The Biosynthesis of Porphyrins 



255 



Although no evidence has yet been obtained concerning the bio- 

 logical mechanism of conversion of the monopyrrole to the tetrapyr- 

 role structure, several suggestions have been advanced.-' 1 " We would 

 like to suggest still another possibility which may explain the distri- 

 bution of the a-carbon atom of glycine or the 8-carbon atom of 8-amino- 

 levulinic acid in the porphyrin molecule of the I and III series. This 



Ac 



Ac 



Ac 



H B 



■CM- 



NH,C"H., N 

 " H 



C*H.. N' 

 'NH. h 



Ac 



A 



NH 



Ac 



Ac 



Fig. 9. A mechanism of porphyrin formation from the monopyrrole. Ac = acetic 



acid side chain. P = propionic acid side chain. • = a-carbon atom of glycine 



and 5-carbon atom of 5-aminolevulinic acid. 



mechanism is based on the synthetic mechanism of dipyrrole and 

 tetrapyrrole formation demonstrated by Corwin and Andrews, 31 and 

 by Andrews, Corwin, and Sharp. 32 



Condensation of three moles of the precursor pyrrole (porphobilino- 

 gen), or of a closely related derivative, would lead to a tripyrryl- 

 methane compound, as schematically represented in Fig. 9. The tri- 

 pyrrylmethane then breaks down into a dipyrrylmethane and a mono- 

 pyrrole. The structure of the clipyrrylmethane is dependent on the 

 place of splitting. An A split would give rise to dipyrrylmethane A, 

 and a B split would give rise to dipyrrylmethane B. Condensation 

 of two moles of dipyrrylmethane A would give rise to a porphyrin 

 of the I series, and condensation of a mole of A and a mole of B would 

 give rise to a porphyrin of the III series. In the formation of the 



