172 



GENERAL BIOCHEMISTRY 



of carbon dioxide, three of DPNH, and one molecule of TPNH by 

 the reactions of the citric acid cycle. The energy thus accumulated 

 in the DPNH and TPNH is made available by the reactions of the 

 next section. Whenever the supply of hexoses exceeds that required 

 for immediate needs, some of it is metabolized as far as acetyl co- 

 enzyme A, then converted to fatty acids. The proteins required for 

 growth and tissue repair depend in part upon three key amino acids 

 derived directly from the pyruvate, a-ketoglutarate, and oxalacetate. 

 The resulting amino acids are alanine, glutamic, and aspartic acids. 

 These various connections with the citric acid cycle are discussed 

 later. 



Electron transfer. The fourth and fifth stages in the oxidation of 

 foods and substrates as outlined on page 167 involve the transfer of 

 electrons from DPNH and TPNH through a series of electron ac- 

 ceptors to oxygen. During these reactions three molecules of ATP 

 are formed from ADP and inorganic phosphate for each DPNH 

 oxidized by this route. The present knowledge of the reactions in this 

 system is quite limited, and probably there are differences in different 

 species. However, the steps known for the higher plants and animals 

 may be similar down to the point of transfer of electrons to oxygen. 

 These oxidations can be summarized as follows, starting with either 

 TPNH or DPNH: 



TPNH + DPN+ ^ TPN+ + DPNH 

 DPNH + FAD + H3O+ ^ DPN+ + FADH2 + H2O 

 FADH2 + 2 cytochrome • Fe+++ + 2H2O ^ 



FAD + 2 cytochrome • Fe++ -f 2H3O+ 

 4 cytochrome • Fe++ + 4H3O+ + O2 <-> 4 cytochrome • Fe+++ + 6H2O 



FAD = flavine adenine dinucleotide and has the structure 



OH OH OH 0~ ,N 



I I I II I </ 



CH2C — C — C — CH2OP — O— P — OCHj O, N 



H H H 0_ 



O 



HO OH 



On reduction the lower left-hand portion of the molecule gains hy- 

 drogen atoms according to 



