DAVID SHEMIN 



ring carbon atom and the methene-bridge carbon atom, led us 

 to conclude that the same derivative of glycine was utilized for 

 both of these positions. 



These findings limit the number of possible ways in which 

 the succinate and glycine combine to form a pyrrole unit. The 

 mechanism of condensation of succinate and glycine becomes a 

 bit more obvious if one considers that the product of this con- 

 densation should not only supply a reasonable mechanism of 



COOH 



COOH 



CHg 



CH; 



CH; 



-t- 



CHo 



I 



NH2 



C'O 



I 



CHo 



I 



C'O -2H2O 



COOH 



CH; 



COOH 



GH2 



CH2 



HgN 



/ 



^"2 \.r 



PROTO- 

 PORPHYRIN 



C'np 

 I 



NH2 



<5^-AMIN0 

 LEVULINIC ACID 



(H) + (IT) 



PRECURSOR 

 PYRROLE 



Fig. 5. The mechanism for the formation of the monopyrrole, 

 porphobiHnogen, by condensation of 2 moles of 5-aminolevulinic acid. 

 The carbon atoms bearing the closed circles (•) were originally the a- 

 carbon atom of glycine. 



pyrrole formation but should also suggest a reasonable mech- 

 anism by which the a-carbon atom of glycine is detached from 

 its carboxyl group and should explain the distribution of the 

 cK-carbon atom of glycine in the porphyrin molecule. The 

 condensation of succinate on the cn-carbon atom of glycine to 

 form a;-amino-/3-ketoadipic acid would appear to agree with the 

 above findings. The compound formed, being a j8-keto acid, 

 could then readily be decarboxylated, and thus provide a mech- 

 anism by which the a-carbon atom of glycine becomes detached 

 from its carboxyl group. It would also resolve the apparent 



524 



