The uric acid/xanthine H+ symporter, UapA, is a high-affinity purine transporter from the filamentous fungus Aspergillus nidulans. Here we present the crystal structure of a genetically stabilized version of UapA (UapA-G411VΔ1–11) in complex with xanthine. UapA is formed from two domains, a core domain and a gate domain, similar to the previously solved uracil transporter UraA, which belongs to the same family. The structure shows UapA in an inward-facing conformation with xanthine bound to residues in the core domain. Unlike UraA, which was observed to be a monomer, UapA forms a dimer in the crystals with dimer interactions formed exclusively through the gate domain. Analysis of dominant negative mutants is consistent with dimerization playing a key role in transport. We postulate that UapA uses an elevator transport mechanism likely to be shared with other structurally homologous transporters including anion exchangers and prestin.

The papain-like protease (PL pro ) of SARS-CoV-2 is a validated antiviral drug target. Through a fluorescence resonance energy transfer-based high-throughput screening and subsequent lead optimization, we identified several PL pro inhibitors including Jun9-72-2 and Jun9-75-4 with improved enzymatic inhibition and antiviral activity compared to GRL0617 , which was reported as a SARS-CoV PL pro inhibitor. Significantly, we developed a cell-based FlipGFP assay that can be applied to predict the cellular antiviral activity of PL pro inhibitors in the BSL-2 setting. X-ray crystal structure of PL pro in complex with GRL0617 showed that binding of GRL0617 to SARS-CoV-2 induced a conformational change in the BL2 loop to a more closed conformation. Molecular dynamics simulations showed that Jun9-72-2 and Jun9-75-4 engaged in more extensive interactions than GRL0617 . Overall, the PL pro inhibitors identified in this study represent promising candidates for further development as SARS-CoV-2 antivirals, and the FlipGFP-PL pro assay is a suitable surrogate for screening PL pro inhibitors in the BSL-2 setting.

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Using the crystal structure of the uracil transporter UraA of Escherichia coli , we constructed a 3D model of the Aspergillus nidulans uric acid-xanthine/H + symporter UapA, which is a prototype member of the Nucleobase-Ascorbate Transporter (NAT) family. The model consists of 14 transmembrane segments (TMSs) divided into a core and a gate domain, the later being distinctly different from that of UraA. By implementing Molecular Mechanics (MM) simulations and quantitative structure-activity relationship (SAR) approaches, we propose a model for the xanthine-UapA complex where the substrate binding site is formed by the polar side chains of residues E356 (TMS8) and Q408 (TMS10) and the backbones of A407 (TMS10) and F155 (TMS3). In addition, our model shows several polar interactions between TMS1-TMS10, TMS1-TMS3, TMS8-TMS10, which seem critical for UapA transport activity. Using extensive docking calculations we identify a cytoplasm-facing substrate trajectory (D360, A363, G411, T416, R417, V463 and A469) connecting the proposed substrate binding site with the cytoplasm, as well as, a possible outward-facing gate leading towards the substrate major binding site. Most importantly, re-evaluation of the plethora of available and analysis of a number of herein constructed UapA mutations strongly supports the UapA structural model. Furthermore, modeling and docking approaches with mammalian NAT homologues provided a molecular rationale on how specificity in this family of carriers might be determined, and further support the importance of selectivity gates acting independently from the major central substrate binding site.

Fractionation of the neutral extract of Onobrychis ebenoides (Leguminosae) yielded a new isoflavone, named ebenosin (1), in addition to the known ones, afrormosin (2), formononetin (3) and daidzein (4). Although the relative binding affinities of 1 - 4 for estrogen receptor alpha (ERalpha) were nearly comparable and matched those of 1-3 for ERbeta, that of 4 for the latter receptor was significantly higher than any of the other. Compounds 1 - 4 induced cell proliferation and gene expression in breast and endometrial cancer cells in an ER-dependent manner. Nonetheless, the rank order of induction potencies ( 4 > 3 >or= 2 >or= 1) matched better that of affinities for ERbeta ( 4 > 3 >or= 2 >or= 1) rather than ERalpha ( 4 >or= 3 >or= 2 >or= 1). While the antiestrogen ICI 182,780 could inhibit the induction of proliferation of ER-positive breast cancer cells by 1-4, it could not prevent 1 from exhibiting significant ER-independent cytotoxicity at 10 microM. By contrast, 1 was much less cytotoxic and only weakly estrogenic for ER-positive endometrial adenocarcinoma cells. In conclusion, our data suggest that the C-8 isoprenyl substituent of 1 renders it cytotoxic and/or estrogenic in a cell-dependent manner.

An intense effort is made by pharmaceutical and academic research laboratories to identify and develop selective antagonists for each adenosine receptor (AR) subtype as potential clinical candidates for "soft" treatment of various diseases. Crystal structures of subtypes A and AARs offer exciting opportunities for structure-based drug design. In the first part of the present work, Maybridge HitFinder library of 14400 compounds was utilized to apply a combination of structure-based against the crystal structure of AAR and ligand-based methodologies. The docking poses were rescored by CHARMM energy minimization and calculation of the desolvation energy using Poisson-Boltzmann equation electrostatics. Out of the eight selected and tested compounds, five were found positive hits (63% success). Although the project was initially focused on targeting AAR, the identified antagonists exhibited low micromolar or micromolar affinity against A/A, ARs, or AAR, respectively. Based on these results, 19 compounds characterized by novel chemotypes were purchased and tested. Sixteen of them were identified as AR antagonists with affinity toward combinations of the AR family isoforms (A/A, A/A, A/A/A, and A). The second part of this work involves the performance of hundreds of molecular dynamics (MD) simulations of complexes between the ARs and a total of 27 ligands to resolve the binding interactions of the active compounds, which were not achieved by docking calculations alone. This computational work allowed the prediction of stable and unstable complexes which agree with the experimental results of potent and inactive compounds, respectively. Of particular interest is that the 2-amino-thiophene-3-carboxamides, 3-acylamino-5-aryl-thiophene-2-carboxamides, and carbonyloxycarboximidamide derivatives were found to be selective and possess a micromolar to low micromolar affinity for the A receptor.

In therapeutic interventions associated with melanin hyperpigmentation, tyrosinase is regarded as a target enzyme as it catalyzes the rate-limiting steps in mammalian melanogenesis. Since many known agents have been proven to be toxic, there has been increasing impetus to identify alternative tyrosinase inhibitors, especially from natural sources. In this study, we investigated 900 extracts from Greek plants for potential tyrosinase inhibitive properties. Among the five most potent extracts, the methanol extract of Morus alba wood (MAM) demonstrated a significant reduction in intracellular tyrosinase and melanin content in B16F10 melanoma cells. Bioassay-guided isolation led to the acquisition of twelve compounds: oxyresveratrol (1), kuwanon C (2), mulberroside A (3), resorcinol (4), dihydrooxyresveratol (5), trans-dihydromorin (6), 2,4,3′-trihydroxydihydrostilbene (7), kuwanon H (8), 2,4-dihydroxybenzaldehyde (9), morusin (10), moracin M (11) and kuwanon G (12). Among these, 2,4,3′-trihydroxydihydrostilbene (7) is isolated for the first time from Morus alba and constitutes a novel potent tyrosinase inhibitor (IC50 0.8 ± 0.15). We report here for the first time dihydrooxyresveratrol (5) as a potent natural tyrosinase inhibitor (IC50 0.3 ± 0.05). Computational docking analysis indicated the binding modes of six tyrosinase inhibitors with the aminoacids of the active centre of tyrosinase. Finally, we found both MAM extract and compounds 1, 6 and 7 to significantly suppress in vivo melanogenesis during zebrafish embryogenesis.

SummaryThe xanthine permease XanQ of Escherichia coli is a paradigm for transporters of the evolutionarily broad family nucleobase-cation symporter-2 (NCS2) that transport key metabolites or anti-metabolite analogs. Most functionally known members are xanthine/uric acid transporters related to XanQ and belong to a distinct phylogenetic cluster of the family. Here, we present a comprehensive mutagenesis of XanQ based on the identification and Cys-scanning analysis of conserved sequence motifs in this cluster. Results are interpreted in relation to homology modeling on the structurally known template of UraA and previous data on critical binding-site residues in transmembrane segments (TMs) 3, 8 and 10. The current analysis, of motifs distant to the binding site, revealed a set of functionally important residues in TMs 2, 5, 12 and 13, including seven irreplaceable ones, of which six are Gly residues in the gate domain (159, 369, 370, 383, 409) and in TM2 (Gly-71), and one is polar (Gln-75). Gln-75 (TM2) is probably crucial in a network of hydrogen-bonding interactions in the middle of the core domain involving another essential residue, Asp-304 (TM9). Although the two residues are irreplaceable individually, combinatorial replacement of Gln-75 with Asn and of Asp-304 with Glu rescues significant transport activity.