MODES OF LINKAGE BY COVALENT BONDS 



71 



H 



AH2 + 



H 



H 



A 



N 

 I 

 R 



HH 



CONH, 



H 



H 



A + 



H 



\ / 



N 



I 



R 



CONH2 



+ H 



+ 



A stoichiometric transfer of hydrogen takes place, from the substrate to 

 the coenzyme, with the hberation of one equivalent of acid. In effect, a 

 highly basic quaternary nitrogen atom is transformed into a feebly basic 

 ternary nitrogen. The reduction takes place, as can be seen from the for- 

 mula, in the para position of the pyridine ring. 



(6) Flavin adenine dinucleotide {FAD) — Like FMN, FAD is a pseudo- 

 nucleotide containing adenylic acid joined via a pyrophosphate linkage 

 to FMN. 



H,NC = N 



HC 



,N-C CH 



II U 

 N-C — N 



OHOHOH 



I I I 



O 



o 



O- 



OH 



I 



CH5-C-C-C-CH,-0-P-0-P-0-CH,-C— C- 



I 



I 



OH OH 



I I 

 H H 



OH 



I 



-c-c 



I I 



H H 



FAD 



It is the coenzyme of xanthine oxidase, aldehyde oxidase and other aerobic 

 dehydrogenases. Like riboflavin and FMN, FAD is universally present 

 in the biosphere. It is reddish-yellow in colour but, like FMN and ribo- 

 flavin, its solutions are a yellow-green. 



The formula above is that of the oxidized dinucleotide, reduction takes 

 place in the isoalloxazine ring as follows : 



H3C 



H,C 



