336 



RIBOFLAVIN 



The reaction catalyzed by this enzyme is: 



Riboflavin + ATP -^ Riboflavin-5-phosphate + ADP 

 The mechanism of transformation of riboflavin phosphate to flavin ade- 

 nine dinucleotide is not known, but it has been shown to occur in human 

 blood cells. '^ 



1. General Properties of Flavin Coenzymes 



Riboflavin phosphate and flavin adenine dinucleotide resemble the 

 parent vitamin, riboflavin, in many respects. They exhibit the same char- 

 acteristic yellow color and yellow-green fluorescence. Reduction with hypo- 

 sulfite, platinum, or H2 will reduce riboflavin and its coenzymes to colorless 

 compounds which will reoxidize to their original state when shaken with 

 air. If reduced in strongly acidic solution, a red intermediate is formed 



H 





 CH2-O-P-OH 



HCOH 



I 

 HCOH 







HCOH 



I 

 HCH 



NyN^c=o 



II 

 



Riboflavin-5-phosphate 



H3C 

 H3C 



Basic 

 group 



Acid 

 group 



Protein 



Apoenzyme 



which has the properties of a semiquinoid radical. ^^ In solution they are 

 essentiafly unstable. This decomposition is influenced by hght, heat, and 

 pH, riboflavin being rapidly decomposed in strongly alkaline solutions. 



Riboflavin phosphate is considerably more soluble in water than free 

 riboflavin and can be precipitated by various salts. It is hydrolyzed quite 

 slowly in weakly alkaline solutions, quite rapidly in acid solutions, and by 

 phosphatases such as a-glycerophosphatase. Riboflavin phosphate com- 

 bines with specific proteins," the apoenzymes, by attachment at the phos- 

 phoric acid group and at the slightly acidic imino group in the 3 position. 



The typical fluorescence of riboflavin is dependent upon the presence of 

 a free 3-imino group, and neither 3-substituted riboflavin nor the enzyme 

 systems will fluoresce. 



"^ J. K. Klein and II. I. Kolui, ./. Biol. Chciii. 136, 177 (1940). 



1" ]-:. Haas, Biochem. Z. 290, 291 (1937). 



" R. Kuhn and II. Rudy, Ber. 69, 2557 (1936). 



