A semisynthetic glutathione peroxidase with high catalytic efficiency. Selenoglutathione transferase

Repurposing Glutathione Transferases: Directed Evolution Combined with Chemical Modification for the Creation of a Semisynthetic Enzyme with High Hydroperoxidase Activity

Glutathione peroxidases (GPXs) are antioxidant selenoenzymes, which catalyze the reduction of hydroperoxides via glutathione (GSH), providing protection to cells against oxidative stress metabolites. The present study aims to create an efficient semisynthetic GPX based on the scaffold of tau class glutathione transferase (GSTU). A library of GSTs was constructed via DNA shuffling, using three homologue GSTUs from Glycine max as parent sequences. The DNA library of the shuffled genes was expressed in E. coli and the catalytic activity of the shuffled enzymes was screened using cumene hydroperoxide (CuOOH) as substrate. A chimeric enzyme variant (named Sh14) with 4-fold enhanced GPX activity, compared to the wild-type enzyme, was identified and selected for further study. Selenocysteine (Sec) was substituted for the active-site Ser13 residue of the Sh14 variant via chemical modification. The GPX activity (kcat) and the specificity constant (kcat/Κm) of the evolved seleno-Sh14 enzyme (SeSh14) was increased 177- and 2746-fold, respectively, compared to that of the wild-type enzyme for CuOOH. Furthermore, SeSh14 effectively catalyzed the reduction of hydrogen peroxide, an activity that is completely undetectable in all GSTs. Such an engineered GPX-like biocatalyst based on the GSTU scaffold might serve as a catalytic bioscavenger for the detoxification of hazardous hydroperoxides. Furthermore, our results shed light on the evolution of GPXs and their structural and functional link with GSTs.

Genomic footprints of a biological invasion: Introduction from Asia and dispersal in Europe of the topmouth gudgeon (Pseudorasbora parva)

Facilitated by the intensification of global trading, the introduction and dispersal of species to areas in which they are historically non-native is nowadays common. From an evolutionary standpoint, invasions are paradoxical: not only non-native environments could be different from native ones for which introduced individuals would be ill-adapted, but also small founding population size should be associated with reduced adaptive potential. As such, biological invasions are considered valuable real-time evolutionary experiments. Here, we investigated the population structure and adaptive potential of the highly invasive topmouth gudgeon (Pseudorasbora parva) across Europe and East Asia. We RAD-sequenced 301 specimens from sixteen populations and three distinct within-catchment invaded regions as well as two locations in the native range. With 13,785 single nucleotide polymorphisms, we provide conclusive evidence for a genome-wide signature of two distinct invasion events, in Slovakia and Turkey, each originating from a specific area in the native range. A third invaded area, in France, appears to be the result of dispersal within the invasive range. Few loci showed signs of selection, the vast majority of which being identified in the Slovakian region. Functional annotation suggests that faster early stage development, resistance to pollution and immunocompetence contribute to the invasion success of the local habitats. By showing that populations in the invasive range have different evolutionary histories, our study reinforces the idea that populations, rather than species, are the units to consider in invasion biology.

Selenium and Selenocysteine in Protein Chemistry

Selenocysteine, the selenium-containing analogue of cysteine, is the twenty-first proteinogenic amino acid. Since its discovery almost fifty years ago, it has been exploited in unnatural systems even more often than in natural systems. Selenocysteine chemistry has attracted the attention of many chemists in the field of chemical biology owing to its high reactivity and resulting potential for various applications such as chemical modification, chemical protein (semi)synthesis, and protein folding, to name a few. In this Minireview, we will focus on the chemistry of selenium and selenocysteine and their utility in protein chemistry.

Recent advances in protein engineering and biotechnological applications of glutathione transferases

Glutathione transferases (GSTs, EC 2.5.1.18) are a widespread family of enzymes that play a central role in the detoxification, metabolism, and transport or sequestration of endogenous or xenobiotic compounds. During the last two decades, delineation of the important structural and catalytic features of GSTs has laid the groundwork for engineering GSTs, involving both rational and random approaches, aiming to create new variants with new or altered properties. These approaches have expanded the usefulness of native GSTs, not only for understanding the fundamentals of molecular detoxification mechanisms, but also for the development medical, analytical, environmental, and agricultural applications. This review article attempts to summarize successful examples and current developments on GST engineering, highlighting in parallel the recent knowledge gained on their phylogenetic relationships, structural/catalytic features, and biotechnological applications.

Why Nature Chose Selenium

The authors were asked by the Editors of ACS Chemical Biology to write an article titled "Why Nature Chose Selenium" for the occasion of the upcoming bicentennial of the discovery of selenium by the Swedish chemist Jöns Jacob Berzelius in 1817 and styled after the famous work of Frank Westheimer on the biological chemistry of phosphate [Westheimer, F. H. (1987) Why Nature Chose Phosphates, Science 235, 1173-1178]. This work gives a history of the important discoveries of the biological processes that selenium participates in, and a point-by-point comparison of the chemistry of selenium with the atom it replaces in biology, sulfur. This analysis shows that redox chemistry is the largest chemical difference between the two chalcogens. This difference is very large for both one-electron and two-electron redox reactions. Much of this difference is due to the inability of selenium to form π bonds of all types. The outer valence electrons of selenium are also more loosely held than those of sulfur. As a result, selenium is a better nucleophile and will react with reactive oxygen species faster than sulfur, but the resulting lack of π-bond character in the Se-O bond means that the Se-oxide can be much more readily reduced in comparison to S-oxides. The combination of these properties means that replacement of sulfur with selenium in nature results in a selenium-containing biomolecule that resists permanent oxidation. Multiple examples of this gain of function behavior from the literature are discussed.

Enantioselectivity and catalysis improvements of Pseudomonas cepacia lipase with Tyr and Asp modification

A concise strategy to improve the p-nitrophenyl palmitate catalytic activity and enantioselectivity towards secondary alcohols of PcL is described.

A mechanistic mathematical model for the catalytic action of glutathione peroxidase

Glutathione peroxidase (GPx) is a well-known seleno-enzyme that protects cells from oxidative stress (e.g., lipid peroxidation and oxidation of other cellular proteins and macromolecules), by catalyzing the reduction of harmful peroxides (e.g., hydrogen peroxide: H₂O₂) with reduced glutathione (GSH). However, the catalytic mechanism of GPx kinetics is not well characterized in terms of a mathematical model. We developed here a mechanistic mathematical model of GPx kinetics by considering a unified catalytic scheme and estimated the unknown model parameters based on different experimental data from the literature on the kinetics of the enzyme. The model predictions are consistent with the consensus that GPx operates via a ping-pong mechanism. The unified catalytic scheme proposed here for GPx kinetics clarifies various anomalies, such as what are the individual steps in the catalytic scheme by estimating their associated rate constant values and a plausible rationale for the contradicting experimental results. The developed model presents a unique opportunity to understand the effects of pH and product GSSG on the GPx activity under both physiological and pathophysiological conditions. Although model parameters related to the product GSSG were not identifiable due to lack of product-inhibition data, the preliminary model simulations with the assumed range of parameters show that the inhibition by the product GSSG is negligible, consistent with what is known in the literature. In addition, the model is able to simulate the bi-modal behavior of the GPx activity with respect to pH with the pH-range for maximal GPx activity decreasing significantly as the GSH levels decrease and H₂O₂ levels increase (characteristics of oxidative stress). The model provides a key component for an integrated model of H₂O₂ balance under normal and oxidative stress conditions.

Expression of selenocysteine-containing glutathione S-transferase in eukaryote

Glutathione peroxidase (GPX) is a crucial antioxidant selenocysteine (Sec) containing enzyme which plays a significant role in protecting cells against oxidative damage by catalyzing the reduction of hydroperoxides with glutathione (GSH). Several methods have been used to generate GPX mimics, however, only a few of these methods involved genetic engineering and none of them have achieved specific site-directed incorporation of Sec without other modifications, which has hampered further structure-function studies. Here, we report for the first time the conversion of human glutathione transferase Zeta (hGSTZ1-1) into seleno-hGSTZ1-1 by means of genetic engineering in eukaryotes. Fluorescence microscopy images of the expression of Seleno-GST-green fluorescent protein chimaera indicated that we successfully achieved the read-through of the UGA codon to specifically incorporate Sec. Therefore, we achieved the conversion of human glutathione transferase Zeta (hGSTZ1-1) into a seleno-GST (seleno-hGSTZ1-1) by means of genetic engineering in eukaryotes. These results show that recombinant selenoproteins with incorporation of specific selenocysteine residues may be heterologously produced in eukaryotes by using a Sec insertion sequence in the 3' untranslated region (3'-UTR) of the mRNA, and the recombinant selenoproteins is single catalytically active residue and well-characterized structure. In this case a novel GPX activity of 2050±225 U/μmol was introduced into hGSTZ1-1 by substitution of serine 15 by Sec 15. This result will lay a foundation for preparing much smaller GPX mimics with higher activity.

Susceptibility to oxidative stress and modulated expression of antioxidant genes in the copper-exposed polychaete Perinereis nuntia

To identify and evaluate potentially useful biomarkers for oxidative stress as early warning indices in the polychaete, Perinereis nuntia, we exposed P. nuntia to copper (Cu) and measured several biomarker enzymes (glutathione S-transferase; GST, glutathione peroxidase; GPx, Metallothionein-like protein; MTLPs, and catalase; CAT) and genes (Pn-GSTs, Pn-CAT, and Pn-MT) with a cellular oxidative index, reactive oxygen species (ROS) level. Accumulated Cu concentrations in P. nuntia increased in a time-dependent manner. Intracellular ROS reached high levels 6h after exposure in P. nuntia with an increase of GST activity and glutathione (GSH) content. Particularly, GSH in polychaetes showed a positive correlation with Cu contents accumulated in P. nuntia. Messenger RNA expressions of GST sigma and GST omega showed relatively high expressions at 50 μg/L of Cu exposure, even though the moderate increase of rest of GST isoforms was also observed. Also regarding long-term exposure, we reared P. nuntia in sediments for 15 days, and found that there was an obvious increase of Pn-GSTs, Pn-CAT, and Pn-MT genes with elevated concentrations of Cu and Cd in polychaete body, compared to initial levels, suggesting that P. nuntia in sediment was affected by metals as well as by other organic pollutants to induce oxidative stress genes and enzymes. These findings suggest that oxidative stress is a potential modulator of defense system of P. nuntia. Several potential biomarker genes are available as early warning signals for environmental biomonitoring.

Cyclodextrin-derived mimic of glutathione peroxidase exhibiting enzymatic specificity and high catalytic efficiency

To elucidate the relationships between molecular recognition and catalytic ability, we chose three assay systems using three different thiol substrates, glutathione (GSH), 3-carboxyl-4-nitrobenzenethiol (CNBSH), and 4-nitrobenzenethiol (NBSH), to investigate the glutathione peroxidase (GPx) activities of 2,2'-ditellurobis(2-deoxy-beta-cyclodextrin) (2-TeCD) in the presence of a variety of structurally distinct hydroperoxides (ROOH), H2O2, tert-butyl peroxide (tBuOOH), and cumene peroxide (CuOOH), as the oxidative reagent. A comparative study of the three assay systems revealed that the cyclodextrin moiety of the GPx mimic 2-TeCD endows the molecule with selectivity for ROOH and thiol substrates, and hydrophobic interactions are the most important driving forces in 2-TeCD complexation. Furthermore, in the novel NBSH assay system, 2-TeCD can catalyze the reduction of ROOH about 3.4 x 10(5) times more efficiently than diphenyl diselenide (PhSeSePh), and its second-order rate constants for thiol are similar to some of those of native GPx. This comparative study confirms that efficient binding of the substrate is essential for the catalytic ability of the GPx mimic, and that NBSH is the preferred thiol substrate of 2-TeCD among the chosen thiol substrates. Importantly, the proposed mode of action of 2-TeCD imitates the role played by several possible noncovalent interactions between enzymes and substrates in influencing catalysis and binding.

A novel dicyclodextrinyl diselenide compound with glutathione peroxidase activity

A 6A,6A'-dicyclohexylamine-6B,6B'-diselenide-bis-beta-cyclodextrin (6-CySeCD) was designed and synthesized to imitate the antioxidant enzyme glutathione peroxidase (GPX). In this novel GPX model, beta-cyclodextrin provided a hydrophobic environment for substrate binding within its cavity, and a cyclohexylamine group was incorporated into cyclodextrin in proximity to the catalytic selenium in order to increase the stability of the nucleophilic intermediate selenolate. 6-CySeCD exhibits better GPX activity than 6,6'-diselenide-bis-cyclodextrin (6-SeCD) and 2-phenyl-1,2-benzoisoselenazol-3(2H)-one (Ebselen) in the reduction of H(2)O(2), tert-butyl hydroperoxide and cumenyl hydroperoxide by glutathione, respectively. A ping-pong mechanism was observed in steady-state kinetic studies on 6-CySeCD-catalyzed reactions. The enzymatic properties showed that there are two major factors for improving the catalytic efficiency of GPX mimics. First, the substrate-binding site should match the size and shape of the substrate and second, incorporation of an imido-group increases the stability of selenolate in the catalytic cycle. More efficient antioxidant ability compared with 6-SeCD and Ebselen was also seen in the ferrous sulfate/ascorbate-induced mitochondria damage system, and this implies its prospective therapeutic application.

Single Chain Antibody Displays Glutathione S-Transferase Activity

Substrate binding and the subsequent reaction are the two principal phenomena that underlie the activity of enzymes, and many enzyme-like catalysts were generated based on the phenomena. The single chain variable region fragment of antibody 2F3 (scFv2F3) was elicited against hapten GSH-S-DN2phBu, a conjugate of glutathione (GSH), butyl alcohol, and 1-chloro-2,4-dinitrobenzene (CDNB); it can therefore bind both GSH and CDNB, the substrates of native glutathione S-transferases (GSTs). It was shown previously that there is a serine residue that is the catalytic group of GST in the CDR regions of scFv2F3 close to the sulfhydryl of GSH. Thus, we anticipated that scFv2F3 will display GST activity. The experimental results showed that scFv2F3 indeed displayed GST activity that is equivalent to the rat-class GST T-2-2 and exhibited pH- and temperature-dependent catalytic activity. Steady-state kinetic studies showed that the Km values for the substrates are close to those of native GSTs, indicating that scFv2F3 has strong affinities for the substrates. Compared with some other GSTs, its kcat value was found to be low, which could be caused by the similarity between the GSH-S-DN2phBu and the reaction product of GSH and CDNB. These results showed that our approach to imitating enzymes is correct, which is that an active site may catalyze a chemical reaction when a catalytic group locates beside a substrate-binding site of a receptor. It is important to consider product inhibition in hapten design in order to obtain a mimic with a high catalytic efficiency.

Ligand-Directed Labeling of a Single Lysine Residue in hGST A1-1 Mutants

Previously, we discovered that human glutathione transferase (hGST) A1-1 could be site-specifically acylated on a tyrosine residue (Y9) to form ester products using thiolesters of glutathione (GS-thiolesters) as acylating reagents. Out of a total of 20 GS-thiolester reagents tested, 15 (75%) are accepted by hGST A1-1 and thus this is a very versatile reaction. The present investigation was aimed at obtaining a more stable product, an amide bond, between the acyl group and the protein, in order to further increase the value of the reaction. Three lysine mutants (Y9K, A216K, and Y9F/A216K) were therefore prepared and screened against a panel of 18 GS-thiolesters. The Y9K mutant did not react with any of the reagents. The double mutant Y9F/A216K reacted with only one reagent, but in contrast, the A216K mutant could be acylated at the introduced lysine 216 with eight (44%) of the GS-thiolesters. The reaction can take place in the presence of glutathione and even in a crude cell lysate for five (28%) of the reagents. Through the screening process we obtained some basic rules relating to reagent requirements. We have thus produced a mutant (A216K) that can be rapidly and site-specifically modified at a lysine residue to form a stable amide linkage with a range of acyl groups. One of the successful reagents is a fluorophore that potentially can be used in downstream protein purification and protein fusion applications.

Kinetic studies on the glutathione peroxidase activity of selenium-containing glutathione transferase

Selenium-containing glutathione transferase (seleno-GST) was generated by biologically incorporating selenocysteine into the active site of glutathione transferase (GST) from a blowfly Lucilia cuprina (Diptera: Calliphoridae). Seleno-GST mimicked the antioxidant enzyme glutathione peroxidase (GPx) and catalyzed the reduction of structurally different hydroperoxides by glutathione. Kinetic investigations reveal a ping-pong kinetic mechanism in analogy with that of the natural GPx cycle as opposed to the sequential one of the wild type GST. This difference of the mechanisms might result from the intrinsic chemical properties of the incorporated residue selenocysteine, and the selenium-dependent mechanism is suggested to contribute to enhancement of the enzymatic efficiency.

Asymmetric organotellurides as potent antioxidants and building blocks of protein conjugates

New asymmetric organotellurides exhibiting good antioxidant properties in vitro and in cell culture can be attached to human serum albumin.

Engineering glutathione transferase to a novel glutathione peroxidase mimic with high catalytic efficiency. Incorporation of selenocysteine into a glutathione-binding scaffold using an auxotrophic expression system

Glutathione peroxidase (GPx, EC 1.11.1.9) protects cells against oxidative damage by catalyzing the reduction of hydroperoxides with glutathione (GSH). Several attempts have been made to imitate its function for mechanical study and for its pharmacological development as an antioxidant. By replacing the active site serine 9 with a cysteine and then substituting it with selenocysteine in a cysteine auxotrophic system, catalytically essential residue selenocysteine was bioincorporated into GSH-specific binding scaffold, and thus, glutathione S-transferase (GST, EC 2.5.1.18) from Lucilia cuprina was converted into a selenium-containing enzyme, seleno-LuGST1-1, by genetic engineering. Taking advantage of the important structure similarities between seleno-LuGST1-1 and naturally occurring GPx in the specific GSH binding sites and the geometric conformation for the active selenocysteine in their common GSH binding domain-adopted thioredoxin fold, the as-generated selenoenzyme displayed a significantly high efficiency for catalyzing the reduction of hydrogen peroxide by glutathione, being comparable with those of natural GPxs. The catalytic behaviors of this engineered selenoenzyme were found to be similar to those of naturally occurring GPx. It exhibited pH and temperature-dependent catalytic activity and a typical ping-pong kinetic mechanism. Engineering GST into an efficient GPx-like biocatalyst provided new proof for the previous assumption that both GPx and GST were evolved from a common thioredoxin-like ancestor to accommodate different functions throughout evolution.

Aryl Thiol Substrate 3-Carboxy-4-Nitrobenzenethiol Strongly Stimulating Thiol Peroxidase Activity of Glutathione Peroxidase Mimic 2, 2'-Ditellurobis(2-Deoxy-β-Cyclodextrin)

Artificial glutathione peroxidase (GPx) model 2, 2'-ditellurobis(2-deoxy-beta-cyclodextrin) (2-TeCD) which has the desirable properties exhibited high substrate specificity and remarkably catalytic efficiency when 3-carboxy-4-nitrobenzenethiol (ArSH) was used as a preferential thiol substrate. The complexation of ArSH with beta-cyclodextrin was investigated through UV spectral titrations, fluorescence spectroscopy, 1H NMR and molecular simulation, and these results indicated that ArSH fits well to the size of the cavity of beta-cyclodextrin. Furthermore, 2-TeCD was found to catalyze the reduction of cumene peroxide (CuOOH) by ArSH 200,000-fold more efficiently than diphenyl diselenide (PhSeSePh). Its steady-state kinetics was studied and the second rate constant kmax/KArSH was found to be 1.05 x 10(7) M(-1) min(-1) and similar to that of natural GPx. Moreover, the kinetic data revealed that the catalytic efficiency of 2-TeCD depended strongly upon the competitive recognition of both substrates for 2-TeCD. The catalytic mechanism of 2-TeCD catalysis agreed well with a ping-pong mechanism, in analogy with natural GPx, and might exert its thiol peroxidase activity via tellurol, tellurenic acid, and tellurosulfide.

Expression of selenocysteine-containing glutathione S-transferase in Escherichia coli

Evolution of a probable 'glutathione-binding ancestor' resulting in a common thioredoxin-fold for glutathione S-transferases and glutathione peroxidases may possibly suggest that a glutathione S-transferase could be engineered into a selenium-containing glutathione S-transferase (seleno-GST), having glutathione peroxidase (GPX) activity. Here, we addressed this question by production of such protein. In order to obtain a recombinant seleno-GST produced in Escherichia coli, we introduced a variant bacterial-type selenocysteine insertion sequence (SECIS) element which afforded substitution with selenocysteine for the catalytic Tyr residue in the active site of GST from Schistosoma japonica. Utilizing coexpression with the bacterial selA, selB, and selC genes (encoding selenocysteine synthase, SelB, and tRNA(Sec), respectively) the yield of recombinant seleno-GST was about 2.9 mg/L bacterial culture, concomitant with formation of approximately 85% truncation product as a result of termination of translation at the selenocysteine-encoding UGA codon. The mutations inferred as a result of the introduction of a SECIS element did not affect the glutathione-binding capacity (Km = 53 microM for glutathione as compared to 63 microM for the wild-type enzyme) nor the GST activity (kcat = 14.3 s(-1) vs. 16.6 s(-1)), provided that the catalytic Tyr residue was intact. When this residue was changed to selenocysteine, however, the resulting seleno-GST lost the GST activity. It also failed to display any novel GPX activity towards three standard peroxide substrates (hydrogen peroxide, butyl hydroperoxide or cumene hydroperoxide). These results show that recombinant selenoproteins with internal selenocysteine residues may be heterologously produced in E. coli at sufficient amounts for purification. We also conclude that introduction of a selenocysteine residue into the catalytic site of a glutathione S-transferase is not sufficient to induce GPX activity in spite of a maintained glutathione-binding capacity.

Combinatorial Chemical Reengineering of the Alpha Class Glutathione Transferases

Previously, we discovered that human glutathione transferases (hGSTs) from the alpha class can be rapidly and quantitatively modified on a single tyrosine residue (Y9) using thioesters of glutathione (GS-thioesters) as acylating reagents. The current work was aimed at exploring the potential of this site-directed acylation using a combinatorial approach, and for this purpose a panel of 17 GS-thioesters were synthesized in parallel and used in screening experiments with the isoforms hGSTs A1-1, A2-2, A3-3, and A4-4. Through analytical HPLC and MALDI-MS experiments, we found that between 70 and 80% of the reagents are accepted and this is thus a very versatile reaction. The range of ligands that can be used to covalently reprogram these proteins is now expanded to include functionalities such as fluorescent groups, a photochemical probe, and an aldehyde as a handle for further chemical derivatization. This site-specific modification reaction thus allows us to create novel functional proteins with a great variety of artificial chemical groups in order to, for example, specifically tag GSTs in biological samples or create novel enzymatic function using appropriate GS-thioesters.

Programmed delivery of novel functional groups to the alpha class glutathione transferases

Here we describe a new route to site- and class-specific protein modification that will allow us to create novel functional proteins with artificial chemical groups. Glutathione transferases from the alpha but not the mu, pi, omega, or theta classes can be rapidly and site-specifically acylated with thioesters of glutathione (GS-thioesters) that are similar to compounds that have been demonstrated to occur in vivo. The human isoforms A1-1, A2-2, A3-3, and A4-4 from the alpha class all react with the reagent at a conserved tyrosine residue (Y9) that is crucial in catalysis of detoxication reactions. The yield of modified protein is virtually quantitative in less than 30 min under optimized conditions. The acylated product is stable for more than 24 h at pH 7 and 25 degrees C. The modification is reversible in the presence of excess glutathione, but the labeled protein can be protected by adding S-methylglutathione. The stability of the ester with respect to added glutathione depends on the acyl moiety. The reaction can also take place in Escherichia coli lysates doped with alpha class glutathione transferases. A control substance that lacks the peptidyl backbone required for binding to the glutathione transferases acylates surface-exposed lysines. There is some acyl group specificity since one out of the three different GS-thioesters that we tried was not able to acylate Y9.

Chemical modification of biocatalysts

Although several powerful methods exist for the redesign of enzyme structure and function these are typically limited to the 20 most abundant proteinogenic amino acids. The use of chemical modification overcomes this limitation to allow virtually unlimited alteration of amino acid sidechain structures. If heterogeneous mixtures of enzyme products are to be avoided, however, the required chemistry should be efficient, selective and compatible with aqueous conditions. Recent advances have been made in the modification of proteinases, aminotransferases and redox enzymes.