ENZYMES 



277 



When the oxidized coenzyme is reduced, the nitrogen changes to a 

 valence of three, the double bond between the nitrogen and the adjacent 

 carbon atom is reduced, and a hydrogen ion is formed. These changes 

 may be represented as follows: 



+N* +2(H) ^^=^ N* +H+ 



I I 



The trivalent nitrogen is much less basic and no longer neutralizes a 

 phosphate group. Consequently the hydrogen ion formed might add to 

 this group: 



-0— P— + H+ :^=^ HO— P— 



II II 



O O 



However, the phosphate group is strongly acidic and in the physiological 

 pH range is almost completely dissociated {i.e., the equilibrium point of 

 the above reaction lies far to the left). What actually happens is that 

 the newly formed hydrogen ion is picked up by the buffer systems of 

 the cell and is used later in the reaction of reduced cytochrome c with 

 oxygen (see below) . 



It should be remembered in this connection that an atom of hydrogen 

 consists of a proton (hydrogen ion, H + ) and an electron. The electron 

 corresponding to the proton set free in the reduction of DPN or TPN 

 becomes attached to the coenzyme, neutralizing the positive charge on 

 the nitrogen atom. The reduced coenzyme thus actually carries one 

 hydrogen atom and one electron, the proton of the second hydrogen being 

 carried in the cell buffers. 



4. Riboflavin Coenzymes. These substances are also coenzymes of 

 oxidation-reduction reactions. There are two of these: the so-called ribo- 

 flavin mononucleotide (FMN) , which is more accurately named riboflavin- 

 5'-phosphate, and riboflavin adenine dinucleotide (FAD) . The abbrevia- 

 tions start with "F" because riboflavin is often called simply "flavin." 

 The formulas of these compounds are given in Fig. 10-7. Since the union 

 between the isoalloxazine and ribitol (alcohol corresponding to ribose) 

 residues is not glycosidic, neither substance strictly speaking is a nucleo- 

 tide, but the above names are in common use and are likely to be retained. 

 These coenzymes act by taking up and giving off two hydrogens. In 

 each case the hydrogen atoms are attached to positions 1 and 10 in the 

 flavin part of the molecule (Fig. 10-7) . FMN is a coenzyme for TPN- 

 cytochrome reductase and L-amino acid oxidase. FAD is required by 

 xanthine oxidase and glycine oxidase. 



