294 



MISCELLANEOUS NITROGEN AND SULFUR COMPOUNDS 



HN 

 I 



HC 

 I 





 li 



NH 



I 



CH 



CH(CH2)4C00H 



biotin 



Folic acid is only one of a series of related compounds which are found widely dis- 

 tributed in nature. It can be regarded as made up of a pteridine nucleus, a molecule of 

 p-aminobenzoic acid, and a molecule of glutamic acid. The exact coenzyme form of folic 

 acid may vary from reaction to reaction, but in all cases the coenzymes are tetrahydro 

 derivatives to which one-carbon units may be attached at position 5 or 10. N^ N^°-anhy- 

 droformyltetrahydrofolic acid is active in one step of purine biosynthesis, N'°-formyltetra- 

 hydrofolic acid at another step. N^-formyltetrahydrofolic acid (folinic acid, leucovorin, 

 citrovorum factor) has been given much prominence since it was the first formylated folic 

 acid to be discovered, but it must be converted to one of the other derivatives before serv- 

 ing as a coenzyme. 







li 



CNHCHCHgCHjCOOH 

 COOH 



folic acid 



OCNHCHCHgCHgCOOH 

 COOH 



folinic acid 



Sometimes a polypeptide of glutamic acid is present rather than a single amino acid mole- 

 cule. Other forms of the vitamin are tightly bound to cellular structure. The active form 

 of folic acid functions as a coenzyme in one carbon metabolism involving formate, such 

 as in the interconversion of glycine and serine and in the synthesis of methyl groups. 

 Transformations of folic acid and one-carbon units have been investigated in germinating 

 pea extracts (62). 



