Common Pathways of Cellular Metabolism 



145 



equivalent reduction. What we commonly 

 call an oxidation is, therefore, essentially an 

 oxidation-reduction reaction. 



The oxidation of hydrogen (H 2 ) will serve 

 as an example. In uniting with oxygen (0 2 ), 

 each hydrogen atom loses one electron and 

 each oxygen atom gains two, thus forming 

 the compound FLO. In essence, therefore, 

 while the hydrogen is being oxidized, the 

 oxygen is simultaneously being reduced. 



Oxygen, however, is not the only substance 

 in the cell that is able to accept, or gain, 

 electrons — thus to act as an oxidizing agent. 

 Eventually, the electrons given up while 

 various organic substances are undergoing 

 oxidative metabolism are delivered to oxy- 

 gen. But meanwhile, the electrons are passed 

 serially from one organic acceptor to another 

 and the cell derives useful energy at virtually 

 every step. 



The tendency of a substance to be oxidized 

 — that is, to yield up electrons — can be meas- 

 ured by bringing a solution of the substance 

 into contact with an electrode that has a 

 standardized potential for giving up or taking 

 on electrons. By such measurements the oxi- 

 dation-reduction potential of a particular 

 substance may be specified. Thus it has been 

 found that the total flow of electrons from 

 compound to compound in the cell is always 

 downhill — from compounds of higher to 

 compounds of lower potential. There is an 

 ever-flowing cascade of electrons that con- 

 tinues to liberate energy, and much of this 

 energy goes to maintain and expand the 

 high-energy phosphate reserves of the cell. 

 Accordingly, oxidative metabolism, in large 

 measure, may be described as oxidative phos- 

 phorylation. 



It is important to realize that in giving up 

 an electron an organic compound likewise 

 tends to give up hydrogen: the hydrogen that 

 previously was bonded to the molecule by 

 the now missing electron. Thus biological 

 oxidations usually involve dehydrogenation. 

 Furthermore, the electron acceptor com- 

 pound likewise serves, at least usually, as a 

 hydrogen acceptor, and there are a number 



of important acceptor compounds in every 

 cell (below). Many biological oxidations, ac- 

 cordingly, are dehydrogenation reactions, 

 and a variety of specific enzymes, the dehy- 

 drogenases, are present in every cell. In ac- 

 cepting hydrogen, along with one or more 

 electrons, the acceptor becomes reduced, as 

 is shown in Figures 8-2 and 8-3. 



H 



I 



H-C-H 



H — C— O H 

 I 

 COOH 



lactic acid 



lactic 

 dehydrogenase 



H 

 I 

 -C-H 



I 



c=o 



I 



COOH 



+ 



oxidized 



acceptor 



TPN or DPN 



-2e~ 

 -2H + 



H- 



+. 



reduced 

 acceptor 

 TPN- H 2 or DPN- H, 



pyruvic acid 

 oxidation product 



Fig. 8-2. Certain dehydrogenases (e.g., lactic de- 

 hydrogenase) act only upon substrates possessing a 



I 

 particular molecular configuration, namely H — C — OH. 



I 

 This group transmits electrons (and hydrogen) mainly 

 to DPN or TPN (p. 145). Pyruvic acid, the oxidation 

 product, is a very important intermediary metabolic 

 derivative (see Figs. 8-5 and 8-7). Above reaction, in 

 reverse, represents a reduction, rather than an oxi- 

 dation. 



Primary Acceptors of the Cell. The primary 

 oxidizing agents in the cell — that is, sub- 

 stances that pick up electrons and hydrogen 

 as these are discharged from various catabo- 

 lizing substrates — are a group of four im- 

 portant nucleotides: (1) Diphosphopyridinc 

 Nucleotide (DPN); (2) Triphosphopyridine 

 Nucleotide (TPN); (3) Flavin Adenine Di- 

 nucleotide (FAD); and (4) Flavin Mononu- 



