Crystal structures of a new class of pyrimidine/purine nucleoside phosphorylase (ppnP) revealed a Cupin fold

Nucleotides metabolism is a fundamental process in all organisms. Two families of nucleoside phosphorylases (NP) that catalyze the phosphorolytic cleavage of the glycosidic bond in nucleosides have been found, including the trimeric or hexameric NP-I and dimeric NP-II family enzymes. Recently studies revealed another class of NP protein in E. coli named Pyrimidine/purine nucleoside phosphorylase (ppnP), which can catalyze the phosphorolysis of diverse nucleosides and yield D-ribose 1-phosphate and the respective free bases. Here, we solve the crystal structures of ppnP from E. coli and the other three species. Our studies revealed that the structure of ppnP belongs to the Rlmc-like cupin fold and showed as a rigid dimeric conformation. Detail analysis revealed a potential nucleoside binding pocket full of hydrophobic residues. And the residues involved in the dimer and pocket formation are all well conserved in bacteria. Since the cupin fold is a large superfamily in the biosynthesis of natural products, our studies provide the structural basis for understanding and the directed evolution of NP proteins.

Early Diagnosis and Treatment of Purine Nucleoside Phosphorylase (PNP) Deficiency through TREC-Based Newborn Screening

Purine nucleoside phosphorylase (PNP) deficiency is a rare inherited disorder, resulting in severe combined immunodeficiency. To date, PNP deficiency has been detected in newborn screening only through the use of liquid chromatography tandem mass spectrometry. We report the first case in which PNP deficiency was detected by TREC analysis.

Single tryptophan Y160W mutant of homooligomeric E. coli purine nucleoside phosphorylase implies that dimers forming the hexamer are functionally not equivalent

E. coli purine nucleoside phosphorylase is a homohexamer, which structure, in the apo form, can be described as a trimer of dimers. Earlier studies suggested that ligand binding and kinetic properties are well described by two binding constants and two sets of kinetic constants. However, most of the crystal structures of this enzyme complexes with ligands do not hold the three-fold symmetry, but only two-fold symmetry, as one of the three dimers is different (both active sites in the open conformation) from the other two (one active site in the open and one in the closed conformation). Our recent detailed studies conducted over broad ligand concentration range suggest that protein–ligand complex formation in solution actually deviates from the two-binding-site model. To reveal the details of interactions present in the hexameric molecule we have engineered a single tryptophan Y160W mutant, responding with substantial intrinsic fluorescence change upon ligand binding. By observing various physical properties of the protein and its various complexes with substrate and substrate analogues we have shown that indeed three-binding-site model is necessary to properly describe binding of ligands by both the wild type enzyme and the Y160W mutant. Thus we have pointed out that a symmetrical dimer with both active sites in the open conformation is not forced to adopt this conformation by interactions in the crystal, but most probably the dimers forming the hexamer in solution are not equivalent as well. This, in turn, implies that an allosteric cooperation occurs not only within a dimer, but also among all three dimers forming a hexameric molecule.

Regioselective synthesis of triazolo[3,4-e]purine derivatives and their anti-cancer activity against NCI-60 cell-lines

Introduction: Due to the vast medicinal importance of purine nucleoside, a hybrid molecule of triazole with purine ring might explode a lead molecule in the pharma sector and based on the last decade’s studies suggested that the nitrogen-rich molecules possess a wide range of medicinal importance. Aim: Due to the vast application of purine nucleoside itself in the field of cancer research, we synthesized triazolo[3,4-e]purines and screened them for their anti-cancer study against NCI-60 cell lines by the protocol used by NIH. Materials and methods: The targeted molecules, 4-chloro-5a,6-dihydro-8-substitutedphenyl-1H-[1,2,4]triazolo[3,4-e]purine derivatives (4a-4h) were synthesized in a two-step procedure by nucleophilic substitution (SN) at C-2 chlorine followed by formation of the triazole ring by acid-catalyzed reaction in the polar protic solvent. Results: It was observed that the regioselective approach followed in C-2 chlorine replacement instead of C-6 chlorine during SN reaction. One-dose response of selected three molecules (4a, 4b, and 4c) showed that 4b (K-562: 64.47 µM & SR: 63.38 µM; mean GI50: 99.09 µM) was found to be more potent than 4a and 4c. Conclusions: We have described in this study the general synthetic method for triazolo[3,4-e]purines as an innovative class of potential anticancer agents. The dose-response curve in the sense of mean GI50 for three compounds across all 60 cell lines, 4b can be served as lead after necessary modification.

Investigating the Role of Guanosine on Human Neuroblastoma Cell Differentiation and the Underlying Molecular Mechanisms

Neuroblastoma arises from neural crest cell precursors failing to complete the process of differentiation. Thus, agents helping tumor cells to differentiate into normal cells can represent a valid therapeutic strategy. Here, we evaluated whether guanosine (GUO), a natural purine nucleoside, which is able to induce differentiation of many cell types, may cause the differentiation of human neuroblastoma SH-SY5Y cells and the molecular mechanisms involved. We found that GUO, added to the cell culture medium, promoted neuron-like cell differentiation in a time- and concentration-dependent manner. This effect was mainly due to an extracellular GUO action since nucleoside transporter inhibitors reduced but not abolished it. Importantly, GUO-mediated neuron-like cell differentiation was independent of adenosine receptor activation as it was not altered by the blockade of these receptors. Noteworthy, the neuritogenic activity of GUO was not affected by blocking the phosphoinositide 3-kinase pathway, while it was reduced by inhibitors of protein kinase C or soluble guanylate cyclase. Furthermore, the inhibitor of the enzyme heme oxygenase-1 but not that of nitric oxide synthase reduced GUO-induced neurite outgrowth. Interestingly, we found that GUO was largely metabolized into guanine by the purine nucleoside phosphorylase (PNP) enzyme released from cells. Taken together, our results suggest that GUO, promoting neuroblastoma cell differentiation, may represent a potential therapeutic agent; however, due to its spontaneous extracellular metabolism, the role played by the GUO-PNP-guanine system needs to be further investigated.