238 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



Fig. 62 (Bloch) — Likely distribution of carbon isotopes in squalene. 



This hypothesis has been tested using labelled squalene and the results 

 have been favourable. However the ultimate mechanism of sterol formation 

 from squalene still remains undiscovered. The scheme shown in Fig. 63 

 has been proposed but part of this is still conjecture. 



(g) Biosynthesis of Porphyrins 



These substances are synthesized from 8-amino-levulinic acid formed 

 in the Shemin cycle (p. 222). Two molecules of the acid are condensed to 

 porphobilinogen, which is the precursor ring of the porphyrins. 



COOH 

 I 



COOH 



-2H20 



COOH 



I 



CH, 



NHa 



# = originally C^ of glycine. 



Several suggestions have been made as to the mode of conversion of the 

 precursor to the different porphyrins. In order to account for the distri- 

 bution of the a-carbon of glycine and the S-carbon of amino-levulinic acid 

 in the porphyrins of series I and II, Shemin has proposed the following : 

 the condensation of three molecules of porphobilinogen forms a tri- 

 pyrrylmethane which is then split into a dipyrrylmethane and a mono- 

 pyrrole. The structure of the dipyrrylmethane will differ depending on 

 the place where scission has occurred. Fission at A or at B will produce the 

 types of dipyrrylmethane indicated by the letters A and B (Fig. 64). 



J 



