150 THE BIOCHEMISTRY OF B VITAMINS 



to a cytochrome system, and although extensive study has been devoted 

 to this problem, as yet the proof of such a mechanism is lacking. 



The oxidation of the reduced triphosphopyridine nucleotide is carried 

 out by cytochrome c reductase. This enzyme has been isolated from yeast 

 and differs from the diaphorases mentioned above in that (1) its coen- 

 zyme is the riboflavin mononucleotide, (2) its substrate is the triphospho- 

 pyridine coenzyme, and (3) that the cytochrome system to which it is 

 linked has been identified. 87 



Two flavin enzymes: (1) the "old yellow enzyme" 55 which was isolated 

 from yeast and contains riboflavin phosphate as the prosthetic group, 

 and (2) the synthetic yellow enzyme 66 in which the dinucleotide is 

 substituted for riboflavin phosphate upon the same apoenzyme, may act 

 upon either the di- or triphosphopyridine nucleotides. No cytochrome 

 system has been shown to function with either of these enzymes; the 

 flavoproteins in this case pass the hydrogen atoms directly to oxygen 

 at a sluggish rate and the protein is believed to be a "derived" enzyme 

 rather than a "native" one. 



The other secondary hydrogen donor that is coupled with flavoproteins 

 is the reduced thiamine system. When thiamine functions in oxidative 

 decarboxylation reactions involving the removal from a-keto acids of 

 carbon dioxide and two hydrogen atoms (see p. 165) , either the thiamine 

 coenzyme or some group of the apoenzyme must be temporarily reduced. 

 It has been shown, using a crude protein extract from bacteria, that a 

 flavin dinucleotide must be present when this type of reaction is carried 

 out aerobically. 88 On this basis it has been postulated that a yellow 

 enzyme is essential for the reoxidation of the reduced thiamine systems. 



The mechanism by which energy is conserved in aerobic respiration 

 is still not clear. Energy balances indicate that there are approximately 

 three high-energy phosphate bonds formed for every two hydrogen atoms 

 passed from a substrate via the phosphopyridine and riboflavin nucleo- 

 tides to oxygen. 89 



The flavin-containing enzymes which have been enumerated function 

 primarily as carriers of hydrogen atoms in processes which are believed 

 to be unidirectional in their natural environment, i.e., the net process is 

 always one in which organic substrates are the hydrogen donors, and 

 molecular oxygen (in vivo) is the final acceptor of these atoms. Does 

 riboflavin function in anaerobic systems? Undoubtedly flavoproteins 

 catalyze reactions carried out by anaerobic organisms, and in these in- 

 stances the enzymes must have some organic substrate as a hydrogen 

 acceptor. To date only one example of this type of reaction has been 

 disclosed i.e., fumaric dehydrogenase, a flavin adenine dinucleotide 



