some 

 (for example, lactic dehydrogenase, Fig. 8-2) 

 require one type of configuration, namely 



I 

 H — C — OH, and this group ol enzymes trans- 



I 

 acts business mainly with the pyridine nu- 

 cleotides (DPN and TPN). But another 

 group, represented by succinic dehydrogen- 

 ase (Fig. 8-3), requires substrates of different 



I 

 H— C— H 



configuration, | , and these en- 



H— C— H 



I 

 zymes generally employ one of the flavin 



nucleotides (FAD or FMN) as an electron- 

 hydrogen accepting agency. Finally, it should 

 be noted that the pyridine acceptors (DPN 

 and TPN) generally transmit their electrons 

 (and hydrogen) to the flavin acceptors (FAD 

 and FMN) — as is shown in Figure 8-4. 



The Cytochromes: a Secondary Electron- 

 Hydrogen Transmitting System. To a very 

 limited extent the flavin acceptors can trans- 

 mit hydrogen directly to molecular oxygen. 1 

 This relatively unimportant pathway leads 

 to the production of hydrogen peroxide 

 (FLO;,), which is catabolized through the 

 agency of catalase (p. 100). 



By far the greater flow electrons, however, 

 passes into the cytochrome system (Fig. 8-4). 

 This important electron-hydrogen transmit- 

 ting mechanism, first studied by the great 

 English biochemist, David Keilin, of Cam- 

 bridge University, consists mainly of a series 

 of hemoprotein (p. 321) enzymes, although 

 one of the more recently identified compo- 

 nents (ubiquinone) is not an iron (Fe) com- 

 pound. Each successive member of the series 

 (Fig. 8-4) displays a greater tendency to ac- 

 cept electrons. Thus, as electrons drop from 

 level to level in this cascading system, energy 

 is generated and utilized for building up the 

 high-energy phosphate (ATP) reserves of the 



1 Enzymes that catalyze a direct transfer of elec- 

 trons and hydrogen to oxygen are called oxidases, 

 rather than dehydrogenases. Here, either FAD oxidase 

 or FMN oxidase would he involved. 



