Pfizer Handbook of Microbial Metabolites 564 



When an electron is transferred from the N-atom of the 

 reduced pyridine ring to an unoccupied 7r-orbital of the 

 isoalloxazine ring of FAD, the N-atom assumes a positive 

 charge, which is neutrahzed by attraction of a proximate, 

 ionized phosphate hydroxyl oxygen. The increased elec- 

 tron density on the 0-atom at position 2 in the riboflavin 

 nucleus might cause formation of a bond to phosphorus 

 as shown in the activated complex above, the reaction 

 being : 



DPNH + FAD + H2O3PO® + H® ^ DPN® + H2O3P— FADH + OH® 



When this substance is oxidized by the subsequent carrier 

 (probably a cytochrome), two electrons, perhaps dislocal- 

 ized TT-electrons, are withdrawn from the FAD-complex 

 thus permitting dissociation of a proton and activation of 

 the phosphoryl group. In the presence of ADP, then, ATP 

 could be formed according to the equation: 



FADH— PO3H2 + ADP + 2Fe€I5) ^ FAD + H© + ATP + 2Fe<£5 



Other flavoprotein dehydrogenase substrates are: alde- 

 hydes, a-amino acids, a-hydroxy acids, purines, fatty acid- 

 coenzyme A esters and certain amines. Flavine enzymes 

 also participate in bacterial hydrogenase systems, in ni- 

 trate reduction and assimilation by fungi and higher 

 plants and in photosynthesis and bioluminescence. There 

 is currently much study of flavoprotein reactions, which 

 can often be followed by spectrophotometry and EPR tech- 

 niques. 



Reviews of the flavine coenzymes and their biosynthesis 

 are available. ^^' ^^ 



1048 Xanthopterin, CgHjOoNg, yellow amorphous substance, isolated 

 as barium or sodium salts. 



OH 



I .OH 



HoN 



N Y 1 



^2 Paul D. Boyer, Henry Lardy and Karl Myrback (Eds.), "The 

 Enzymes" Vol. II, 2nd ed., Helmut Beinert, Flavin coenzymes. 

 Academic Press, New York, 1960, pp. 340-416. 



