THE BIOSYNTHESIS OF PORPHYRINS; 

 THE SUCCINATE-GLYCINE CYCLE 



DAVID SHEMIN, The Department of Biochemistry, College of Physicians 

 and Surgeons, Columbia University, New York 



There has been tremendous progress in the last twenty 

 years in the elucidation of the biochemical reactions and trans- 

 formations which occur in living organisms. Some of the general 

 concepts which have emerged, summarized very briefly, are that 

 the basic reactions in the cells are surprisingly simple, that the 

 cell synthesizes its complex molecules from relatively simple and 

 available substances, and that there is a biochemical unity in 

 living matter. 



Although these concepts were perhaps not fully appreciated 

 when a study of the biosynthesis of porphyrins was begun ten 

 years ago, the picture which has emerged is rather a good il- 

 lustration of these basic concepts. The studies have revealed 

 that the complicated looking molecule, protoporphyrin, is syn- 

 thesized from two simple and available compounds, glycine and 

 succinate, by relatively simple reactions. Furthermore it has 

 been established that the synthesis of this esoteric looking mole- 

 cule is intimately related to the citric acid cycle, since the "ac- 

 tive" succinate utilized in porphyrin synthesis is produced in 

 these cyclic reactions. Although most of the investigations have 

 been concerned with the biosynthesis of protoporphyrin, it 



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