242 UNITY AND DIVERSITY IN BIOCHEMISTRY 



(/) Biosynthesis of Short Chain Amino Acids 



Alanine is formed by transamination of pyruvate, and this latter sub- 

 stance is the starting material for the synthesis of other short chain amino 

 acids : glycine, serine, cysteine and cystine. According to Shemin, serine 

 is converted into glycine by way of formylglycine. 



CHoOH 



CHO 



— 2H 



+ H20 



NHo— CH— COOH -^ NHo— CH— COOH 



Serine 



HCOOH + NH2CH2COOK 



Formylglycine Glycine 



The serine is synthesized from "formate" and glycine. If the formate is 

 tagged with C^* and the carboxyl of the glycine with C^^, doubly-labelled 

 serine is obtained : 



HC^OOH + NHoCH^C^'OOH ^ NH^— CH— C^'OOH 



The formation of serine from glycine and "formate" implies the partici- 

 pation of folic acid and pyridoxal, and this has been demonstrated by 

 several experiments with bacteria. A number of observations has shown 

 that the serine is derived from the glycolysis pathway and finally yields 

 glycine. The glycine can be formed not only from serine but also from 

 threonine with formation of acetate. 



The carbon skeleton of cystine is also derived from serine, both in 

 mammals and in micro-organisms, as has been demonstrated by experi- 

 ments carried out with isotopes. Cysteine is actually an intermediate. The 

 sulphur of cysteine and cystine is derived from sulphates in micro- 

 organisms and from methionine in mammals. 



Cysteine is readily transformed into cystine in the cell, according to the 

 equation 



r^ 



OOH 



2H 



COOH 



COOH 



2 HCNH, 



HCNH2 



CHoSH + 2 H 



Cysteine 



HCNH2 



CH2 — S — S — CH2 



Cystine 



