Phosphoribosyl aminoimidazole

In this way, the nitrogen from several amino adds can be transferred to aspartate, glutamate or glutamine The ketoacid acceptors may originate from glucose or gluconeogenic precursors. The C-1 atom on formyltetrahydrofolate may arise from serine or methionine. In addition HC07 is required for the carboxylation of 5 -phosphoribosyl-5-aminoimidazole. 

FIGURE 14. Structural comparison (from left to right) of Amitrole, 5 -phosphoribosyl-5-aminoimidazole-4-carboxylic acid, Virazole, and 2-amino-l,3,4-thiadiazole. 

Similar results were obtained in the conversion of S -phosphoribosyl-aminoimidazole carboxamide 13 to inosine-5 -monophosphate 14 (S -IMP, Scheme 9). 

Phosphoribosyl glycineamide + 5,10-methenyl H -folate Phosphoribosyl aminoimidazole carboxamide -+ 10-formyl Hi-folate... 

Phosphoribosyl aminoimidazole succinocarboxamide Phosphoribosyl aminoimidazole carboxamide... 

It was subsequently shown that radioactive aminoimidazole carboxamide was converted to inosinate by pigeon liver extracts, and it seemed logical to assume that a ribose phosphate derivative of aminoimidazole carboxamide must be an intermediate. The ribonucleotide, phosphoribosyl aminoimidazole carboxamide, was synthesized enzymatically and was indeed converted to inosinate by pigeon liver fractions. 

These results indicated not only that phosphoribosyl aminoimidazole carboxamide was a precursor of purine nucleotides, but also that the imidazole portion of the purine was formed prior to the pyrimidine moiety, and that this nonpurine intermediate was involved in the form of a ribose phosphate derivative. 

When formate was omitted from this system, a compound accumulated which gave a Bratton-Marshall color reaction identifying it as an aromatic amine. As predicted, this compound was phosphoribosyl aminoimidazole carboxamide. 

When CO2 and aspartate were omitted from the system in addition to formate, a second aromatic amine accumulated, as shown by a Bratton-Marshall test color different from that given with phosphoribosyl aminoimidazole carboxamide. This amine was identified as phosphoribosyl aminoimidazole. These reaction sequences were then dissected to reveal their component parts. 

The fixation of COj or HCO3 into phosphoribosyl aminoimidazole led to the formation of a new intermediate because CO2 was knoivn to furnish the C-6 position, it was identified as phosphoribosyl aminoimidazole carboxylate. The reaction is readily reversible. 

For several years there was some confusion concerning the possible role of biotin in this reaction. Phosphoribosyl aminoimidazole accumulated in biotin-deficient yeast, and because this vitamin is knoivn to be involved in... 

Aspartate is required for the subsequent metabolism of phosphoribosyl aminomidazole carboxylate, and phosphoribosyl aminoimidazole accumulates because of the ready reversibility of the carboxylation reaction. 

The reaction of aspartate with phosphoribosyl aminoimidazole carboxylate produced a compound whose acidity indicated that the dicarboxylic... 

Phosphoribosyl aminoimidazole succinocarboxamide is cleaved in a reversible manner to fumarate and the well-characterized intermediate, phosphoribosyl aminoimidazole carboxamide. The enzyme catalyzing this coo-... 

Phosphoribosyl aminoimidazole carboxamide formyltransferase catalyzes the addition of a second formate group it becomes the C-2 position of the purine ring. 

Phosphoribosyl aminoimidazole synthetase Carbon-nitrogen Cyclization No Yes... 

Phosphoribosyl aminoimidazole carboxylase Carbon-carbon CO2 fixation Yes No... 

Phosphoribosyl aminoimidazole succinocarboxamide synthetase Carbon-nitrogen Aspartate amino transfer Yes Yes... 

Phosphoribosyl aminoimidazole carboxylase Phosphoribosyl aminoimidazole succinocarboxamide 15 19... 

Pho8phoribosyl-formyl-glycineamidine cyclo-ligase (ADP) Phosphoribosyl aminoimidazole synthetase... 

Phosphoribosyl-5-amino-4- imidazolecarboxylate carboi lyase Phosphoribosyl aminoimidazole carboxylase... 

Phosphoribosyl-5-form-amide-4-imidazolecarbox-amide tetrahydrofolate 10-formyltransferase Phosphoribosyl aminoimidazole carboxamide formyltransferase... 

The substrate specificity of adenylosuccinate synthetase is strict inosi-nate cannot be replaced by hypoxanthine, inosine, or phosphoribosyl aminoimidazole carboxamide L-aspartate cannot be replaced by ammonia or other d- or L-amino acids and other nucleoside mono-, di-, and triphosphates cannot replace GTP. Divalent cations are required. Bacterial, plant, and animal enzymes have similar specificities and general properties, although Michaelis constants differ somewhat. These are approximately 10 to 10 Af for GTP and aspartate, and 3 X Iff" M for ino-sinate. 

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