ANIMAL METABOLISM 



333 



or TPN • H2 (see Chap. 10 for exact formulas) . Next, the hydrogen is 

 most probably transferred to one of the flavin nucleotides. This may 

 be represented, for example, as follows: 



TPN -Hi + FAIN 



(26) 



TPN + FMN-Hi 



Note that the pyridine nucleotide is returned to its original condition, 

 ready to take up more hydrogen. The reduced flavin coenzyme then 

 hands its hydrogen to cytochrome c (Cyt. c) : 



FMN-Hj + 2Cyt. c Fe+++ 



(27) 



FMN + 2Cyt. c Fe++ + 2W 



The final reaction is the reoxidation of the reduced cytochrome c by mo- 

 lecular oxygen (from oxyhemoglobin) under the influence of cytochrome 

 oxidase, with the formation of water: 



2Cyt. c Fe++ + 2H+ + ^Os 



(28) cytochrome 

 oxidase 



2 Cyt. c Fe+++ + H2O 



The transport of hydrogen through the cytochrome system may be 

 represented by the scheme shown in Fig. 13-5. The light curved arrows 

 are used to indicate the alternate reduction and reoxidation of the hydro- 



Hydrogen transport system 



^ 



FMN-H. 



FMN 



2Cyt c Fe+++ 



,2Cyt c Fe++ 

 2H+ 



HoO 



Net result 



H 

 I 

 HOOC— C-OH 



HC— COOH 



I 

 HjC— COOH 



Isocitric acid 

 (metabolite) 



+ 10, 







II 



C— COOH 



I +H,0 



HC-COOH 



I 

 H.C-COOH 



Oxalosuccinic acid 

 (oxidized metabolite) 



Fig. 13-5. Transport of hydrogen through the cytochrome system. Al- 

 ternate oxidation and reoxidation of the hydrogen carriers is indicated by 

 light curved arrows. Heavy arrows show path of hydrogen from metabolite 

 to o.'cygen. > 



