Studying temporal variability of GRS1739-278 during the 2014 outburst

We report a discovery of low-frequency quasi periodic oscillation at 0.3-0.7 Hz in the power spectra of the accreting black hole GRS1739-278 in the hard-intermediate state during its 2014 outburst based on the ${\it NuSTAR}$ and Swift/XRT data. The QPO frequency strongly evolved with the source flux during the NuSTAR observation. The source spectrum became softer with rising QPO frequency and simultaneous increasing of the power-law index and decreasing of the cut-off energy. In the power spectrum, a prominent harmonic is clearly seen together with the main QPO peak. The fluxes in the soft and the hard X-ray bands are coherent, however, the coherence drops for the energy bands separated by larger gaps. The phase-lags are generally positive (hard) in the 0.1-3 Hz frequency range, and negative below 0.1 Hz. The accretion disc inner radius estimated with the relativistic reflection spectral model appears to be $R_{\rm in} < 7.3 R_{\rm g}$. In the framework of the relativistic precession model, in order to satisfy the constraints from the observed QPO frequency and the accretion disc truncation radius, a massive black hole with $M_{\rm BH} \approx 100$M$_\odot$ is required.Comment: 15 pages, 12 figures; accepted for publication in MNRA

Substrate-Induced Allosteric Change in the Quaternary Structure of the Spermidine N-Acetyltransferase SpeG

AbstractThe spermidine N-acetyltransferase SpeG is a dodecameric enzyme that catalyzes the transfer of an acetyl group from acetyl coenzyme A to polyamines such as spermidine and spermine. SpeG has an allosteric polyamine-binding site and acetylating polyamines regulate their intracellular concentrations. The structures of SpeG from Vibrio cholerae in complexes with polyamines and cofactor have been characterized earlier. Here, we present the dodecameric structure of SpeG from V. cholerae in a ligand-free form in three different conformational states: open, intermediate and closed. All structures were crystallized in C2 space group symmetry and contain six monomers in the asymmetric unit cell. Two hexamers related by crystallographic 2-fold symmetry form the SpeG dodecamer. The open and intermediate states have a unique open dodecameric ring. This SpeG dodecamer is asymmetric except for the one 2-fold axis and is unlike any known dodecameric structure. Using a fluorescence thermal shift assay, size-exclusion chromatography with multi-angle light scattering, small-angle X-ray scattering analysis, negative-stain electron microscopy and structural analysis, we demonstrate that this unique open dodecameric state exists in solution. Our combined results indicate that polyamines trigger conformational changes and induce the symmetric closed dodecameric state of the protein when they bind to their allosteric sites

Structural characterization of a hypothetical protein: a potential agent involved in trimethylamine metabolism in Catenulispora acidiphila

Catenulispora acidiphila is a newly identified lineage of actinomycetes that produces antimicrobial activities and represents a promising source of novel antibiotics and secondary metabolites. Among the discovered protein coding genes, 68 % were assigned a putative function, while the remaining 32 % are genes encoding “hypothetical” proteins. Caci_0382 is one of the “hypothetical” proteins that has very few homologs. Sequence analysis shows that the protein belongs to the NTF2-like protein family. The structure of Caci_0382 demonstrates that it shares the same fold and has a similar active site as limonene-1,2-epoxide hydrolase, which suggests that it may have a related function. Using a fluorescence thermal shift assay, we identified stabilizing compounds that suggest potential natural ligands of Caci_0382. Using this information, we determined the crystal structure in complex with trimethylamine to provide a better understanding of the function of this uncharacterized protein. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s10969-014-9176-z) contains supplementary material, which is available to authorized users

Large-scale Structural Rearrangement of a Serine Hydrolase from Francisella Tularensis Facilitates Catalysis

Tularemia is a deadly, febrile disease caused by infection by the gram-negative bacterium, Francisella tularensis. Members of the ubiquitous serine hydrolase protein family are among current targets to treat diverse bacterial infections. Herein we present a structural and functional study of a novel bacterial carboxylesterase (FTT258) from F. tularensis, a homologue of human acyl protein thioesterase (hAPT1). The structure of FTT258 has been determined in multiple forms, and unexpectedly large conformational changes of a peripheral flexible loop occur in the presence of a mechanistic cyclobutanone ligand. The concomitant changes in this hydrophobic loop and the newly exposed hydrophobic substrate binding pocket suggest that the observed structural changes are essential to the biological function and catalytic activity of FTT258. Using diverse substrate libraries, site-directed mutagenesis, and liposome binding assays, we determined the importance of these structural changes to the catalytic activity and membrane binding activity of FTT258. Residues within the newly exposed hydrophobic binding pocket and within the peripheral flexible loop proved essential to the hydrolytic activity of FTT258, indicating that structural rearrangement is required for catalytic activity. Both FTT258 and hAPT1 also showed significant association with liposomes designed to mimic bacterial or human membranes, respectively, even though similar structural rearrangements for hAPT1 have not been reported. The necessity for acyl protein thioesterases to have maximal catalytic activity near the membrane surface suggests that these conformational changes in the protein may dually regulate catalytic activity and membrane association in bacterial and human homologues

Research on the antiviral activity of water-soluble melanin from the pharmaceutical chaga mushroom (Inonotus obliquus) against influenza А virus subtypes H5N1, H3N2 and H1N1pdm09 in experiments in vitro

Introduction. Influenza A virus is the cause of epidemics and pandemics that severely affect the health and socioeconomic status of the world's population. The need to develop new methods of etiotropic therapy, and the increasing ability of viruses to counteract the antiviral drugs makes extremely relevant the search for new pharmacologically active substances and the subsequent study of their medicinal properties. The aim of the study is to conduct research into the antiviral properties of melanin obtained from the pharmaceutical chaga mushroom in relation to different subtypes of the influenza A virus. Materials and methods. A sample of water-soluble melanin from Inonotus obliquus obtained by alkaline hydrolysis and dried at 40C was tested for toxicity and antiviral activity. The commercial anti-influenza drug Tamiflu was used as a reference drug. Statistical processing of the obtained data was carried out according to the Spearman-Kerber method. Results. Inonotus obliquus melanin (sample 20-24) toxicity markers, such as a maximum tolerable concentration (MTC) of 237.0 g/mL, and a 50% cytotoxic concentration (CC50) of 153,45 g/mL were established for MDCK cell culture. The assessment of antiviral activity of test sample against three subtypes of the influenza A virus (H5N1, H3N2 and H1N1pdm09) demonstrated a decrease in the infectivity of the influenza virus by 2.53.5 lg with 50% virus-inhibiting concentrations (IC50) of 1.559.52 g/mL. Based on the obtained values of CC50 and IC50, the selectivity indices (SI) of the sample were calculated, characterizing its prospects for further research. Conclusions. Melanin obtained from the pharmaceutical chaga mushroom showed the highest activity against the strain of the human pandemic influenza virus A/California/04/2009 (H1N1)pdm09, caused a decrease in its infectivity by 3.5 lg and had an IC50 of 1.6 g/ml. The obtained results indicate the prospects for creating an antiviral drug based on Inonotus obliquus melanins against the influenza virus

Studying temporal variability of GRS1739-278 during the 2014 outburst

We report a discovery of low-frequency quasi-periodic oscillation at 0.3–0.7 Hz in the power spectra of the accreting black hole GRS 1739–278 in the hard-intermediate state during its 2014 outburst based on the NuSTAR and Swift/XRT data. The QPO frequency strongly evolved with the source flux during the NuSTAR observation. The source spectrum became softer with rising QPO frequency and simultaneous increasing of the power-law index and decreasing of the cut-off energy. In the power spectrum, a prominent harmonic is clearly seen together with the main QPO peak. The fluxes in the soft and the hard X-ray bands are coherent, however, the coherence drops for the energy bands separated by larger gaps. The phase lags are generally positive (hard) in the 0.1–3 Hz frequency range, and negative below 0.1 Hz. The accretion disc inner radius estimated with the relativistic reflection spectral model appears to be R_(in) < 7.3R_g. In the framework of the relativistic precession model, in order to satisfy the constraints from the observed QPO frequency and the accretion disc truncation radius, a massive black hole with M_(BH) ≈ 100 M⊙ is required