VITAMINS, ANTIBIOTICS, AND GROWTH 341 



. Ribose 



C II I > II I I 







Purine 6,7-DimethYl-6-RibitYl 



- E,4-Dioxypberidine 



Figure 5. Biological synthesis of riboflavin. 



ortho-xylene ring, is added. This can be formed from acetate or, as 

 shown by recent experiments by Plant ( 1960 ) , from two additional 

 molecules of compound G, which presumably break down to form the 

 fragments necessary for completion of the ring. 



Pantothenic Acid. Pantothenic acid is a vitamin because it is the 

 only portion of the molecule of coenzyme A that cannot be synthesized 

 by animals. Coenzyme A is the only known metabolically functional 

 form of pantothenic acid. Animals are unable to use a mixture of pan- 

 toic acid and beta-alanine as a replacement for pantothenic acid, al- 

 though microbiological systems are able to couple this system to form 

 pantothenic acid. The reaction has been studied by Maas ( 1955 ) in 

 cell-free extracts of E. coli, and he has outlined the following mecha- 

 r.ism for the reaction as carried out by the enzyme pantothenate syn- 

 thase : 



K+ Mg++ 



Enzyme + ATP + pantoate > Enzyme-pantoyl adenylate + PP; 



Enzyme-pantoyl adenylate + beta-alanine > pantothenate + 



AMP -f enzyme; 

 Sum: Pantoate + beta-alanine + ATP ^ pantothenate + AMP 



+ PP. 



The synthesis of pantoate proceeds in E. coU from alpha-keto 

 isovaleric acid ("ketovaline") which is formed from glucose by re- 



