COENZYMES DERIVED FROM B VITAMINS 143 



ponent of the yellow enzyme; but the addition of pure riboflavin to the 

 protein component did not result in reconstitution of the flavoprotein. 

 Isolation of the pure coenzyme from the enzyme showed the coenzyme to 

 be a phosphoric acid ester of riboflavin. 58 



Riboflavin forms two coenzymes. The simpler one is the phosphoric 

 acid ester just mentioned. This ester is often referred to as a mono- 

 nucleotide, although in a strict sense this is incorrect since the compound 

 is derived not from the sugar, D-ribose, but from the corresponding 

 alcohol, D-ribitol. The more complex coenzyme, called the dinucleotide, 

 can be described as a molecule in which adenylic acid (muscle) and 

 riboflavin phosphate are united by the formation of a pyrophosphate bond. 



The location of the phosphoric acid at the 5 position on the ribityl unit 

 has been definitely established. Oxidation of the mononucleotide does not 

 yield formaldehyde, a product that would be formed were the primary 

 hydroxyl group unesterified. 59 A synthesis of riboflavin-5'-phosphate by 

 a method establishing the location of the phosphate confirmed the struc- 

 ture of the mononucleotide, since the synthetic product had the same 

 coenzymatic activity as the naturally occurring compound. 58 



The structure of the dinucleotide has not been proved by synthesis. 

 However, the identity of the two mononucleotides obtained upon hydrol- 

 ysis is certain. 60 The structure indicated in the formula above has been 

 accepted as the most likely one for the dinucleotide. 



Enzymes containing either of the two coenzymes are often referred to 

 as yellow enzymes or flavoproteins. The coenzymes are sometimes desig- 

 nated by the method of classification in which the name of the aromatic 

 nucleus, alloxazine (or more properly, isoalloxazine) , is specified. The 

 simpler expression, flavin adenine dinucleotide (FAD), is now more 

 popular than the cumbersome expression isoalloxazine adenine dinucleo- 

 tide. 



The coenzymes of riboflavin resemble the parent vitamin in many of 

 their physical and chemical characteristics: they exhibit the same charac- 

 teristic color and fluorescence ; they are decomposed when irradiated ; they 

 can be reduced by chemical reagents to leuco derivatives; they are auto- 

 oxidizable {i.e., the reduced leuco compounds can be oxidized by air back 

 to the original pigment) ; during reduction in strongly acidic solution 

 there is formed a red intermediate having the properties of a semiquinoid 

 radical; G1 and the redox potential of the riboflavin phosphate system is 

 comparable to that of a simple riboflavin system. 62 



The introduction of the acidic phosphate groups, however, makes the 

 coenzymes acidic and profoundly affects the capacity of the molecule for 

 combining with the apoenzyme (p. 145) . 



