222 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



Ci fragments (formaldehyde or 

 formate) from C-a of glycine 

 and used for the synthesis of 

 purines, etc. 



a — ketoglutaric -<- 

 acid 



Ac. 



uccmyl — CoA 

 + glycine 



H 



HOOC— CH,— CH,— C— CHO 



& 



ketoglutaraldehyde 



HOOC— CH,— CHs— C— C— COOH 



II I 

 ONHj 

 a — amino — P — ketoadipic acid 



—CO, 



HOOC— CH,— CH,— C— CH,— CH, NH, 



II 

 O 



— aminolevulinic acid 



— NH, •" 



+ }0, 



porphyrins 



Fig. 54 (Shemin) — The succinate-glycine cycle. 



(j) Cystine and Cysteine 

 Among various other metabolic fates these two acids can yield pyruvic acid. 



HOOC— HC— HjC— S— S— CH,— CH-COOH 



I cystine I 



NHa NH2 



Ha i T Oj 



HS— CHa— CH— COOH -* HjS + NH., + CH,— C— COOH 



I desulphydrase 



NHa O 



cysteine pyruvic acid 



Quantitatively the most important path is the following : 



SH SO2H 



CH, 



CHNH2 



CH2 

 CHNH2 

 COOH COOH 



cysteine cysteine-sulphinic acid 



Cysteine-sulphinic acid, by transamination, can yield j8-sulphinyl- 

 pyruvic acid which is decomposed into pyruvic acid and SO, — which 



