250 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



When we come to tryptophan, which lies after shikimic acid, anthranilic 

 acid and indole are found to be intermediates in the synthesis. The con- 

 densation of indole with serine to give tryptophan is brought about by the 

 action of a specific enzyme, indole-ligase or tryptophan desmolase, whose 

 coenzyme is pyridoxal phosphate (this enzyme is not to be confused with 

 tryptophanase which splits tryptophan into indole and pyruvic acid). 



In addition to the pathway just outlined, tryptophan can result from the 

 transamination of indole-pyruvic acid, but it seems unlikely that this 

 reaction makes any important contribution to the biosynthesis, A trypto- 

 phan-kynurenine-anthranilate-indole-tryptophan cycle has also been 

 proposed i.e. the reverse of the catabolic pathway described in Fig. 58. 

 However this sequence of reactions is only traversed if an excess of trypto- 

 phan is present and its function appears to be purely degradative. 



(r) Biosynthesis of Histidi?ie and the Imidazole Ring 

 CH— NH CH— NH 



Pentose (?) 



^ C 



-N 



/ 



CH 



CH 



-N 



/ 



CHOH 



I 

 CHOH 



I 

 CHaOPCH, 



CH, 



C=0 



CH2OPO3H2 



Imidazole-glycerolphosphate Imidazole-hydroxyacetonephosphate 



CH— NH CH— NH CH— NH 



CH 



-N 



/ 



^C 



CH 



-N 



/ 



CH 



-N 



CH, 



CH. 



CH, 



CHNH2 



I 

 CHgOPO^H 



CHNH, 



30.12 CH2OH 



Histidinolphosphate Histidinol 



CHNHo 



COOH 



Histidine 



Fig. 70 (Davis) — Biosynthesis of histidine. 



Mutants of E. coli which are auxotrophic for histidine accumulate 

 L-histidinol, the corresponding aminoalcohol. Mutants of Neurospora have 

 been discovered since, which accumulate not only histidinol but also 



