Aminoimidazole succinocarboxamide ribonucleotide

Aminoimidazole ribonucleotide (AIR) Aminoimidazolecarboxylate ribonucleotide (CAIR) Aminoimidazole succinocarboxamide ribonucleotide (SAICAR) Aminoimidazolecarboxamide ribonucleotide (AICAR) Formamidoimidazolecarboxamide ribonucleotide (FAICAR)... 

A few steps convert inosinate into either AMP or GMP (Figure 25,9). Adenylate is synthesized from inosinate by the substitution of an amino group for the carbonyl oxygen atom at C-6. Again, the addition of aspartate followed by the elimination of fumarate contributes the amino group. GXP, rather than ATP, is the phosphoryl-group donor in the synthesis of the adenylosuccinate intermediate from inosinate and aspartate. In accord with the use of GTP, the enzyme that promotes this conversion, adenylsuccinate synthase, is structurally related to the G-protein family and does not contain an ATP-grasp domain. The same enzyme catalyzes the removal of fumarate from adenylosuccinate in the synthesis of adenylate and from 5-aminoimidazole-4-jV-succinocarboxamide ribonucleotide in the synthesis of inosinate. 

Adenylosuccinate formed by adenylosuccinate synthetase is cleaved by adenylosuccinate lyase to form AMP. The reaction steps are illustrated in Fig. 1. Included in the sequence is the additional reaction catalyzed by AMP deaminase. These three enzymes have been suggested to function in a cyclic process termed the purine nucleotide cycle 7,8). The two-step conversion of IMP to AMP is very similar to both the conversion of citrulline to arginine, which involves formation of argininosuccinate as an intermediate, and formation of 5-amino-imidazole 4-carboxamide ribonucleotide from 5-aminoimidazole 4-carboxylate ribonucleotide as part of IMP biosynthesis. Adenylosuccinate lyase is a dual function enzyme catalyzing the cleavage of both adenylosuccinate and 5-aminoimidazole 4-N-succinocarboxamide ribonucleotide. 

As mentioned in Chapter 7, adenylosuccinate lyase not only functions in purine ribonucleotide interconversion, but also converts phosphori-bosyl aminoimidazole succinocarboxamide to phosphoribosyl amino-imidazole carboxamide. Kinetic studies show that these compounds are alternative substrates and products, respectively, with adenylosuccinate and adenylate, and attempts physically to separate the two activities have consistently failed. Genetic studies have shown that both activities are governed by the same locus and that mutant enzymes behave similarly in both reactions 20, 21). 

Phosphoribosyl-1-pyrophosphate synthetase 2 aminophosphoribosyltransferase 3 phospho-ribosylglycinamide synthetase 4 glycinamide ribonucleotide transformylase 5 iV-formylglycin-amidine ribonucleotide amidoligase 6 5-aminoimidazole ribonucleotide synthetase 7 5-amino-imidazole ribonucleotide carboxylase 8 5-aminoimidazole-4-iV-succinocarboxamide ribonucleotide synthetase 9 adenylosuccinate lyase 10 5-aminoimidazole-4-carboxamide ribonucleotide transformylase 11 inosinicase... 

ATP paid as often seen, for that transfer Makes formyl glycinamidine, ribonucleotide Dehydration s now its role, and ring closure 5-aminoimidazole, still a nucleotide And now a COj adds on the C-4 side Then aspartate adds, makes carboxylate Succinocarboxamide. 

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