BIOSYNTHESIS 243 



The oxidation of cysteine to cystine is catalysed by cytochrome-c and 

 cytochrome-oxidase, whilst the reduction of cystine is accomplished by a 

 number of reducing agents : HgS, glutathione, SH-enzymes, etc. 



The conversion of serine into cysteine is brought about by conden- 

 sation of the serine with homocysteine, formed by the demethylation of 

 methionine, or more exactly of S-adenosylmethionine, in the course of 

 transmethylation reactions. 



CH3— S— CH,-CH,— CHNH,— COOH — HS CHj- CH^-CHNH.-COOH 



methionine homocysteine 



N CHj - CH, - CHNHi— COOH 



"^ CH2 — CHNH2 - COOH HOCHj — CH NH, — COOH 



cystathionine serine 



HS CH2 - CHNHj - COOH HOCHj — CH,— CH NH, - COOH 



cysteine homosenne 



(m) Biosynthesis of Glutamic Acid 



The chief source of glutamic acid is a-ketoglutaric acid produced in the 

 Krebs cycle. This acid can itself only arise in the oxidative decarboxylation 

 of isocitric acid, for the decarbox}dation of a-ketoglutarate to give succinate 

 is irreversible. This is the sole pathway for the formation of glutamic acid 

 from carbohydrate. 



In certain micro-organisms like Escherichia colt, it is the only source, 

 whilst in other organisms other substances can contribute to the formation 

 of glutamic acid. These substances are proline, arginine, histidine, glycine 

 and succinic acid, by way of S -aminolevulinic acid. 



The amino group is derived from ammonia fixed to a-ketoglutaric acid 

 by glutamic dehydrogenase, or from the amino group of another amino 

 acid, transferred by transamination. 



[n) From Ghitamate to Glutamtne, Proline^ Hydroxyproline 



and Arginine 

 Here, we have a biosynthetic pathway which, starting from a-keto- 

 glutaric acid in the Krebs cycle, divides at the level of glutamic acid 

 into three branches, one leading to glutamine, a second to proline and 

 hydroxyproline, and the third to ornithine, citruUine and arginine. 



1. Ghitamine 



Starting from glutamic acid and ammonia, in the presence of ATP, 

 glutamine is formed in a reaction whose mechanism is unknown. 



