35. BIOSYNTHESIS OF PURINE NUCLEOTIDES 



319 



N lo -formyl- FH 4 



ADP+P, 

 ATP 

 Forma te 

 + FH 4 



Fig. 7. The enzymic formation of the formylated derivatives of tetrahydrofolic 

 acid. 



isolated from natural sources as the citrovorum factor. 73 The N b derivative 

 may be converted enzymically in the presence of ATP into the N 5 ,N 10 - 

 anhydroformyl derivative 57 • 74 or a compound closely related to the cyclic 

 derivative so that iV 5 -formyltetrahydrofolic acid may serve indirectly as a 

 source of formyl groups in purine biosynthesis. However, iV 5 -formyltetra- 

 hydrofolic acid may donate formyl groups directly to glutamic acid to yield 

 formylglutamic acid. 75 It is thus seen that the nature of the formyl donor 

 in any given instance depends upon the enzymic system involved. 



V. Synthesis of Adenylic Acid from Inosinic Acid 



The precursors of the extra-ring nitrogens of adenylic and guanylic acids 

 have been shown to be aspartic acid and the amide nitrogen of glutamine, 

 respectively. 76 The enzymic synthesis of adenylic acid (Fig. 3) has been 

 studied in bone marrow, yeast, and Escherichia coli. The synthesis of ad- 



73 H. E. Sauberlich, J. Biol. Chem. 195, 337 (1952). 



74 J. M. Peters and D. M. Greenberg, J. Am. Chem. Soc. 80, 2719 (1958). 



75 M. Silverman, J. C. Keresztesy, G. J. Koval, and R. C. Gardiner, J . Biol. Chem. 

 226, 83 (1957). 



76 R. Abrams and M. Bentley, J. Am. Chem. Soc. 77, 4179 (1955). 



