Purine-binding enzymes

The P-site of adenylyl cyclase inhibits cyclic AMP accumulation. Since P, and P2 receptors are located on the cell surface, they bind purines or pyrimidines in the extracellular space. There also is an adenosine binding site located intracellularly on the enzyme adenylyl cyclase (see Ch. 21). This is referred to as the P-site of adenylyl cyclase. Binding of adenosine and other purines, notably 3 AMP, 2 deoxy-3 -ATP and 2, 5 -dideoxyadenosine to this site, inhibits adenylyl cyclase activity [8]. The P-site of adenylyl cyclase and other intracellular purine binding sites are not classified as purinergic receptors. 

Several other purine nucleotide derivatives with alkyl halide substituents have appeared in the literature, including adenosine S -chloromethane phosphonate, adenosine 5 -chloromethylpyrophosphate, and adenosine 5 -()8-bromoethane phosphonate) (287-291). These compounds have been evaluated as affinity labels of such nucleotide-binding enzymes as leucyl- and tryptophanyl-tRNA synthetases (leucine- and tryptophan-tRNA ligases), phosphorylase b, and cAMP-... 

The process of RNA synthesis in bacteria—depicted in Figure 37-3—involves first the binding of the RNA holopolymerase molecule to the template at the promoter site to form a PIC. Binding is followed by a conformational change of the RNAP, and the first nucleotide (almost always a purine) then associates with the initiation site on the 3 subunit of the enzyme. In the presence of the appropriate nucleotide, the RNAP catalyzes the formation of a phosphodiester bond, and the nascent chain is now attached to the polymerization site on the P subunit of RNAP. (The analogy to the A and P sites on the ribosome should be noted see Figure... 

The association between RNase A and 3 -UMP or 3 -dUMP has been studied by P n.m.r. and kinetic methods, respectively. In both cases the participation of two dissociable groups at the active site of the enzyme was demonstrated, in agreement with n.m.r. and A -ray - studies on the binding of 3 -CMP to RNase. In the binding of Tj RNase to purine nucleotide monophosphates, the phosphate group appears to have an important effect while the ribose ring is relatively unimportant. ... 

Most in vitro studies of xanthines have centered around the enzyme xanthine oxidase. Bergmann and co-workers 40-4)) have examined the main oxidative pathways in the xanthine oxidase catalyzed oxidation of purines. The mechanism proposed by these workers 41 > is that the enzyme binds a specific tautomeric form of the substrate, regardless of whether or not that form represents the major structure present in solution. It is then proposed that the purine, e.g., xanthine, undergoes hydration at the N7=C8 double bond either prior to or simultaneously with dehydrogenation of the same position. Accordingly, the process would involve either pathway a or b. Fig. 15. Route a would give a lactim form of the oxidized purine, while b would give the cor-... 

Glutamol adenylate (8) (Table 3) is a competitive inhibitor (A)= 3 pmol 1 ) of GluRS from E. coli The A -benzoyl adenine derivative is also an inhibitor (A) = 60pmolG ). Replacing adenine with other bases (purine, cytosine, dihydrocytosine, uridine) resulted in a more than 1000-fold loss of activity, indicating the importance of the contribution of the adenine ring to enzyme binding. 

Nucleoside phosphorylases that catalyse the reversible cleavage of purine nucleosides to the free bases and ribose-1-phosphate are found in most cells, although a phosphorylase that will cleave adenosine has so far been identified only in bacteria. Recent studies have shown that ribo- and 2 -deoxyribofurano-syltransferase activity is associated with phosphorylase activity [19, 23., 222] and that both activities reside in one enzyme, which can be converted from one form to the other by substrate or product binding [20]. Upon crystallization of the enzyme from human erythrocytes a marked decrease in the ribosyl transfer reaction was observed [21b]. 

As the first committed step in the biosynthesis of AMP from IMP, AMPSase plays a central role in de novo purine nucleotide biosynthesis. A 6-phosphoryl-IMP intermediate appears to be formed during catalysis, and kinetic studies of E. coli AMPSase demonstrated that the substrates bind to the enzyme active sites randomly. With mammalian AMPSase, aspartate exhibits preferred binding to the E GTPTMP complex rather than to the free enzyme. Other kinetic data support the inference that Mg-aspartate complex formation occurs within the adenylosuccinate synthetase active site and that such a... 

The answer is E. Methotrexate is an analog of folic acid that binds with very high affinity to the substrate-binding site of dihydrofolate reductase, the enzyme that catalyzes conversion of DHF to THE, which is used in various forms by enzymes of both the purine and pyrimidine de novo synthetic pathways. Thus, synthesis of dTMP from dUMP catalyzed by thymidylate synthetase and several steps in purine synthesis catalyzed by formyltransferase are indirectly blocked by the action of methotrexate because both those enzymes require THE coenzymes. 

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