42 



David Shemin and Jonathan Wittenberg 



numbered so that similar side chains occur on the same 

 numbered ^-carbon of each pyrrole ring. Since uroporphyrin 

 is the biological porphyrin containing the largest number of 

 carbon atoms, it is used as the parent compound. The carbon 

 atoms of protoporphyrin, the porphyrin found in haemoglobin, 

 would therefore be numbered as shown in Fig. 2. It is now 

 easy to designate a particular carbon atom; e.g. carbon atom 



ioCOOH 



7 COOH 9CH2 



r 



/ 

 HC6 



W 



H 



I 



D 



6 CHp ^ CHo 

 7C00H 9 CH2 

 IOCOOH 



H 



G- 



a 



:G- 



H 



10 QOOH 

 7.CC0H 9 CH? 

 &QH2 sQHa 



S 



I 



N 



H 



2 



N 



I 



2 ! 



^h 



6 CH< 



8 CH2 



9 CH2 7 COOH 

 loCOOH 



UROPORPHYRIN HI 

 Fig. 1. Numbering System for Porphyrins. 



A4 refers to the /8-carbon atom of pyrrole ring A to which 

 the methyl group is attached, etc. 



For the purpose of clarity this discussion is divided into 

 two parts: the first part dealing with the role of glycine in 

 porphyrin formation, and the second with the role of a four 

 carbon unsymmetric compound which appears to arise in the 

 tricarboxylic acid cycle. It will be shown below that glycine 

 and this four carbon compound account for all thirty-four 

 carbon atoms of protoporphyrin. 



