Biosynthesis of Porphyrins 



63 



chain would be converted to coproporphyrin III. The latter by 

 decarboxylation and dehydrogenation of the propionic acid side 

 chains of pyrroles A and B would yield protoporphyrin. An 

 alternative pathway may be suggested in which the first decar- 

 boxylation occurs at the mono-pyrrole stage. Decarboxylation 



• 



I I Methyl group of Acetic Acid 

 9 Corboxyl group of Acetic Acid 

 X Of-Corbon atom of Glycine 



Fig. 9. Schematic representation of the origin of the carbon 



atoms of protoporf)hyrin. 



of the carboxymethyl group and condensation of these derived 

 pyrrole compounds would result in the formation of copropor- 

 phyrin, by-passing uroporphyrin. It is significant that in all of 

 the naturally occurring porphyrins the side chains can theoretic- 

 ally be derived from carboxymethyl and carboxyethyl groups. 

 A summary of the source of each of the carbon atoms of 

 protoporphyrin is given in Fig. 9, where the X is the a-carbon 



