Biochemical characterization of selenium-containing catalytic antibody as a cytosolic glutathione peroxidase mimic

Catalytic Antibodies: Design, Expression, and Their Applications in Medicine

Catalytic antibodies made it feasible to develop new catalysts, which had previously been the subject of research. Scientists have discovered natural antibodies that can hydrolyze substrates such as nucleic acids, proteins, and polysaccharides during decades of research, as well as several ways of producing antibodies with specialized characteristics and catalytic functions. These antibodies are widely used in chemistry, biology, and medicine. Catalytic antibodies can continue to play a role and even fully prevent the emergence of autoimmune disorders, especially in the field of infection and immunity, where the process of its occurrence and development often takes a long time. In this work, the development, design and evolution methodologies, and the expression systems and applications of catalytic antibodies, are discussed. Trial registration: not applicable.

Essential Protective Role of Catalytically Active Antibodies (Abzymes) with Redox Antioxidant Functions in Animals and Humans

During the life of aerobic organisms, the oxygen resulting from numerous reactions is converted into reactive oxygen species (ROS). Many ROS are dangerous due to their high reactivity; they are strong oxidants, and react with various cell components, leading to their damage. To protect against ROS overproduction, enzymatic and non-enzymatic systems are evolved in aerobic cells. Several known non-enzymatic antioxidants have a relatively low specific antioxidant activity. Superoxide dismutases, catalase, glutathione peroxidase, glutathione S-transferase, thioredoxin, and the peroxiredoxin families are the most important enzyme antioxidants. Artificial antibodies catalyzing redox reactions using different approaches have been created. During the past several decades, it has been shown that the blood and various biological fluids of humans and animals contain natural antibodies that catalyze different redox reactions, such as classical enzymes. This review, for the first time, summarizes data on existing non-enzymatic antioxidants, canonical enzymes, and artificial or natural antibodies (abzymes) with redox functions. Comparing abzymes with superoxide dismutase, catalase, peroxide-dependent peroxidase, and H₂O₂-independent oxidoreductase activities with the same activities as classical enzymes was carried out. The features of abzymes with the redox activities are described, including their exceptional diversity in the optimal pH values, dependency and independence on various metal ions, and the reaction rate constants for healthy donors and patients with different autoimmune diseases. The entire body of evidence indicates that abzymes with redox antioxidant activities existing in the blood for a long time compared to enzymes are an essential part of the protection system of humans and animals from oxidative stress.

Site-Specific Selenocysteine Incorporation into Proteins by Genetic Engineering

Selenocysteine (Sec), a rare naturally proteinogenic amino acid, is the major form of essential trace element selenium in living organisms. Selenoproteins, with one or several Sec residues, are found in all three domains of life. Many selenoproteins play a role in critical cellular functions such as maintaining cell redox homeostasis. Sec is usually encoded by an in-frame stop codon UGA in the selenoprotein mRNA, and its incorporation in vivo is highly species-dependent and requires the reprogramming of translation. This mechanistic complexity of selenoprotein synthesis poses a big challenge to produce synthetic selenoproteins. To understand the functions of natural as well as engineered selenoproteins, many strategies have recently been developed to overcome the inherent barrier for recombinant selenoprotein production. In this review, we will describe the progress in selenoprotein production methodology.

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.

IgG abzymes with peroxidase and oxidoreductase activities from the sera of healthy humans

We present the evidence showing that small fractions of electrophoretically homogeneous immunoglobulin G (IgGs) from the sera of healthy humans and their Fab and F(ab)2 fragments oxidize 3,3'-diaminobenzidine through a peroxidase activity in the presence of H2 O2 and through an oxidoreductase activity in the absence of H2 O2 . During purification on protein G-Sepharose and gel filtration, the polyclonal IgGs partially lose the Me(2+) ions. After extensive dialysis of purified Abs against agents chelating metal ions, the relative peroxidase activity decreased dependently of IgG analyzed from 100 to ~10-85%, while oxidoreductase activity from 100 to 14-83%. Addition of external metal ions to dialyzed and non-dialyzed IgGs leads to a significant increase in their activity. Chromatography of the IgGs on Chelex non-charged with Cu(2+) ions results in the adsorption of a small IgG fraction bound with metal ions (~5%), while Chelex charged with Cu(2+) ions bind additionally ~38% of the total IgGs. Separation of Abs on both sorbents results in IgG separation to many different subfractions demonstrating various affinities to the chelating resin and different levels of the specific oxidoreductase and peroxidase activities. In the presence of external Cu(2+) ions, the specific peroxidase activity of several IgG subfractions achieves 20-27 % as compared with horseradish peroxidase (HRP, taken for 100%). The oxidoreductase activity of these fractions is ~4-6-fold higher than that for HRP. Antioxidant enzymes such as superoxide dismutases, catalases, and glutathione peroxidases are known to represent critical defence mechanisms for preventing oxidative modifications of DNA, proteins, and lipids. Peroxidase and oxidoreductase activities of human IgGs could also play an important role in the protection of organisms from oxidative stress and toxic compounds.

Comparison of glutathione peroxidase 1 and peroxiredoxin 6 in protection against oxidative stress in the mouse lung

Peroxiredoxin 6 (Prdx6) and cytosolic GSH peroxidase (GPx1), both GSH-dependent peroxidases, were compared for the effects of their knockout on injury and lipid peroxidation in: (a) lungs of mice exposed to 0.85 or 1.0atm O(2), (b) isolated perfused mouse lungs exposed to 5mM tert-butylhydroperoxide (t-BOOH) or 1mM paraquat, and (c) primary mouse pulmonary microvascular endothelial cells exposed to 50muM t-BOOH. Derangements in GPx1 null were similar or slightly greater than in wild type for all parameters in the various models of oxidant stress, whereas Prdx6 null showed markedly increased effects. GSH peroxidase activity with phosphatidylcholine hydroperoxide as substrate in GPx1-null lung homogenate was decreased only slightly vs wild type, whereas activity in Prdx6-null lungs was decreased by ~95%, indicating that Prdx6 is the major enzyme for reduction of oxidized lung phospholipids. Expression levels of oxidant-related genes measured with a PCR-based gene array indicated no significant differences between the Prdx6 and the GPx1 null except for the target genes and IL-19. Thus, Prdx6-null mice are significantly more sensitive to oxidant stress compared to GPx1 null, suggesting that scavenging of phospholipid hydroperoxides by Prdx6 plays a major role in lung antioxidant defense.

Incorporation of tellurocysteine into glutathione transferase generates high glutathione peroxidase efficiency

A rival to native peroxidase! An existing binding site for glutathione was combined with the catalytic residue tellurocysteine by using an auxotrophic expression system to create an engineered enzyme that functions as a glutathione peroxidase from the scaffold of a glutathione transferase (see picture). The catalytic activity of the telluroenzyme in the reduction of hydroperoxides by glutathione is comparable to that of native glutathione peroxidase.

Oxidoreductase activities of polyclonal IgGs from the sera of Wistar rats are better activated by combinations of different metal ions

It was shown that IgGs purified from the sera of healthy Wistar rats contain several different bound Me2+ ions and oxidize 3,3'-diaminobenzidine through a H2O2-dependent peroxidase and H2O2-independent oxidoreductase activity. IgGs have lost these activities after removing the internal metal ions by dialysis against EDTA. External Cu2+ or Fe2+ activated significantly both activities of non-dialysed IgGs containing different internal metals (Fe > or = Pb > or = Zn > or = Cu > or = Al > or = Ca > or = Ni > or = Mn > Co > or = Mg) showing pronounced biphasic dependencies corresponding to approximately 0.1-2 and approximately 2-5 mM of Me2+, while the curves for Mn2+ were nearly linear. Cu2+ alone significantly stimulated both the peroxidase and oxidoreductase activities of dialysed IgGs only at high concentration (> or = 2 mM), while Mn2+ weakly activated peroxidase activity at concentration >3 mM but was active in the oxidoreductase oxidation at a low concentration (<1 mM). Fe2+-dependent peroxidase activity of dialysed IgGs was observed at 0.1-5 mM, but Fe2+ was completely inactive in the oxidoreductase reaction. Mg2+, Ca2+, Zn2+, Al2+ and especially Co2+ and Ni2+ were not able to activate dialysed IgGs, but slightly activated non-dialysed IgGs. The use of the combinations of Cu2+ + Mn2+, Cu2+ + Zn2+, Fe2+ + Mn2+, Fe2+ + Zn2+ led to a conversion of the biphasic curves to hyperbolic ones and in parallel to a significant increase in the activity as compared with Cu2+, Fe2+ or Mn2+ ions taken separately; the rates of the oxidation reactions, catalysed by non-dialysed and dialysed IgGs, became comparable. Mg2+, Co2+ and Ni2+ markedly activated the Cu2+-dependent oxidation reactions catalysed by dialysed IgGs, while Ca2+ inhibited these reactions. A possible role of the second metal in the oxidation reactions is discussed.

Catalase and glutathione peroxidase mimics

Overproduction of the reactive oxygen species (ROS) superoxide (O(2)(-)) and hydrogen peroxide (H(2)O(2)) are increasingly implicated in human disease and aging. ROS are also being explored as important modulating agents in a number of cell signaling pathways. Earlier work has focused on development of small catalytic scavengers of O(2)(-), commonly referred to as superoxide dismutase (SOD) mimetics. Many of these compounds also have substantial abilities to catalytically scavenge H(2)O(2) and peroxynitrite (ONOO(-)). Peroxides have been increasingly shown to disrupt cell signaling cascades associated with excessive inflammation associated with a wide variety of human diseases. Early studies with enzymatic scavengers like SOD frequently reported little or no beneficial effect in biologic models unless SOD was combined with catalase or a peroxidase. Increasing attention has been devoted to developing catalase or peroxidase mimetics as a way to treat overt inflammation associated with the pathophysiology of many human disorders. This review will focus on recent development of catalytic scavengers of peroxides and their potential use as therapeutic agents for pulmonary, cardiovascular, neurodegenerative and inflammatory disorders.

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.

A fused selenium-containing protein with both GPx and SOD activities

As a safeguard against oxidative stress, the balance between the main antioxidant enzymes including superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT) was believed to be more important than any single one, for example, dual-functional SOD/CAT enzyme has been proved to have better antioxidant ability than either single enzyme. By combining traditional fusion protein technology with amino acid auxotrophic expression system, we generated a bifunctional enzyme with both GPx and SOD activities. It displayed better antioxidant ability than GPx or SOD. Such dual-functional enzymes could facilitate further studies of the cooperation of GPx and SOD and generation of better therapeutic agents.

Substrate specificity of rat sera IgG antibodies with peroxidase and oxidoreductase activities

We have recently shown that intact IgGs from the sera of healthy Wistar rats oxidize 3,3'-diaminobenzidine (DAB) in the presence and in the absence of H(2)O(2) similar to horseradish peroxidase (HRP). Here we demonstrate for the first time that the peroxidase and oxidoreductase activities of IgGs can efficiently oxidize not only DAB but also o-phenylendiamine, phenol, p-dihydroquinone, alpha-naphthol, and NADH but, in contrast to HRP, cannot oxidize adrenalin. In contrast to IgGs, HRP cannot oxidize phenol, p-dihydroquinone, or alpha-naphthol in the absence of H(2)O(2). In contrast to plant and mammalian peroxidases, IgGs were more universal in their metal dependence. The specific wide repertoire of polyclonal peroxidase and oxidoreductase IgGs oxidizing various substances could play an important role in protecting the organism from oxidative stress and serve as an additional natural system destroying different toxic, carcinogenic, and mutagenic compounds.

Peroxiredoxins in the lung with emphasis on peroxiredoxin VI

All six mammalian peroxiredoxins are expressed in the lung. Peroxiredoxin (Prx) VI is the isoform expressed at the highest level and its lung expression exceeds that for other organs. The predominant location of Prx VI is the cytosol and acidic organelles of Clara cells of the conducting airways and type II epithelial cells and macrophages in the alveoli. Prx I and VI show developmental induction of transcription at birth. PrxVI shares structural homology with other peroxiredoxins exhibiting a thioredoxin fold and a conserved catalytic Cys residue in the N-terminus of the protein. This enzyme is highly inducible by oxidative stress in both the neonatal and adult lung consistent with a role in antioxidant defense. Prx VI has several properties that distinguish its peroxidase activity from other peroxiredoxins: it can reduce phospholipid hydroperoxides in addition to other organic hydroperoxides and H2O2; the electron donor that serves to reduce the oxidized peroxidatic cysteine is not thioredoxin but GSH; instead of homodimerization, heterodimerization with pi-glutathione S-transferase is required for regeneration of the active enzyme. Prx VI also expresses a phospholipase A2 activity that is Ca2+-independent, maximal at acidic pH, and dependent on a serine-based catalytic triad and nucleophilic elbow at the surface of the protein. Models of altered Prx VI expression at the cellular, organ and whole animal levels have demonstrated that Prx VI functions as an important anti-oxidant enzyme with levels of protection that exceed those ascribed to GSH peroxidase (GPx1). The phospholipase A2 activity plays an important role in lung surfactant homeostasis and is responsible for the bulk of the degradation of internalized phosphatidylcholine and its resynthesis by the reacylation pathway. Expression of peroxiredoxins is elevated in several lung diseases including lung cancer, mesothelioma and sarcoidosis, although the mechanism for these alterations is not known. The unique properties of Prx VI enable it to play an important role in lung cell function.

Peroxidase activity of selenoprotein GrdB of glycine reductase and stabilisation of its integrity by components of proprotein GrdE from Eubacterium acidaminophilum

The anaerobe Eubacterium acidaminophilum has been shown to contain an uncharacterized peroxidase, which may serve to protect the sensitive selenoproteins in that organism. We purified this peroxidase and found that it was identical with the substrate-specific "protein B"-complex of glycine reductase. The "protein B"-complex consists of the selenocysteine-containing GrdB subunit and two subunits, which derive from the GrdE proprotein. The specific peroxidase activity was 1.7 U (mg protein)(-1) with DTT and cumene hydroperoxide as substrates. Immunoprecipitation experiments revealed that GrdB was important for DTT- and NADH-dependent peroxidase activities in crude extracts, whereas the selenoperoxiredoxin PrxU could be depleted without affecting these peroxidase activities. GrdB could be heterologously produced in Escherichia coli with coexpression of selB and selC from E. acidaminophilum for selenocysteine insertion. Although GrdB was sensitive to proteolysis, some full-size protein was present which accounted for a peroxidase activity of about 0.5 U (mg protein)(-1) in these extracts. Mutation of the potentially redox-active UxxCxxC motif in GrdB resulted in still significant, but decreased activity. Heterologous GrdB was protected from degradation by full-length GrdE or by GrdE-domains. The GrdB-GrdE interaction was confirmed by copurification of GrdE with Strep-tagged GrdB. The data suggest that GrdE domains serve to stabilise GrdB.

Metal ions-dependent peroxidase and oxidoreductase activities of polyclonal IgGs from the sera of Wistar rats

We present evidence showing that a small fraction of electrophoretically homogeneous IgGs from the sera of healthy Wistar rats is bound with several different Me2+ ions and oxidizes 3,3'-diaminobenzidine through a peroxidase activity in the presence of H2O2 and through an oxidoreductase activity in the absence of H2O2. During purification on Protein A-Sepharose and gel filtration, the polyclonal IgGs partially lose the Me2+ ions. Therefore, in the absence of external metal ions, the specific peroxidase activity of IgGs from the sera of different rats varied in the range 1.6-26% and increased up to 13-198% after addition of Fe2+ or Cu2+ ions as compared with horseradish peroxidase (HRP, taken for 100%). The oxidoreductase activity of HRP is 24-fold lower than its peroxidase activity, while oxidoreductase and peroxidase activities of IgGs are comparable. Oxidoreductase activities of different IgGs in the absence of external metal ions varied from 22 to 800%, and in the presence of Fe2+ or Cu2+ ions, from 37 to 1100% in comparison with the HRP oxidoreductase activity (100%). Chromatography of the IgGs on Chelex-100 leads to the adsorption of a small IgG fraction bound with metal ions and to its separation to many different subfractions demonstrating various affinities to the chelating resin and increased levels of the specific oxidoreductase and peroxidase activities. Antioxidant enzymes such as superoxide dismutases, catalases, and glutathione peroxidases are known to represent critical defense mechanisms for preventing oxidative modifications of DNA, proteins, and lipids. Peroxidase and oxidoreductase activity of antibodies may play an important role in the protection of organisms from oxidative stress and toxic compounds.

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.

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.

Selenocysteine in proteins-properties and biotechnological use

Selenocysteine (Sec), the 21st amino acid, exists naturally in all kingdoms of life as the defining entity of selenoproteins. Sec is a cysteine (Cys) residue analogue with a selenium-containing selenol group in place of the sulfur-containing thiol group in Cys. The selenium atom gives Sec quite different properties from Cys. The most obvious difference is the lower pK(a) of Sec, and Sec is also a stronger nucleophile than Cys. Proteins naturally containing Sec are often enzymes, employing the reactivity of the Sec residue during the catalytic cycle and therefore Sec is normally essential for their catalytic efficiencies. Other unique features of Sec, not shared by any of the other 20 common amino acids, derive from the atomic weight and chemical properties of selenium and the particular occurrence and properties of its stable and radioactive isotopes. Sec is, moreover, incorporated into proteins by an expansion of the genetic code as the translation of selenoproteins involves the decoding of a UGA codon, otherwise being a termination codon. In this review, we will describe the different unique properties of Sec and we will discuss the prerequisites for selenoprotein production as well as the possible use of Sec introduction into proteins for biotechnological applications. These include residue-specific radiolabeling with gamma or positron emitters, the use of Sec as a reactive handle for electophilic probes introducing fluorescence or other peptide conjugates, as the basis for affinity purification of recombinant proteins, the trapping of folding intermediates, improved phasing in X-ray crystallography, introduction of 77Se for NMR spectroscopy, or, finally, the analysis or tailoring of enzymatic reactions involving thiol or oxidoreductase (redox) selenolate chemistry.

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.

Selenium-containing 15-Mer Peptides with High Glutathione Peroxidase-like Activity

Glutathione peroxidase (GPX) is one of the most crucial antioxidant enzymes in a variety of organisms. Here we described a new strategy for generating a novel GPX mimic by combination of a phage-displayed random 15-mer peptide library followed by computer-aided rational design and chemical mutation. The novel GPX mimic is a homodimer consisting of a 15-mer selenopeptide with an appropriate catalytic center, a specific binding site for substrates, and high catalytic efficiency. Its steady state kinetics was also studied, and the values of k(cat)/K(m)(GSH) and k(cat)/ K(mH(2)O(2)) were found to be similar to that of native GPX and the highest among the existing GPX mimics. Moreover, the novel GPX mimic was confirmed to have a strong antioxidant ability to inhibit lipid peroxidation by measuring the content of malondialdehyde, cell viability, and lactate dehydrogenase activity. Importantly, the novel GPX mimic can penetrate into the cell membrane because of its small molecular size. These characteristics endue the novel mimic with potential perspective for pharmaceutical applications.

A selenium-containing single-chain abzyme with potent antioxidant activity

Reactive oxygen species (ROS) are products of normal metabolic activities and are thought to be the cause of many diseases. A selenium-containing single-chain abzyme 2F3 (Se-2F3-scFv) that imitates glutathione peroxidase has been produced which has the capacity to remove ROS. To evaluate the antioxidant ability of Se-2F3-scFv, we constructed a ferrous sulfate/ascorbate (Vc/Fe2+)-induced mitochondrial damage model system and investigated the capacity of Se-2F3-scFv to protect mitochondria from oxidative damage. Se-2F3-scFv markedly decreased mitochondrial swelling, inhibited lipid peroxidation, and maintained the activity of cytochrome c oxidase, in comparison with Ebselen, a well-studied glutathione peroxidase mimic, indicating that Se-2F3-scFv has potential for treating diseases mediated by ROS.

A semisynthetic glutathione peroxidase with high catalytic efficiency. Selenoglutathione transferase

Glutathione peroxidase (GPX) protects cells against oxidative damage by catalyzing the reduction of hydroperoxides by glutathione (GSH). GPX therefore has potential therapeutic value as an antioxidant, but its pharmacological development has been limited because GPX uses a selenocysteine as its catalytic group and it is difficult to generate selenium-containing proteins with traditional recombinant DNA technology. Here, we show that naturally occurring proteins can be modified to generate GPX activity. The rat theta-class glutathione transferase T2-2 (rGST T2-2) presents an ideal scaffold for the design of a novel GPX catalyst because it already binds GSH and contains a serine close to the substrate binding site, which can be chemically modified to bind selenium. The modified Se-rGST T2-2 efficiently catalyzes the reduction of hydrogen peroxide, and the GPX activity surpasses the activities of some natural GPXs.

An engineered azurin variant containing a selenocysteine copper ligand

Modulating the properties of proteins through de novo design or redesign of existing proteins has been a longstanding goal in protein chemistry. Over the past two decades, site-directed mutagenesis has been a powerful tool to probe the role of certain residues and to fine-tune the activity of proteins. A limitation of this approach has been the accessibility of only a restricted number of functional groups through the 20 amino acids in the genetic code. The more recent technique of expressed protein ligation (EPL) provides an alternative route that allows efficient incorporation of nonnatural residues into proteins. We report here the preparation and spectroscopic characterization of an azurin variant in which a cysteine ligand to the blue copper center has been replaced by EPL with selenocysteine (Sec). This reports marks the first time that selenocysteine is artificially incorporated into the active site of a metalloprotein. The variant displays a significantly increased A(parallel) (from 56 to 104 G) and red-shifted CT band (from 625 to 677 nm), while maintaining the general type 1 copper characteristics, including similarity in reduction potentials. This study illustrates that iso-structural substitution using EPL can fine-tune the structural and functional properties of a metal-binding site without loss of most of its characteristics. Further spectroscopic and X-ray crystallographic studies of this and other EPL variants will provide new insights into the fine-control of the structure and function of metalloproteins.

Generation of three selenium-containing catalytic antibodies with high catalytic efficiency using a novel hapten design method

A novel strategy for design of haptens that were used to produce catalytic antibodies was developed and three monoclonal antibodies, 3G5, 2F3, and 5C9, were generated using this strategy. These monoclonal antibodies were converted into selenium-containing abzymes by chemically modifying the hydroxyl group of serines followed by sodium hydrogen selenide displacement. These selenium-containing abzymes exhibited remarkable glutathione peroxidase activity, which surpasses the activity of some native glutathione peroxidases. The activities of the selenium-containing abzymes Se-3G5, Se-2F3, and Se-5C9 which catalyzed reduction of hydroperoxides by glutathione were 2.23, 4.20, and 3.79 times that of rabbit liver glutathione peroxidase, respectively. Detailed steady-state kinetics study on Se-2F3 was carried out and the value of k(cat)/K(m) (H(2)O(2)) was found to be 2.11 x 10(7) M(-1) min(-1) which was supposed to be one of the highest among the known catalytic antibodies. The data of association constants and glutathione peroxidase activities of these catalytic antibodies and the steady-state kinetics of Se-2F3 showed that the method might be a remarkably efficient one for generating catalytic antibodies with glutathione peroxidase activity.

Cloning and expression of a single-chain catalytic antibody that acts as a glutathione peroxidase mimic with high catalytic efficiency

Glutathione peroxidase (GPX) has a powerful role in scavenging reactive oxygen species. In previous papers we have developed a new strategy for generating abzymes: the monoclonal antibody with a substrate-binding site is first prepared, then a catalytic group is incorporated into the monoclonal antibody's binding site by using chemical mutation [Luo, Zhu, Ding, Gao, Sun, Liu, Yang and Shen (1994) Biochem. Biophys. Res. Commun. 198, 1240-1247; Ding, Liu, Zhu, Luo, Zhao and Ni (1998) Biochem. J. 332, 251-255]. Since then we have established a series of catalytic antibodies capable of catalysing the decomposition of hydroperoxides by GSH. The monoclonal antibody 2F3 was raised against GSH-S-2,4-dinitrophenyl t-butyl ester and exhibited high catalytic efficiency, exceeding that of rabbit liver GPX, after chemical mutation. To produce pharmaceutical proteins and to study the reason why it exhibits high catalytic efficiency, we sequenced, cloned and expressed the variable regions of 2F3 antibody as a single-chain Fv fragment (2F3-scFv) in different bacterial strains. The amounts of 2F3-scFv proteins expressed from JM109 (DE3), BL21 (DE3), and BL21 (coden plus) were 5-10%, 15-20% and 25-30% of total bacterial proteins respectively. The 2F3-scFv was expressed as inclusion bodies, purified in the presence of 8 M urea by Co(2+)-immobilized metal-affinity chromatography (IMAC) and renatured to the active form in vitro by gel filtration. The binding constants of the active 2F3-scFv for GSH and GSSG were 2.46 x 10(5) M(-1) and 1.03 x 10(5) M(-1) respectively, which were less by one order of magnitude than that of the intact 2F3 antibody. The active 2F3-scFv was converted into selenium-containing 2F3-scFv (Se-2F3-scFv) by chemical modification of the reactive serine; the GPX activity of the Se-2F3-scFv was 3394 units/micromol, which approaches the activity of rabbit liver GPX.

Kinetics study of a selenium-containing ScFv catalytic antibody that mimics glutathione peroxidase

The steady-state kinetics study and some enzymatic characterization of a selenium-containing scFv catalytic antibody (Se-scFv2F3) were carried out. A novel reaction formula of this abzyme-catalyzed reaction was proposed and a rate equation was obtained according to the formula. The constants in the equation were compared with Dalziel's parameters and the exact meanings of these constants were analyzed. The obtained kinetics parameters from the kinetics study of Se-scFv2F3 were analyzed and compared with those of native glutathione peroxidase.

Kinetics study of a selenium-containing ScFv catalytic antibody that mimics glutathione peroxidase

The steady state kinetic study and some enzymic characterization of a selenium-containing scFv catalytic antibody (Se-scFv2F3) was carried out. A novel reaction formula of this abzyme-catalyzed reaction was proposed and a rate equation was gotten according to the formula. The constants in the equation were compared with Dalziel's parameters and the exact meanings of these constants were analyzed. The gotten kinetics parameters from the kinetics study of Se-scFv2F3 were analyzed and compared with that of native glutathione peroxidase.

Preparation and Properties of a Selenium-containing Catalytic Antibody as Type I Deiodinase Mimic

Conversion of thyroxine (T4) to 3,5,3'-triiodothyronine is an essential first step in controlling thyroid hormone action. Type I deiodinase (DI) can catalyze the conversion to produce the bulk of serum 3,5,3'-triiodothyronine. Acting as a mimic of DI, a selenium-containing catalytic antibody (Se-4C5) prepared by converting the serine residues of monoclonal antibody 4C5 raised against T4 into selenocysteines, can catalyze the deiodination of T4 with dithiothreitol (DTT) as cosubstrate. The mimic enzyme Se-4C5 exhibited a much greater deiodinase activity than model compound ebselen and another selenium-containing antibody Se-Hp4 against GSH. The coupling of selenocysteine with the combining pocket of antibody 4C5 endowed Se-4C5 with enzymatic activity. To probe the catalytic mechanism of the catalytic antibody, detailed kinetic studies were carried out in this paper. Investigations into the deiodinative reaction revealed the relationship between the initial velocity and substrate concentration. The characteristic parallel Dalziel plots demonstrated that Se-4C5-catalyzed reaction mechanism was ping-pong one, involving at least one covalent enzyme intermediate. The kinetic properties of the catalytic antibody were similar to those of DI, with Km values for T4 and DTT of approximately 0.8 microm and 1.8 mm, respectively, and a Vm value of 270 pmol per mg of protein per min. The activity could be sensitively inhibited by 6-propyl-2-thiouracil (PTU) with a K(i) value of approximately 120 microm at 2.0 microm T4 concentration. The PTU inhibition was progressively alleviated with the increasing concentration of added DTT, revealing that PTU was a competitive inhibitor for DTT.

Human catalytic antibodies with glutathione peroxidase activity

In order to generate catalytic antibodies with glutathione peroxidase (GPx) activity, we prepared GSH-S-DNP butyl ester and GSH-S-DNP benzyl ester as the haptens. Two ScFvs that bound specifically to the haptens were selected from the human phage-displayed antibody library. The two ScFv genes were highly homologous, consisting of 786 bps and belonging to the same VH family-DP25. In the premise of maintaining the amino acid sequence, mutated plasmids were constructed by use of the mutated primers in PCR, and they were over-expressed in E. coli. After the active site serine was converted into selenocysteine with the chemical modifying method, we obtained two human catalytic antibodies with GPx activity of 72.2U/micromol and 28.8U/micromol, respectively. With the aid of computer mimicking, it can be assumed that the antibodies can form dimers and the mutated selenocysteine residue is located in the binding site. Furthermore, the same Ping-Pong mechanism as the natural GPx was observed when the kinetic behavior of the antibody with the higher activity was studied.

A Selenium-Containing Catalytic Antibody with Type I Deiodinase Activity

Acting as a mimic of type I deiodinase (DI), a selenium-containing catalytic antibody (Se-4C5) prepared by converting the serine residues of monoclonal antibody 4C5 raised against thyroxine (T4) into selenocysteines, can catalyze the deiodination of T(4) to 3,5,3'-triiodothyronine (T(3)) with dithiothreitol (DTT) as cosubstrate. Investigations into the deiodinative reaction by Se-4C5 revealed the relationship between the initial velocity and substrate concentration was subjected to Michaelis-Menten equation and the reaction mechanism was ping-pong one. The kinetic properties of the catalytic antibody were a little similar to those of DI, with Km values for T(4) and DTT of approximately 0.8 microM and 1.8 mM, respectively, and V(m) value of 270 pmol per mg protein per min. The activity could be sensitively inhibited by PTU with a Ki value of approximately 120 microM at 2.0 microM of T(4) concentration, revealing that PTU was a competitive inhibitor for DTT.