Histidine from purine nucleotides

PRPP is the activated intermediate in the synthesis of phosphoribosylamine in the de novo pathway of purine formation of purine nucleotides from free bases by the salvage pathway of orotidylate in the formation of pyrimidines of nicotinate ribonucleotide of phosphoribosyl ATP in the pathway leading to histidine and of phosphoribosylanthranilate in the pathway leading to tryptophan. 

Histidine is derived from three biomolecules PRPP (five carbons), the adenine ring from ATP (one nitrogen and one carbon), and glutamine (one nitrogen). The ATP used in the first reaction in the pathway is regenerated when 5-phosphoribosyl-4-carboxamide-5-aminoimidazole (released in a subsequent reaction) is diverted into the purine nucleotide biosynthetic pathway. 

Histidine alone, however, is not capable of reversing the growth effects of amitrole on green plants nucleotide bases are more effective. " Purine biosynthesis has been suggested as a more important site of action than IGP dehydratase.Since histidine is ultimately biosynthesized from a purine nucleotide, adenosine triphosphate, the inhibition of histidine formation may only be indirect. Amitrole causes the bleaching of newly formed plant tissue, and pigment biosynthesis has been suggested as an alternative site of herbicide action.Effects of 1,2,4-triazole on cysteine biosynthesis have been noted in bacteria, but no studies have been carried out on plants to assess the relevance of this to amitrole-phytotoxicity. ... 

Discussing the biosynthetic pathways for amino acids and nucleotides together is a sound approach, not only because both classes of molecules contain nitrogen (which arises from common biological sources) but because the two sets of pathways are extensively intertwined, with several key intermediates in common. Certain amino acids or parts of amino acids are incorporated into the structure of purines and pyrimidines, and in one case part of a purine ring is incorporated into an amino acid (histidine). The two sets of pathways also share... 

Sources of the Purines.—The hydroxy and amino purines are derived chiefly from the nucleoproteins of the diet, although there is evidence that the organism can synthesise the purine ring from the amino acid histidine. Nucleoproteins are compound proteins, the non-protein being nucleic acid, which is liberated during alimentary digestion. Nucleic acid is composed of four nucleotide units, each of which is a phosphoric ester of a nucleoside, or compound of sugar and a nucleo purine or a pyrimidine (p. 130). 

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