Homology modeling and substrate binding study of Nudix hydrolase Ndx1 from Thermos thermophilus HB8

Insight into the Binding and Hydrolytic Preferences of hNudt16 Based on Nucleotide Diphosphate Substrates

Nudt16 is a member of the NUDIX family of hydrolases that show specificity towards substrates consisting of a nucleoside diphosphate linked to another moiety X. Several substrates for hNudt16 and various possible biological functions have been reported. However, some of these reports contradict each other and studies comparing the substrate specificity of the hNudt16 protein are limited. Therefore, we quantitatively compared the affinity of hNudt16 towards a set of previously published substrates, as well as identified novel potential substrates. Here, we show that hNudt16 has the highest affinity towards IDP and GppG, with K_d below 100 nM. Other tested ligands exhibited a weaker affinity of several orders of magnitude. Among the investigated compounds, only IDP, GppG, m⁷GppG, AppA, dpCoA, and NADH were hydrolyzed by hNudt16 with a strong substrate preference for inosine or guanosine containing compounds. A new identified substrate for hNudt16, GppG, which binds the enzyme with an affinity comparable to that of IDP, suggests another potential regulatory role of this protein. Molecular docking of hNudt16-ligand binding inside the hNudt16 pocket revealed two binding modes for representative substrates. Nucleobase stabilization by Π stacking interactions with His24 has been associated with strong binding of hNudt16 substrates.

A preliminary study on the effects of lanthanum (III) on plant vitronectin-like protein and its toxicological basis

Vitronectin-like protein (VN) is widely found outside plant plasma membranes. The VN molecular surface contains a large number of active groups that combine strongly with rare earth elements (REEs), which means that VN is a preferential binding target for REEs exhibiting their toxic effects, but the toxicological mechanism remains unknown. This study used transmission electron microscopy, circular dichroism, fluorescence spectrometry, ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, and calculational chemistry (homology modeling, molecular dynamics simulation and quantum chemical calculation) to preliminarily investigate the effect of lanthanum [La(III)] as an REE, on the structure of VN and its toxicological mechanism. The results showed that low-concentration La(III) could cause micro-interference to the VN molecular structure through weak interactions, such as electrostatic attraction. High-concentration La(III) formed stable complexes with VN, which changed the average binding energy and electron cloud density of VN, loosened the molecular structure and increased the disorder of VN molecule. The results of building a 3D model of VN and simulating the interaction between La(III) and VN using calculational chemistry showed that La(H₂O)₇³⁺ in solution could coordinately bind to the carboxyl-/carbonyl-O groups in the negatively charged areas on the VN molecular surface. Furthermore, one or more strong H-bonds were formed to enhance the stability of the La(H₂O)₇³⁺-VN complexes. In summary, low La(III) concentrations could cause micro-interference to the VN molecular structure, whereas high La(III) concentrations could coordinately bind to VN to form stable La-VN complexes, which destroyed the molecular structure of VN; thus the toxicological basis by which La(III) exhibits its toxic effects is its binding to VN.

Probing the structure of Mycobacterium tuberculosis MbtA: model validation using molecular dynamics simulations and docking studies

Multidrug resistance capacity of Mycobacterium tuberculosis demands urgent need for developing new antitubercular drugs. The present work is on M. tuberculosis-MbtA, an enzyme involved in the biosynthesis of siderophores, having a critical role in bacterial growth and virulence. The molecular models of both holo and apo forms of M. tuberculosis-MbtA have been constructed and validated. A docking study with a series of 42 5'-O-[N-(salicyl) sulfamoyl] adenosine derivatives, using GOLD software, revealed significant correlation (R(2) = 0.8611) between Goldscore and the reported binding affinity data. Further, binding energies of the docked poses were calculated and compared with the observed binding affinities (R(2) = 0.901). All-atom molecular dynamics simulation was performed for apo form, holo form without ligand and holo form with ligands. The holo form without ligand on molecular dynamics simulation for 20 ns converged to the apo form and the apo form upon induced fit docking of the natural substrate, 2,3-dihydroxybenzoic acid-adenylate, yielded the holo structure. The molecular dynamics simulation of the holo form with ligands across the time period of 20 ns provided with the insights into ligand-receptor interactions for inhibition of the enzyme. A thorough study involving interaction energy calculation between the ligands and the active site residues of MbtA model identified the key residues implicated in ligand binding. The holo model was capable to differentiate active compounds from decoys. In the absence of experimental structure of MbtA, the homology models together with the insights gained from this study will promote the rational design of potent and selective MbtA inhibitors as antitubercular therapeutics. An animated interactive 3D complement (I3DC) is available in Proteopedia at http://proteopedia.org/w/Journal:JBSD:33.

HdrC2 from Acidithiobacillus ferrooxidans owns two iron-sulfur binding motifs but binds only one variable cluster between [4Fe-4S] and [3Fe-4S]

The heterodisulfide reductase complex HdrABC from Acidithiobacillus ferrooxidans was suggested to own novel features that act in reverse to convert the sulfane sulfur of GS( n )H species (n > 1) into sulfite in sulfur oxidation. The HdrC subunit is potentially encoded by two different highly upregulated genes sharing only 29 % identity in A. ferrooxidans grown in sulfur-containing medium, which were named as HdrC1 and HdrC2, respectively and had been confirmed to contain iron-sulfur cluster by expression and characterization, especially the HdrC1 which had been showed to bind only one [4Fe-4S] cluster by mutations. However, the mutations of the HdrC2 remain to be done and the detailed binding information of it is still unclear. Here, we report the expression, mutations, and molecular modeling of the HdrC2 from A. ferrooxidans. This HdrC2 had two identical motifs (Cx(2)Cx(2)Cx(3)C) containing total of eight cysteine residues potentially for iron-sulfur cluster binding. This purified HdrC2 was exhibited to contain one variable cluster converted between [4Fe-4S] and [3Fe-4S] according to different conditions by the UV-scanning and EPR spectra. The site-directed mutagenesis results of these eight residues further confirmed that the HdrC2 in reduction with Fe(2+) condition loaded only one [4Fe-4S](+) with spin S = 1/2 ligated by the residues of Cys73, Cys109, Cys112, and Cys115; the HdrC2 in natural aeration condition lost the Fe atom ligated by the residue of Cys73 and loaded only one [3Fe-4S](0) with spin S = 0; the HdrC2 in oxidation condition loaded only one [3Fe-4S](+) with spin S = 1/2. Molecular modeling results were also in line with the experiment results.

Expression, purification and molecular modeling of another HdrC from Acidithiobacillus ferrooxidans which binds only one [4Fe-4S] cluster

The heterodisulfide reductase complex HdrABC from Acidithiobacillus ferrooxidans was predicted to have novel features that work in reverse to catalyse the sulfane sulfur of GSnH species (n > 1) into sulfite in sulfur oxidation. There are two different highly upregulated genes potentially encoding the HdrC subunit in A. ferrooxidans grown in sulfur-containing medium. An HdrC containing iron-sulfur cluster from A. ferrooxidans corresponding to one of the genes had been expressed and biophysically characterized. Comparatively, here we report the cloning, expression, and characterization of another HdrC from A. ferrooxidans. This HdrC was expressed in inclusion bodies in all conditions tested. This purified HdrC displayed brown color and contained the [4Fe-4S] cluster confirmed by the UV-scanning and EPR spectra. This HdrC owned two identical motifs (Cx(2)Cx(2)Cx(3)C) including total of eight cysteine residues for [4Fe-4S] cluster binding. To our surprise, the site-directed mutagenesis results of these eight residues revealed that respective removal of the sulfhydryl group of Cys73, Cys76, Cys79, and Cys37 resulted in the cluster loss, but those of Cys27, Cys30, Cys33, and Cys83 had no influence, which demonstrated that this HdrC bound only one cluster, and it might be responsible for causing the HdrABC in A. ferrooxidans working in reverse. Molecular modeling results also supported the above results and showed that this cluster was ligated by Cys73, Cys76, and Cys79 in one motif and Cys37, however, in another motif.

Modulation of redox homeostasis under suboptimal conditions by Arabidopsis nudix hydrolase 7

BACKGROUND

Nudix hydrolases play a key role in maintaining cellular homeostasis by hydrolyzing various nuceloside diphosphate derivatives and capped mRNAs. Several independent studies have demonstrated that Arabidopsis nudix hydrolase 7 (AtNUDT7) hydrolyzes NADH and ADP-ribose. Loss of function Atnudt7-1 mutant plants (SALK_046441) exhibit stunted growth, higher levels of reactive oxygen species, enhanced resistance to pathogens. However, using the same T-DNA line, two other groups reported that mutant plants do not exhibit any visible phenotypes. In this study we analyze plausible factors that account for differences in the observed phenotypes in Atnudt7. Secondly, we evaluate the biochemical and molecular consequences of increased NADH levels due to loss of function of AtNUDT7 in Arabidopsis.

RESULTS

We identified a novel conditional phenotype of Atnudt7-1 knockout plants that was contingent upon nutrient composition of potting mix. In nutrient-rich Metro-Mix, there were no phenotypic differences between mutant and wild-type (WT) plants. In the nutrient-poor mix (12 parts vermiculite: 3 parts Redi-earth and 1 part sand), mutant plants showed the characteristic stunted phenotype. Compared with WT plants, levels of glutathione, NAD+, NADH, and in turn NADH:NAD+ ratio were higher in Atnudt7-1 plants growing in 12:3:1 potting mix. Infiltrating NADH and ADP-ribose into WT leaves was sufficient to induce AtNUDT7 protein. Constitutive over-expression of AtNudt7 did not alter NADH levels or resistance to pathogens. Transcriptome analysis identified nearly 700 genes differentially expressed in the Atnudt7-1 mutant compared to WT plants grown in 12:3:1 potting mix. In the Atnudt7-1 mutant, genes associated with defense response, proteolytic activities, and systemic acquired resistance were upregulated, while gene ontologies for transcription and phytohormone signaling were downregulated.

CONCLUSIONS

Based on these observations, we conclude that the differences observed in growth phenotypes of the Atnudt7-1 knockout mutants can be due to differences in the nutrient composition of potting mix. Our data suggests AtNUDT7 plays an important role in maintaining redox homeostasis, particularly for maintaining NADH:NAD+ balance for normal growth and development. During stress conditions, rapid induction of AtNUDT7 is important for regulating the activation of stress/defense signaling and cell death pathways.

Probing the structure of Leishmania donovani chagasi DHFR-TS: comparative protein modeling and protein-ligand interaction studies

Dihydrofolate reductase (DHFR) has been used successfully as a drug target in the area of anti-bacterial, anti-cancer and anti-malarial therapy. It also acts as a drug target for Leishmaniasis. Inhibition of DHFR leads to cell death through lack of thymine (nucleotide metabolism). Although the crystal structures of Leishmania major and Trypanosoma cruzi DHFR-thymidylate synthase (TS) have been resolved, to date there is no three-dimensional (3D)-structural information on DHFR-TS of Leishmania donovani chagasi, which causes visceral leishmaniasis. Our aim in this study was to model the 3D structure of L. donovani chagasi DHFR-TS, and to investigate the structural requirements for its inhibition. In this paper we describe a highly refined homology model of L. donovani chagasi DHFR-TS based on available crystallographic structures by using the Homology module of Insight II. Structural refinement and minimization of the generated L. donovani chagasi DHFR-TS model employed the Discover 3 module of Insight II and molecular dynamic simulations. The model was further validated through use of the PROCHECK, Verify_3D, PROSA, PSQS and ERRAT programs, which confirm that the model is reliable. Superimposition of the model structure with the templates L. major A chain, L. major B chain And T. cruzi A chain showed root mean square deviations of 0.69 A, 0.71 A and 1.11 A, respectively. Docking analysis of the L. donovani chagasi DHFR-TS model with methotrexate enabled us to identify specific residues, viz. Val156, Val30, Lys95, Lys75 and Arg97, within the L. donovani chagasi DHFR-TS binding pocket, that play an important role in ligand or substrate binding. Docking studies clearly indicated that these five residues are important determinants for binding as they have strong hydrogen bonding interactions with the ligand.

Homology modelling and bivalent single-chain Fv construction of anti-HepG2 single-chain immunoglobulin Fv fragments from a phage display library

We prepared single-chain immunoglobulin Fv fragments (scFv) SLH10 specific for the HepG2 cell line after biopanning from a large human-naive phage display library (Griffin. 1 Library). The three-dimensional (3D) structure of SLH10 was modelled by the Insight II molecule simulation software.The structure was refined using the molecular dynamics method.The structures with the least steric clashes and lowest energy were determined finally. The optimized structures of heavy (VH) and light (VL) variable chains of SLH10 scFv were obtained.Then SLH10 bivalent single-chain Fv (BsFv) was constructed that would be suitable for high-affinity targeting.SLH10 BsFv was generated by linking scFvs together and identified by sequencing. Its expression products were confirmed by western blot analysis.The relative molecular masses of scFv and BsFv were approximately 30 kDa and 60 kDa,respectively. Flow cytometry revealed that SLH10 BsFv bound the selected cell lines with greater signal intensity than the parental scFv. The improved antigen binding of SLH10 BsFv may be useful for immunodiagnostics or targeted gene therapy for liver cancer.

Analysis of a three-dimensional structure of human acidic mammalian chitinase obtained by homology modeling and ligand binding studies

The three-dimensional (3D) model of the human acidic mammalian chitinase (hAMCase) was constructed based on the crystal structure of the human chitotriosidase (EC 3.2.1.44, PDB code 1HKK) by using InsightII/Homology module. With the aid of molecular mechanics and molecular dynamics methods, the last refined model was obtained and further assessed by Profile-3D and Procheck, which confirms that the refined model is reliable. Furthermore, the docking results of the ligands (allosamidin and NAG(2)) into the active site of hAMCase indicate that allosamidin is a more preferred ligand than NAG(2), and that Glu119 forms hydrogen bond with allosamidin, which is in good agreement with the experimental results. From the docking studies, we also suggest that Trp10, Glu49, Asp192, and Glu276 in hAMCase are four important determinant residues in binding as they have strong van-der-Waals and electrostatic interactions with the ligand, respectively.

Homology modeling and molecular dynamics study on N-acetylneuraminate lyase

With homology modeling techniques, molecular mechanics and molecular dynamics methods, a 3D structure model of N-acetylneuraminate lyase from human (hNAL, EC 4.1.3.3) was created and refined. This model was further assessed by Profile-3D and PROCHECK, which confirms that the refined model is reliable. Furthermore, the docking results of the substrates (sialic acid and KDO) into the active site of hNAL indicate that hNAL can cleave the sialic acid and KDO. Thr51 and Tyr143 may be the key amino acids residues as they have strong hydrogen bonding interactions with the substrates, which is in good agreement with the experimental results by Izard et al. (Structure 2:361-369. doi:10.1016/S0969-2126(00)00038-1 (1994)). From the docking studies, we also suggest that Asp176 and Ser218 only form hydrogen bonds with sialic acid, therefore, they may help sialic acid interact with hNAL steadly.