561 Pteridines and Flavines 



ganisms. Obligate anaerobes contain relatively large 

 quantities of flavoproteins. Surveys have been made of 

 the flavine content of microorganisms not used in com- 

 mercial production.'' '^ There is variation in the tight- 

 ness of binding of the coenzyme, and the modes of attach- 

 ment are not entirely understood. 



One of the functions of the flavine enzymes has been 

 mentioned already, namely, the dehydrogenation of re- 

 duced DPN in the respiratory chain. Sites of DPNH for- 

 mation were seen earlier, particularly in the glycolysis 

 route and the citric acid cycle. The enzyme succinic de- 

 hydrogenase is a flavoprotein, and the FADH^ formed in 

 this reaction also is fed into the respiratory chain. Besides 

 the direct net synthesis of 2 moles of ATP during glycoly- 

 sis and of 1 mole of ATP in the citric acid cycle, the re- 

 maining energy released during glucose catabolism is 

 transferred in the form of hydrogen or electrons to en- 

 zymes with TPN, DPN or FAD as prosthetic groups. 



These reduced enzymes are, in turn, oxidized by the 

 metal ion-porphyrin enzymes, which are oxidized by gase- 

 ous oxygen. When two hydrogen atoms are passed along 

 the entire respiratory chain, water is formed as well as 3 

 more molecules of ATP. 



The exact number of particles in the chain is not en- 

 tirely clear, and there are variations with different or- 

 ganisms. In lactobacilli, for example, flavines seem to 

 replace heme proteins in electron transport. *^^ Also ob- 

 scure is the exact manner in which ATP is formed during 

 respiration and the precise way in which hydrogen is 

 transferred from one coenzyme to the next. There has 

 been interesting speculation in this area of biophysics. 



The respiratory chain can be shown in a simplified form 

 as in the accompanying diagram. ^^ 



''"J. L. Peel, Biochem. J. 69 403 (1958). 



*^ Chester DeLuca, Morton M. Weber and Nathan O. Kaplan, 

 /. Biol. Chem. 223 559 (1956). 



^s" Cornelius F. Strittmatter, Federation Proc. 17 318 (1958). 

 ■*» Albert L. Lehninger, Scientific American 202 102 (1960). 



