Biosynthesis of Porphyrins 



53 



from the carboxyl group of acetate. These data strongly 

 suggest that not only are the two types of pyrrole unit in 

 protoporphyrin made from the same precursors, but also 

 that in each pyrrole ring the same compound is utilized for the 

 methyl side of the structure and for the vinyl and propionic acid 

 sides of the structure. This conclusion is supported by the 



.14 



C H3COOH Experiment 



COOH 



^^ 



A>' 



/ 



\ 



N 

 H 



CH3C"^00H Experiment 



:ooH('i7o) 

 :h2 



H3C 



y 



c 



/ 



\ 



10 COOH 

 9 CH2 

 6 H3C 8 CH2 



N 

 H 



C 5 2C 



/\;/^ 



H 



Fig. 4. Average activities of comparable carbon atoms in all 

 pyrrole units. The activities are given in parentheses. The 

 pyrrole unit represented above contains a carboxyl group which 

 is found only in Rings C and D of protoporphyrin. In the central 

 figure the carbon atoms are designated according to the num- 

 bering system. 



finding, as pointed out earlier, that the pyruvic acid and 

 a-ketobutyric acid fragments of the pyrrole units have the 

 same activities in each of the experiments using methyl 

 labelled and carboxyl labelled acetate {Table IV). 



If each side of each pyrrole unit utilizes the same com- 

 pound, the precursor which condenses with glycine to form 

 the pyrrole unit must be either a three or a four carbon atom 



