The three-dimensional structure of an avian class-mu glutathione S-transferase, cGSTM1-1 at 1.94 A resolution

The diverse functions of Mu-class Glutathione S-transferase HrGSTm1 during the development of Hyalomma rufipes with a focus on the detoxification metabolism of cyhalothrin

BACKGROUND

Glutathione S-transferases (GSTs) are a superfamily of multifunctional enzymes in living organisms with metabolic and detoxification functions, which can detoxify exogenous and endogenous compounds and thereby reduce the damage caused by toxic substances to the body. Ticks are obligate blood-sucking ectoparasites that can transmit various pathogens, and the characterization of tick-derived GSTs may help improve current understanding of the molecular mechanism of tick resistance to insecticides. In this study, a novel GST gene, named HrGSTm1, was identified from Hyalomma rufipes.

METHODS

Sequence analysis was performed by using bioinformatics techniques. A prokaryotic expression system was used to obtain the recombinant expression protein rHrGSTm1. Detection of spatiotemporal expression patterns of target genes and their response to the toxicity of cyhalothrin on female H. rufipes was performed by using a quantitative PCR platform. The optimal enzymological parameters of rHrGSTm1 using glutathione as substrate were calculated. The antioxidant capacity of the recombinant protein was evaluated by DPPH• (1,1-Diphenyl-2-picrylhydrazyl radical 2,2-Diphenyl-1-(2,4,6-trinitrophenyl) hydrazyl). Knockdown of the HrGSTm1 genes through RNA interference was used to analyze their effects on the physiological parameters of ticks. The changes in HrGSTm1 messenger RNA expression patterns under cypermethrin stress were analyzed.

RESULTS

The complementary DNA sequence of HrGSTm1 contained a 672-bp open reading frame, which potentially encoded 223 amino acids. The predicted molecular weight was 25.62 kDa, and the isoelectric point 8.22. HrGSTm1 is a Mu-class GST, belonging to the cytoplasmic GSTs with no signal peptide observed. The Vmax and Km of rHrGSTm1 were 3.367 ± 0.81 uM and 2.208 ± 0.76 uM, respectively, and its activities were dependent on different temperatures and pH conditions; the scavenging rate of rHrGSTm1 to DPPH• reached 76.4% at 1.25 mg/ml. Variable expressions of HrGSTm1 were observed under various treatment periods and in different tissues, with the highest appearing in eggs (analysis of variance [ANOVA], F(2, 9) = 279.9, P < 0.0001) and Malpighian tubules (ANOVA, F(3, 12) = 290.5, P < 0.0001). After knockdown of HrGSTm1, compared with the control group, the mortality in the treatment group was increased by 16.7%, the average oviposition rate decreased by 33.9%, the average engorged body weight decreased by 287.38 mg and egg weight decreased by 127.46 mg, although only the engorged body weight was significantly different (t-test, t(44) = 2.886, P = 0.006). After exposure to three sublethal concentrations (LC05, LC10, LC50) of cyhalothrin, the expression level of HrGSTm1 in the midgut, ovary and salivary gland was upregulated, whereas in Malpighian tubules, it showed a trend of upregulation at first and then downregulation, implying different functions during the detoxification in different tissues.

CONCLUSIONS

In this study, a novel GST of the Mu-class was successfully isolated from H. rufipes and systematically subjected to bioinformatic analysis and recombination identification. The variation trend of HrGSTm1 expression level in different tissues suggests that the gene has different detoxification functions in different tissues. The potential function of this gene was analyzed to provide basic research for further investigation of its detoxification mechanism.

Elucidating structure and dynamics of glutathione S-transferase from Rhipicephalus (Boophilus) microplus

Rhipicephalus (Boophilus) microplus is tick parasite that affects the cattle industry worldwide. In R. (B.) microplus, acaricide resistance develops rapidly against many commercial acaricides. One of main resistance strategies is to enhance the metabolic detoxification mediated by R. (B.) microplus glutathione-S-transferase (RmGST). RmGST detoxifies acaricides by catalyzing the conjugation of glutathione to acaricides. Although structural and dynamic details of RmGST are expected to elucidate the biologic activity of this molecule, these data have not been available to date. Thus, Molecular Dynamics simulations were employed to study ligand-free RmGST at an atomic level. Like other m-class GSTs, the flexible m loop (m1) of RmGST was observed. M1 seems to shield the active sites from the bulk. A RmGST dimer is stabilized by the lock-and-key motif (F57 as "key") and hydrogen bonds of R82-E91 and R82-D98 at the dimer interface. Without substrates, conserved catalytic Y116 and N209 can interact with V112, G210 (for Y116) and F215 (for N209). Overall, most residues involving in RmGST function and stability are similar to other m-class GSTs. This implies similar structural stability and catalytic activity of RmGST to other GSTs. An insight obtained here will be useful for management of acaricide resistance and tick control.Communicated by Ramaswamy H. Sarma.

Observing How Glutathione and S-Hexyl Glutathione Bind to Glutathione S-Transferase from Rhipicephalus (Boophilus) microplus

Rhipicephalus (Boophilus) microplus is one of the most widespread ticks causing a massive loss to livestock production. The long-term use of acaracides rapidly develops acaracide resistance. In R. microplus, enhancing the metabolic activity of glutathione S-transferase (RmGST) is one of the mechanisms underlying acaracide resistance. RmGST catalyzes the conjugation of glutathione (GSH) to insecticides causing an easy-to-excrete conjugate. The active RmGST dimer contains two active sites (hydrophobic co-substrate binding site (H-site) and GSH binding site (G-site)) in each monomer. To preserve the insecticide efficacy, s-hexyl glutathione (GTX), a GST inhibitor, has been used as a synergist. To date, no molecular information on the RmGST-GSH/GTX complex is available. The insight is important for developing a novel RmGST inhibitor. Therefore, in this work, molecular dynamics simulations (MD) were performed to explore the binding of GTX and GSH to RmGST. GSH binds tighter and sits rigidly inside the G-site, while flexible GTX occupies both active sites. In GSH, the backbone mainly interacts with W8, R43, W46, K50, N59, L60, Q72, and S73, while its thiol group directs to Y7. In contrast, the aliphatic hexyl of GTX protrudes into the H-site and allows a flexible peptide core to form various interactions. Such high GTX flexibility and the protrusion of its hexyl moiety to the H-site suggest the dual role of GTX in preventing the conjugation reaction and the binding of acaracide. This insight can provide a better understanding of an important insecticide-resistance mechanism, which may in turn facilitate the development of novel approaches to tick control.

Structural and Functional Characterization of Camelus dromedarius Glutathione Transferase M1-1

Glutathione transferases (GSTs; EC. 2.5.1.18) are a large family of multifunctional enzymes that play crucial roles in the metabolism and inactivation of a broad range of xenobiotic compounds. In the present work, we report the kinetic and structural characterization of the isoenzyme GSTM1-1 from Camelus dromedarius (CdGSTM1-1). The CdGSΤM1-1 was expressed in E. coli BL21 (DE3) and was purified by affinity chromatography. Kinetics analysis showed that the enzyme displays a relative narrow substrate specificity and restricted ability to bind xenobiotic compounds. The crystal structures of CdGSΤM1-1 were determined by X-ray crystallography in complex with the substrate (GSH) or the reaction product (S-p-nitrobenzyl-GSH), providing snapshots of the induced-fit catalytic mechanism. The thermodynamic stability of CdGSTM1-1 was investigated using differential scanning fluorimetry (DSF) in the absence and in presence of GSH and S-p-nitrobenzyl-GSH and revealed that the enzyme’s structure is significantly stabilized by its ligands. The results of the present study advance the understanding of camelid GST detoxification mechanisms and their contribution to abiotic stress adaptation in harsh desert conditions.

Crystal structure of a class-mu glutathione S-transferase from whiteleg shrimp Litopenaeus vannamei: structural changes in the xenobiotic binding H-site may alter the spectra of molecules bound

Glutathione S-transferases (GSTs) are dimeric proteins that play a key role in phase II cellular detoxification. Here, the first crystal structure of a GST class-mu from marine crustacean shrimp Litopenaeus vannamei is reported at a resolution of 2.0 Å. The coordinates reported here have the lowest sequence identity with previously reported GSTs class-mu deposited at the Protein Data Bank (PDB), although they have subtle conformational differences. One key feature of GST class-mu from L. vannamei is the active site crevice markedly reduced when it is compared with other GSTs class-mu. This finding together with the chemical change of residues into the cavity (F112 and Y210) points to a particular specialization in which smallest xenobiotics with nonstandard chemical characteristics can be bound to the H-site. This suggests that marine organisms have evolved structural strategies to provide efficient selectivity toward xenobiotics to be disposed of by the phase II detoxification process.

A novel biomarker for marine environmental pollution of pi-class glutathione S-transferase from Mytilus coruscus

Glutathione S-transferases (GSTs) are the superfamily of phase II detoxification enzymes that play crucial roles in innate immunity. In this study, a pi-class GST homolog was identified from Mytilus coruscus (named as McGST1, KC525103). The full-length cDNA sequence of McGST1 was 621bp with a 5' untranslated region (UTR) of 70bp and a 3'-UTR of 201bp. The deduced amino acid sequence was 206 residues in length with theoretical pI/MW of 5.60/23.72kDa, containing the conserved G-site and diversiform H-site. BLASTn analysis and phylogenetic relationship strongly suggested that this cDNA sequence was a member of pi class GST family. The prediction of secondary structure displayed a preserved N-terminal and a C-terminal comprised with α-helixes. Quantitative real time RT-PCR showed that constitutive expression of McGST1 was occurred, with increasing order in mantle, muscle, gill, hemocyte, gonad and hepatopancreas. The stimulation of bacterial infection, heavy metals and 180CST could up-regulate McGST1 mRNA expression in hepatopancreas with time-dependent manners. The maximum expression appeared at 6h after pathogenic bacteria injected, with 10-fold in Vibrio alginolyticus and 16-fold in Vibrio harveyi higher than that of the control. The highest point of McGST1 mRNA appeared at different time for exposure to copper (10-fold at day 15), cadmium (9-fold at day10) and 180 CST (10-fold at day 15). These results suggested that McGST1 played a significant role in antioxidation and might potentially be used as indicators and biomarkers for detection of marine environmental pollution.

Aflatoxin B1 in poultry: toxicology, metabolism and prevention

Aflatoxins (AF) are ubiquitous in corn-based animal feed and causes hepatotoxic and hepatocarcinogenic effects. The most important AF in terms of toxic potency and occurrence is aflatoxin B1 (AFB1). Poultry, especially turkeys, are extremely sensitive to the toxic and carcinogenic action of AFB1, resulting in millions of dollars in annual losses to producers due to reduced growth rate, increased susceptibility to disease, reduced egg production and other adverse effects. The extreme sensitivity of turkeys and other poultry to AFB1 is associated with efficient hepatic cytochrome P450-mediated bioactivation and deficient detoxification by glutathione S-transferases (GST). Discerning the biochemical and molecular mechanisms of this extreme sensitivity of poultry to AFB1, will contribute in the development of novel strategies to increase aflatoxin resistance. Since AFB1 is an unavoidable contaminant of corn-based poultry feed, chemoprevention strategies aimed at reducing AFB1 toxicity in poultry and in other animals have been the subject of numerous studies. This brief review summarizes many of the key recent findings regarding the action of aflatoxins in poultry.

Novel omega glutathione S-transferases in disk abalone: Characterization and protective roles against environmental stress

Omega glutathione S-transferases (GSTs) are a newly identified class of GSTs with unique properties compared to other members in GST superfamily. This present study reports the cloning, characterization and stress-induced expression analysis of two omega GST genes in disk abalone, Haliotis discus discus. Two disk abalone omega GST genes, HdGSTO1 and HdGSTO2, encode two polypeptides with calculated molecular mass of 27.4 and 26.9 kDa, respectively. Their deduced amino acid sequences showed highest similarity with another molluscan omega GST from Crassostrea gigas. Three-dimensional structures of two omega GSTs were generated by homology modeling and exhibited typical omega GST structural characteristics. The recombinant proteins of HdGSTO1 and HdGSTO2 showed glutathione-dependent thioltransferase and dehydroascorbate reductase activities; however, no activity towards other common GST substrates was detected. Of the two genes, protein encoded by HdGSTO1 showed much higher catalytic ability than the other one. HdGSTO1 mRNA was expressed ubiquitously with high levels in all examined tissues, while HdGSTO2 showed specific expression in gonad and digestive tract. The transcriptional levels of HdGSTO1 in gill were dramatically elevated when abalones were subjected to heat shock, heavy metals and endocrine-disrupting chemical (EDC) exposure, indicating that HdGSTO1 might play important protective roles against environmental stress. HdGSTO2 expression was also significantly induced by heavy metals and EDCs although with much lower fold change than HdGSTO1. But under thermal stress, HdGSTO2 expression was repressed in a time-dependent pattern, implying its different physiological roles under stress. These results indicate that omega GSTs of the disk abalone, especially HdGSTO1, have great potentials as highly sensitive biomarkers of environmental stress.

A mu-class glutathione S-transferase from the marine shrimp Litopenaeus vannamei: molecular cloning and active-site structural modeling

A cDNA clone coding for a mu-class glutathione S-transferase (GST) was isolated from a hepatopancreas cDNA library from the shrimp Litopenaeus vannamei. The deduced amino acid sequence (215 amino acids) has >50% identity to rodents and other mammals mu-class GSTs. Using RT-PCR, the shrimp GST transcript was detected in hepatopancreas, hemocytes, gills, and muscle, but not in pleopods. The shrimp GST sequence was computer modeled and found to fit the classical two-domain GST structure. Domain I, containing the glutathione (GSH) binding site, is more conserved compared to the flexible C-terminal domain II. Residue Q208 appears to be a key to substrate specificity by comparison with mammalian GST mutants. This position is commonly occupied by serine or threonine in mammalian mu-class GSTs, and shrimp Q208 may affect the affinity to substrates like aminochrome or 1,3-dimethyl-2-cyano-1-nitrosoguanidine. This is the first report of molecular cloning and structural modeling of a crustacean GST and provides new insights into the nature of the detoxification response on marine invertebrates.

Functional sites in protein families uncovered via an objective and automated graph theoretic approach

We report a method for detection of recurring side-chain patterns (DRESPAT) using an unbiased and automated graph theoretic approach. We first list all structural patterns as sub-graphs where the protein is represented as a graph. The patterns from proteins are compared pair-wise to detect patterns common to a protein pair based on content and geometry criteria. The recurring pattern is then detected using an automated search algorithm from the all-against-all pair-wise comparison data of proteins. Intra-protein pattern comparison data are used to enable detection of patterns recurring within a protein. A method has been proposed for empirical calculation of statistical significance of recurring pattern. The method was tested on 17 protein sets of varying size, composed of non-redundant representatives from SCOP superfamilies. Recurring patterns in serine proteases, cysteine proteases, lipases, cupredoxin, ferredoxin, ferritin, cytochrome c, aspartoyl proteases, peroxidases, phospholipase A2, endonuclease, SH3 domain, EF-hand and lectins show additional residues conserved in the vicinity of the known functional sites. On the basis of the recurring patterns in ferritin, EF-hand and lectins, we could separate proteins or domains that are structurally similar yet different in metal ion-binding characteristics. In addition, novel recurring patterns were observed in glutathione-S-transferase, phospholipase A2 and ferredoxin with potential structural/functional roles. The results are discussed in relation to the known functional sites in each family. Between 2000 and 50,000 patterns were enumerated from each protein with between ten and 500 patterns detected as common to an evolutionarily related protein pair. Our results show that unbiased extraction of functional site pattern is not feasible from an evolutionarily related protein pair but is feasible from protein sets comprising five or more proteins. The DRESPAT method does not require a user-defined pattern, size or location of the pattern and therefore, has the potential to uncover new functional sites in protein families.

Tyr115, gln165 and trp209 contribute to the 1, 2-epoxy-3-(p-nitrophenoxy)propane-conjugating activity of glutathione S-transferase cGSTM1-1

We investigated the epoxidase activity of a class mu glutathione S-transferase (cGSTM1-1), using 1,2-epoxy-3-(p-nitrophenoxy)propane (EPNP) as substrate. Trp209 on the C-terminal tail, Arg107 on the alpha4 helix, Asp161 and Gln165 on the alpha6 helix of cGSTM1-1 were selected for mutagenesis and kinetic studies. A hydrophobic side-chain at residue 209 is needed for the epoxidase activity of cGSTM1-1. Replacing Trp209 with histidine, isoleucine or proline resulted in a fivefold to 28-fold decrease in the k(cat)(app) of the enzyme, while a modest 25 % decrease in the k(cat)(app) was observed for the W209F mutant. The rGSTM1-1 enzyme has serine at the correponding position. The k(cat)(app) of the S209W mutant is 2. 5-fold higher than that of the wild-type rGSTM1-1. A charged residue is needed at position 107 of cGSTM1-1. The K(m)(app)(GSH) of the R107L mutant is 38-fold lower than that of the wild-type enzyme. On the contrary, the R107E mutant has a K(m)(app)(GSH) and a k(cat)(app) that are 11-fold and 35 % lower than those of the wild-type cGSTM1-1. The substitutions of Gln165 with Glu or Leu have minimal effect on the affinity of the mutants towards GSH or EPNP. However, a discernible reduction in k(cat)(app) was observed. Asp161 is involved in maintaining the structural integrity of the enzyme. The K(m)(app)(GSH) of the D161L mutant is 616-fold higher than that of the wild-type enzyme. In the hydrogen/deuterium exchange experiments, this mutant has the highest level of deuteration among all the proteins tested. We also elucidated the structure of cGSTM1-1 co-crystallized with the glutathionyl-conjugated 1, 2-epoxy-3-(p-nitrophenoxy)propane (EPNP) at 2.8 A resolution. The product found in the active site was 1-hydroxy-2-(S-glutathionyl)-3-(p-nitrophenoxy)propane, instead of the conventional 2-hydroxy isomer. The EPNP moiety orients towards Arg107 and Gln165 in dimer AB, and protrudes into a hydrophobic region formed by the loop connecting beta1 and alpha1 and part of the C-terminal tail in dimer CD. The phenoxyl ring forms strong ring stacking with the Trp209 side-chain in dimer CD. We hypothesize that these two conformations represent the EPNP moiety close to the initial and final stages of the reaction mechanism, respectively.