Purine pathways

Phosphoribosyl pyrophosphate (PRPP) is important in both, and in these pathways the structure of ribose is retained in the product nucleotide, in contrast to its fate in the tryptophan and histidine biosynthetic pathways discussed earlier. An amino acid is an important precursor in each type of pathway glycine for purines and aspartate for pyrimidines. Glutamine again is the most important source of amino groups—in five different steps in the de novo pathways. Aspartate is also used as the source of an amino group in the purine pathways, in two steps. 

Two other features deserve mention. First, there is evidence, especially in the de novo purine pathway, that the enzymes are present as large, multienzyme complexes in the cell, a recurring theme in our discussion of metabolism. Second, the cellular pools of nucleotides (other than ATP) are quite small, perhaps 1% or less of the amounts required to synthesize the cell s DNA. Therefore, cells must continue to synthesize nucleotides during nucleic acid synthesis, and in some cases nucleotide synthesis may limit the rates of DNA replication and transcription. Because of the importance of these processes in dividing cells, agents that inhibit nucleotide synthesis have become particularly important to modern medicine. 

Histidine is in a family of one. There are nine steps in this pathway which interacts with the purine pathway. 

Henikoff, S., Keene, M. A., Sloan, J. S., Bleskan, J., Hards, R., and Patterson, D. (1986). Multiple purine pathway enzyme activities are encoded at a single genetic locus in Drosophila. Proc. Natl. Acad. Sci. USA, 83, 720-724. 

The enzymatic activity of amido phosphoribosyltransferase (P-Rib-PP— PR A) is low and flux through the de novo pathway in vivo is regulated by the end-products, AMP, IMP and GMP. Inhibition of reaction 1 by dihydrofolate polyglutamates would signal the unavailability of /V1()-formyl tetrahydrofolate, required as a substrate at reactions 3 and 9 of the pathway. The purine pathway is subject to further regulation at the branch point from IMP XMP is a potent inhibitor of IMP cyclohydrolase (FAICAR—> IMP), AMP inhibits adenylosuccinate synthetase (IMP—> sAMP) and GMP inhibits IMP dehydrogenase (IMP— XMP). 

A bifunctional enzyme, comprising the activities of AIR carboxylase and SAICAR synthetase, catalyzes reactions 6 and 7 of the purine pathway (AIR—> CAIR— SAICAR Fig. 15-16). A second bifunctional enzyme, IMP synthase, containing the activities of AICAR transformylase and IMP cyclohydrolase, catalyzes reactions 9 and 10 of the pathway (AICAR — FAICAR— IMP Fig. 15-16). Human IMP synthase has a subunit molecular weight of 62.1 kDa and associates as a dimer. A... 

There is a fifth bifunctional enzyme which catalyzes reactions 8 and 12 of the purine pathway (Fig. 15-16) but adenylosuccinate lyase has one active site with dual specificity, catalyzing both reactions (SA1CAR—> AICAR, sAMP—> AMP Fig. 15-16). All 14 enzymatic activities of Fig. 15-16 are cytosolic and there is a variety of evidence for association of subsets of these activities in vivo. The existence of a pathway particle or metabolon" for de novo purine biosynthesis in intact cells has been proposed. 

Other studies have examined the association between the activity of TPMT and other enzymes in the purine pathway and AZA toxicity. In one study, TPMT, HPRT, 5 -nucleotidase, and purine nucleoside phosphorylase activity in the red blood cells (RBC) of 33 RA patients on AZA (dose of approximately 2 mg/ kg/day) and 66 controls was measured. Fourteen RA patients with low (p = 0.004) and seven patients with intermediate TPMT activity (RR 3.1) developed AZA toxicity when compared to patients with normal TPMT activity [66]. Another study measured TPMT activity in 3 RA patients who had experienced AZA-induced hematologic toxicity and 16 RA patients without AZA toxicity. Two patients with AZA-induced hematologic toxicity were TPMT... 

HGPRT is responsible for the conversion of guanine to guanylic acid and hypoxanthine to inosinic acid. These two conversions require PRPP as the cosubstrate and are important reutilization reactions involved in the synthesis of nucleic acids. A deficiency in the HGPRT enzyme leads to increased metabolism of guanine and hypoxanthine to uric acid, and more PRPP to interact with glutamine in the first step of the purine pathway." Complete absence of HGPRT results in the childhood Lesch-Nyhan syndrome, characterized by choreoathetosis, spasticity, mental retardation, and markedly excessive production of uric acid. A partial deficiency of the enzyme may be responsible for marked hyperuricemia in otherwise normal, healthy individuals. 

The possibility that the iV °-formyl derivative (III.163) might serve as a substrate for two important enzymes of the de novo purine pathway that utilize reduced folates as their natural substrates, namely 5-aminoimidazole-4-car-boxamide ribonucleotide (AICAR) transformylase and glycinamide ribonucleotide (GAR) transformylase, was examined [61]. While the affinity of (III. 163) (AT, (app) = 29/xM) for AICAR transformylase appeared to be greater than that of the natural substrate 10-formyltetrahydrofolate (A (app) = 68 /xM), the reaction was slow, resulting in a 750-fold lower Frei/Am(app) ratio for the quinazoline. The affinity of (III. 163) (/frn(app) = 1.9 /xM) for GAR transformylase was likewise several times greater than that of the natural substrate, in this case 5,10-methenyltetra-hydrofolate (/if,n(app) = 8.9/xM). However, (III. 163) was also used rather efficiently in the reaction by GAR transformylase, resulting in a 4-fold higher V.J Ai, (app) for the quinazoline than for 5,10-methenyltetrahydrofolate. The authors concluded from these results that the 5,10-methenyl structure is not needed for GAR transformylase activity. 

PRPP is an important intermediate in the de novo synthesis of purines pathway (Figure 22.4). Defects in PRPP synthetase may render it insensitive to feedback inhibition by purine nucleotides. Thus, purine nucleotides are overproduced, leading to excessive uric acid synthesis and gout (Figure 22.9). 

AMP synthesis (The two reactions of AMP synthesis minic steps in the purine pathway leading to IMP.) In Step 1, the 6-0 of inosine is displaced by aspartate to yield adenylosuccinate. The energy required to drive this reaction is derived from GTP hydrolysis. The enzyme is adenylosuccinate synthetase. 

In Step 2, adenylosuccinase (also known as adenylosuccinate lyase, the same enzyme that catalyzes one of the steps in the purine pathway) carries out the nonhydrolytic removal of fumarate from adenylosuccinate, leaving AMP. 

Pigment formation can be used to visually detect mutants constitutive for enzymes of the purine pathway (Dorfman, 1969). The technique uses an auxotroph blocked early in the pathway which forms red colonies because of the polymerization into a red pigment of accumulated S-amino-4-imidazole ribonucleotide. Normally, high levels of adenine in the medium inhibit pigment formation because AMP represses the pathway. After mutation of the auxotroph, the population is plated on agar containing adenine derepressed mutant colonies are red. 

A common property of all Bacillus purine production strains is their aux-otrophy for adenine caused by a dysfunctional adenylosuccinate synthase gene (purA) of the AMP-specific branch of the purine pathway. Furthermore, the... 

Shi, S. et al. (2009) Increased production of riboflavin by metabolic engineering of the purine pathway in Bacillus suhtilis. Biochem. Eng. /.,... 

Gene-Enzyme Relationships in the Purine Pathway of Salmonella typhimurium... 

An interesting role of adenylosuccinate synthetase as a possible regulatory protein for the control of early enzymes in the purine pathway has been revealed in studies with yeast mutants [112,112a]. The loss of the synthetase by mutation is associated with an inability of adenine to prevent the synthesis of an early intermediate. It is not yet clear whether this is due to a modification of feedback inhibition or control by repression. If the latter is the case, then at least five different enzymes would come under such a control, since the continued repression of any one would not have allowed isolation of the phenotype. 

Jimanez A, Santos MA, Pompejus M, Revuelta JL. Metabolic engineering of the purine pathway for riboflavin production in Ashbya gossypii. Appl Environ Microbiol 2005 71 5743-51. 

In this study we report on the metabolism of hypoxanthine by parasitized erythrocytes vitro. Emphasis was placed on identified differences in host and parasite purine pathways (5) and on how through use of specific inhibitors a rational approach to the selective destruction of the IE malaria parasite can be achieved. 

Hadacidin, bredinin and mycophenolic acid, each shown to effect specific purine enzymes in other types of cells acted predictably to disrupt purine nucleotide synthesis by PRBC. The lack of a detectable effect on adenylate synthesis by alanosine may be due to an inability to form the active metabolite, L-alanosyl-AICOR, which requires an active de novo purine pathway (10). 

The present study was undertaken to study 3 selected purine pathway enzymes in infectious mononucleosis (IM). The enzymes concerned are adenosine deaminase (ADA), purine nucleoside phosphory-lase (PNP) and 5"-nucleotidase (5"-N). The enzymes were selected since the absence of these enzymes have been associated with clinically defective lymphoid function (1), and because abnormalities in the activities of ADA and PNP have been observed in leukemia (2,3)... 

D. G. Poplack, J. Blatt, and G. Reaman, Purine pathway enzyme abnormalities in acute lymphoblastic leukemia. Cancer Res.. 41 4821 (1981). 

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