334 ANIMAL BIOCHEMISTRY 



Function 



Riboflavin serves by way of one of the above prosthetic groups as a 

 hydrogen-electron carrier, as mentioned earlier. The isoalloxazine 

 portion of the molecule is subject to reversible oxidation and reduc- 

 tion according to the reaction shown, 



R 



CH; 



+ AH2 



O 



where AH2 represents a suitable reducing agent and R the rest of 

 the structure for either FMN or FAD. The R group helps contribute 

 a physiologically suitable oxidation-reduction potential and provides 

 for attachment to the apoenzyme component. 



It is now known that the various flavoproteins are also metallo- 

 protcins containing either Mo+ + , Cu++, or Fe+ + . These ions are 

 important to the activity of the enzymes since their removal blocks 

 the ordinary function. Probably the prosthetic group accepts the 

 electrons and transfers them to the metal ion, which in turn transfers 

 electrons to the next component of the chain. 



2H3O+ + (2^) + FAD • Fe2+++ protein -^ 



2H.>0 + FADHo • Fe2+++ protein 



i 

 FAD • Fe2+++ protein + {2e) <- FAD • Feo++ protein + 2H3O+ 



In this system the electrons come in one end from a reduced substrate 

 and are accepted at the other by an oxidized substrate. It is believed 

 that in at least one flavoprotein there are four iron ions for each 

 FAD instead of the two written here for the sake of convenience. 



The particular electron-transfer system outlined on page 172 in- 

 volves the flavoprotein called cytochrome reductase and functions by 

 the above mechanism. DPNH supplies the electrons to the cytochrome 

 reductase, which transfers them to cytochrome on the way to mo- 

 lecular oxygen. Although this pathway may not represent the only 

 one in animals, it is certainly the major one. Hence, the importance 

 of riboflavin in the diet becomes evident for all those animals unable 

 to synthesize il. 



In addition to cytochrome reductase, there are other important 

 flavoenzymes. Xanthine oxidase, for example, catalyzes the oxidation 

 of a number of different aldehydes as well as purines and pyrimidines 

 like xanthine. This enzyme is widespread and probably functions in 



