Catalytic Application of Selenium and Tellurium Compounds as Glutathione Peroxidase Enzyme Mimetics

A glutationa peroxidase (GPx) é uma importante enzima que faz parte do sistema de defesa do organismo frente a substâncias nocivas formadas durante o metabolismo do oxigênio, como peróxidos e seus derivados. Diversas estratégias para desenvolver novos miméticos, assim como para aumentar a atividade mimética da GPx em compostos como disselenetos, selenetos e teluretos, vêm sendo propostas nos últimos anos. Nesta revisão é apresentado um balanço, dos últimos dez anos, referente ao desenvolvimento e à avaliação de catalisadores organoselênio ou organotelúrio capazes de mimetizar a atividade da GPx. Diferentes mecanismos de ação destes compostos também são apresentados, de acordo com os novos avanços desta relevante área de pesquisa.


Introduction
Interest in organochalcogen compounds has been growing since the 1970s, when many reports described the identification of various selenoproteins, which are involved in a widely number of mammals' biochemistry mechanisms. 1 Synthetic developments and the design of new organoselenium compounds have been attracting considerable attention, particularly due to their ability to mimic natural compounds with important biological properties (e.g., antioxidant, antitumor, anti-inflammatory and anti-infective activity). 2elenoenzymes constitute important mammalian antioxidant enzymes which protect biomembranes and other cellular components from oxidative stress.The oxidative stress is associated to the activity of peroxides and byproducts derived from them, which are produced during the metabolism of oxygen in aerobic cells. 3These substances are known as reactive oxygen species (ROS) and are harmful substances, which destroy key biological components and cause damage to cell membranes.Several diseases, including neurodegenerative diseases, such as Alzheimer's and Parkinson's disease and other physiological and inflammatory processes are linked to the activity of ROS. 4 As part of a complex and sophisticated detoxification system, the enzyme glutathione peroxidase (GPx) plays a pivotal role catalyzing the reduction of hazardous peroxides and their byproducts to water or alcohols. 5The most important amino acid in the active site of the enzyme is L-selenocysteine 1 which is responsible for reducing hydroperoxides at the expense of the tripeptide glutathione GSH 2. 6 The ability that the enzyme has to promote the reduction of peroxides lies in the redox properties of the selenol moiety of L-selenocysteine, the catalytic cycle of GPx is shown in Scheme 1.Initially, the selenol functionality in the enzyme E-SeH 3 reacts with a peroxide molecule to generate the corresponding alcohol or water and selenenic acid E-SeOH 4. The latter then reacts with one equivalent of glutathione to produce water and the corresponding selenenyl sulfide E-Se-SG 5.The last step is the reaction of glutathione with selenenyl sulfide producing the oxidized glutathione (GSSG) and regenerating the reduced enzyme 3 to resume the catalytic cycle. 7e total process consists of the reduction of one equivalent of a reactive oxygen species at the expense of two equivalents of glutathione, producing two equivalents of water (when R = H) and oxidized glutathione (GSSG) Scheme 2.
Since the discovery that selenium plays a pivotal role in GPx enzymes, design and development of new synthetic chalcogen-based catalytic antioxidants have attracted considerable attention.Small molecules of organoselenium and organotellurium compounds have emerged as excellent candidates to act as GPx mimics, due to their well-known ability to undergo two-electron redox cycle between chalcogen (II) and (IV) species. 8Although a wide range of structurally diverse organoselenides and tellurides have been investigated by several groups over the past forty years, 9 the aim of this manuscript is to cover the latest contributions in the field of new organoselenium and organotellurium compounds designed for pronounced glutathione peroxidase like catalytic activity.

Organoselenium compounds
The first synthetic molecule found to be able to mimic the GPx activity was the compound known as ebselen 6, Figure 2.This heterocyclic compound exhibits anti-inflammatory, anti-atherosclerotic and cytoprotective properties with relatively low toxicity. 10ecause of these properties, ebselen is used as a standard for comparison with selenium compounds in terms of GPx like activity.
However, the exact mechanism of its activity has not been elucidated, and this topic is subject of continuous research.Furthermore, its poor solubility in water remains a problem for optimal therapeutic development.In order to enhance its solubility and to increase its activity, research has been focused on the modification of the structure of ebselen.Several structural modifications, including substituent effects and isosteric replacement, have been proposed over the years. 11ecently, Singh et al. 12 have proposed the synthesis and evaluation of ebselen derivatives 7a-c, Figure 2.With exception of compound 7a, these compounds possessing an ortho-coordinating amide group are more active as a GPx mimic than the parent ebselen. 12The enhanced activity of these derivatives is attributed to the intramolecular nonbonding interactions between the oxygen of the amide moiety and selenium.These findings are supported by the elucidation of the crystal X-ray structure of the tested   Catalytic Application of Selenium and Tellurium Compounds as Glutathione Peroxidase Enzyme Mimetics J. Braz.Chem.Soc.2034 compounds.Messali and collaborators reported in 2007 the preparation of six-membered ring homologues of ebselen 8a-c.The authors showed that compound 8b increased the rate of reduction of H 2 O 2 compared to ebselen as a control drug. 13ncouraged by the successful results found for ebselen as a powerful GPx mimic, an intensive research effort has been made toward the design of new classes of organoselenium compounds with catalytic activity as GPx like drugs.In 1989 Spector et al. 14 described the enhanced activity of diphenyl diselenide 9, Figure 3, compared to ebselen. 14The same group also showed that the substituted diselenide 10 with a chelating group is 5-fold more active than PhSe) 2 in this assay.
A further approach was described by Singh and coauthors, 15 diselenide 11 and the selenenate 12, which was synthesized from 11 via halogenation in water, had initial rates approximately 7 and 6-fold enhanced, respectively, compared with the catalyst 10. 15 The same author also studied the behavior of diselenide 13 with intramolecular Se•••O nonbonding interactions and cyclic seleninate ester 14, which is conveniently produced by oxidation of 13 with H 2 O 2 . 16Both substances exhibited better GPx like activity than ebselen.
Mugesh and co-authors, 17 showed that the ability of diaryl diselenides incorporating tertiary amino groups to mimic GPx can be dramatically changed by simple replacement of a hydrogen atom with a methoxy group in compounds 15a-c and 16a-c, Figure 4. 17 The study suggested that methoxy substituents ortho to selenium in N,N-dialkylbenzylamine-based compounds make the basicity of the amino groups better for the catalysis.Moreover, the presence of OMe groups prevents possible Se•••N interactions in the selenols, increasing their zwitterionic characters.The methoxy substituents also protect the selenium in the selenenic acid intermediates from overoxidation to seleninic acids or irreversible inactivation to selenonic acid derivatives.Later, the same group also compared the previously mentioned diselenides possessing a tertiary amino group 15a-c with diselenides assembled with secondary amino substituents 17a-c. 18heir findings revealed that replacement of the tert-amino groups in benzylamine-based diselenides by sec-amino moieties drastically enhances the catalytic activity of the studied compounds.This distinct behavior was attributed to differences in the stability and reactivity of some of the key intermediates which account for the GPx like activity.It was observed that the sec-amino groups were better than the tert-amino moieties for generating the catalytically active selenols.Furthermore, the seleninic acids (RSeO 2 H) derived from the sec-amine based compounds were more stable toward further reactions with peroxides than their tert-amine based analogues.
In 2009, Braga et al. 19 described the synthesis of compounds 18a-c, a new class of chiral diselenides derived from amino acids. 19It was observed that the influence of the chain length between the diselenide moiety and amino acid residue played a crucial role in modulating the GPx like activity of the tested diselenides.Compounds 18c and 18b derived from L-phenylalanine with a longer chain length, showed better results than PhSe) 2 , while compound 18a, with a shorter chain length, was the least effective catalyst screened.
Perhaps in the past decade the most studied organoselenium compounds with ability to mimic the enzyme glutathione peroxidase were the selenides and their derivatives.The first example was designed by Engman and collaborators in 1994. 20They tested compound 19, Figure 5, as a GPx mimic.Actually, 19 is a pro-catalyst, the true active compound is PhSe) 2 , formed by cleavage of the aliphatic carbon-selenium bond of 19.Latter, the same group described the synthesis and evaluation of a series of organochalcogenides 20. 21Although selenide 20 showed some GPx like activity, it was a poorer catalyst as compared to the telluride analogue.More recently, they disclosed their findings in the preparation of a new series of selenides 21a-b, derived from quinoline. 22hile selenide 21b was a catalyst comparable to PhSe) 2 , 21a possessing a ethylseleno moiety was approximately 3-fold more active than PhSe) 2 .
Over the past few years, Back and collaborators have been working extensively on the preparation and screening of a number of organoselenium compounds with enhanced ability to mimic the behavior of the enzyme glutathione peroxidase.In 2002, they reported the preparation of allyl selenide 22, Figure 5.They observed that 22 actually was a pro-catalyst that generates the unusual cyclic seleninate ester 23 through a series of oxidation reactions and sigmatropic rearrangement steps upon the GPx like reaction conditions. 23The novel heterocycle 23 was significantly more effective as a GPx mimic than ebselen 6.Furthermore, the same group also described the synthesis of selenide 24.Similarly to selenide 22 the experiments revealed that selenide 24 was an exceptional catalyst, much better than the widely studied compound ebselen 6 under the same conditions.Notable, they found that selenide 24 was a proactive compound, delivering the unusual spirodioxaselenanonane 25 as the real catalyst during the GPx like cycle. 24ue to the toxicity associated to alkylselenides, the authors reported latter an extension of the earlier investigation with aromatic congeners of 23 and 25, namely 26, 27, 28 and 29, Figure 6. 25 However, the new compounds proved to be inferior catalysts compared to the parent cyclic seleninate ester 23 and spirodioxyselenurane 25.In 2008, after a judicious screening of substituent effects in aromatic seleninate esters 28a-g and spirodioxyselenurane 29a-g Back et al. 26 found that the ability of both classes of compounds to mimic the enzyme GPx could be conveniently enhanced by the addition of electron donating groups in the framework of 28 and 29. 26Especially noteworthy was the fact that the methoxy substituted spirodioxyselenurane 28g displayed a catalytic activity superior to that of all of the other aromatic compounds and approaches that of the aliphatic analogue 23.Another approach was described in the synthesis and characterization of spirodiazaselenurane 30, possessing a covalent N-Se bond, as well as a noncovalent interaction between the selenium atom and the carbonyl oxygen atom of the other amide moiety as determined by spectroscopic and X-ray crystallographic methods. 27The synthesized compound functioned as an effective GPx mimetic, with catalytic activity superior to that of ebselen 6.
Mugesh et al. 28 proposed the synthesis of aryl-substituted L-selenocysteine derivative 31 containing a basic amino group, Figure 7. 28 Their results supported that amino groups in close proximity to selenium could increase the stability of 31 derivatives by intramolecular Se•••N interactions.These interactions appear to modulate the reactivity of selenium toward the GPx reaction conditions.They also suggested that the internally chelated L-Selenocysteine derivative 31 could be used as pro-catalyst for generating catalytically active selenols.The fast and mild oxidative elimination of 31 derivatives to produce dehydroalanines, therefore, may give easy access to biologically active selenium compounds.Kuhn and coauthors 29 have demonstrated that seleninic acid anhydride 32, derived from salicyloylglycine exhibits a higher glutathione peroxidase like activity, fourfold higher than ebselen. 29Very recently, Iwaoka et al. 30 investigated the antioxidative catalytic activities of trans-3,4-dihydroxyselenolane 33, a water soluble cyclic selenide. 30Although the observed GPx like catalytic activities were not so remarkable as those of previous GPx model compounds, the water solubility of 33 certainly would be useful not only for the molecular design of new GPx mimics with enhanced antioxidative catalytic activity but also for biological applications as an efficient catalyst for the formation of sulfur-sulfur linkages in proteins.

Organotellurium compounds
Based on the recognized success that synthetic organoselenium compounds exhibited acting as GPx like substances, and the redox properties of tellurium, several research groups have also investigated the ability of organotellurium compounds to mimic the enzyme glutathione peroxidase.A noteworthy characteristic of organotellurium compounds is their much improved antioxidant activity when compared with their selenium and sulfur analogues.The first example of organotellurium compound 34, Figure 8, described as a possible GPx mimic, was reported by Detty in 1992. 31After that, a series of ditellurides and tellurides have been reported in the literature.Engman et al. 21have shown that the antioxidant capacity of telluride 35 is drastically increased when compared with their selenium congeners 20. 21Later, in 2004, the same group described the synthesis and GPx like behavior of water soluble cyclodextrin derivatives containing tellurium, 36a-e. 32Among organotellurides 36a-e, the cyclodextrin 36e carrying a butyltelluro group was the best catalyst.They also found that the catalytic efficiency of aryl derivatives decreased with decreasing electron density at tellurium (36d > 36c @ 36b > 36a).Furthermore, the selenium analogue, showed poor catalytic efficiency.
Following their initial findings, Engman and co-authors 22 described the preparation and screening of tellurides 37a-b.Similarly to previous reports, they found that telluride possessing an alkyl moiety 37a was a superior GPx mimic compared to the telluride with an aryl substituent 37b.However, both tellurides 37a-b, had an enhanced glutathione peroxidase activity when compared to their selenium analogues. 22he same group recently described the preparation and the structure-reactivity relationship as GPx mimics of two series of tellurides, 38a-b and 39a-d, Figure 9. 33 Their results indicated that both the proximity of chelating oxygen near to the active site and electronic effects are crucial for the catalytic performance of the tested compounds.Telluride 39a, carrying a hydroxyl group ortho to tellurium was a superior GPx mimic than 38a.The increase of the number of electron donating groups attached in the pyridyl ring also had a positive effect in the efficiency of the compounds.The most active was compound 39d, carrying a methoxy group in para position to tellurium, the GPx like activity of this telluride is 25-fold enhanced when compared with PhSe) 2 .
Independently to Engman's results, Detty and coauthors have demonstrated that the GPx like behavior of tellurides is strongly modulated by alkyl groups attached in the tellurium atom. 34Electron rich telluride 41a displayed greater rate acceleration than the diaryl tellurides 40 and 41b as determined by the initial rates.While compounds 41b and 40 are comparable catalysts, with an increased activity of 13-fold and 12-fold higher then PhSe) 2 , respectively, telluride 41a is approximately 5-fold more effective than them, and 55-fold more active than PhSe) 2 .
A common strategy to evaluate the GPx like activity of organotellurides is to compare their efficiency to their selenium congeners.According to this, Singh et al. 12 prepared a series of novel ebselen derivatives, and also a  tellurium analogue 42, Figure 10. 12 Telluride 42 showed a 1.5-fold higher activity than its selenium analogues 7a-c and approximately 5-fold better activity then ebselen.In the same way, Back and collaborators, 25 have prepared several novel organo-selenium and tellurium compounds and evaluated them as mimetics of the selenoenzyme glutathione peroxidase. 25The cyclic tellurinate ester 43 and spirodioxytellurane 44 proved to be a superior catalysts to their selenium analogues 23 and 25, respectively, resulting in the fastest reaction rates by far of all of the compounds that they have investigated to date.
Recently our group described the synthesis and GPx like screening of novel telluroaminoacid derivatives 45a-g, Figure 11. 35For a better understanding of the structure-activity relationship of the compounds and to find a more efficient catalyst, we promoted the variation of amino acid residues and the chain length between the chalcogen atom and the amino acid moiety.Our prime concern in the evaluation of compounds 45a-g as GPx mimics was the influence of the chain length between the tellurium atom and amino acid moiety.We found that compounds 45a and 45b derived from L-valine showed the same activity.Taking advantage of the modular characteristic of our synthetic route, we explored the influence of the amino acid residue in the modulation of the GPx mimetic activity of these telluroaminoacids.Compounds 45c and 45d derived from L-phenylalanine were tested under the same conditions.Compound 45c, with a shorter chain length, showed the same catalytic activity as that of compounds 45a and 45b.However, 45d was less effective in the reduction of peroxide.On the basis of these initial observations, we used telluroaminoacids with n = 2 as standard catalysts, and promoted the variation of amino acid moiety to enhance their ability to mimic the glutathione peroxidase enzyme.Among all compounds tested, we found that telluride 45g derived from L-aspartic acid was the best catalyst.
Encouraged by these results, we next explored the effects of substituents in the aryl group attached to tellurium.A new series of telluroaminoacids were prepared with electron donating 45i-k, electron withdrawing 45l and 45m, as well as with sterically hindered substituents 45n and 45o.We found that the GPx like behavior of these compounds was strongly influenced by steric effects.The activity of para substituted compounds was, on average, 2-fold higher than that of the ortho analogues.Although the electronic environment at tellurium atom did not produce a pronounced difference in the thiol peroxidase activity of these compounds, the best telluride screened in our assay was compound 45k, with a methoxy group attached in the para position of the aromatic ring.

Mechanisms of action of organochalcogen compounds as GPx like catalysts
Additionally to the design of new organochalcogen compounds able to mimic the activity of the enzyme glutathione peroxidase, the pursuit of a deeper understanding of the mechanism of action of these catalysts is an area of intensive research nowadays.Naturally, once different classes of organochalcogen compounds exhibit GPx like behavior, several distinct mechanisms have been postulated to explain their action.
In this context, selenides containing Se-N bonds were one of the first class of compounds investigated.Although ebselen 6, is one of the most studied compound as GPx   like, 36 its catalytic cycle has not been clearly elucidated.A possible mechanism pathway is illustrated in Scheme 3.
Initially, ebselen 6 reacts with a thiol (RSH) to produce the intermediate selenenyl sulfide 46.Reaction of the latter with another equivalent of thiol, delivers the selenol 47, which is the responsible for the reduction of the peroxide and formation of selenenic acid 48.At this point two different pathways are proposed: reaction with another equivalent of thiol to regenerate the selenenyl sulfide 46, or the loss of one molecule of water from selenenic acid 48, to give ebselen 6 to resume the catalytic cycle.Mugesh and coauthors have shown that one of the crucial steps in the catalytic cycle of ebselen is the formation of selenol 47.They found that the presence of strong S•••N or S•••O interactions in the selenenyl sulfide 46 can modulate the attack of an incoming nucleophile (thiol) at the sulfur atom of the Se-S bridge and enhance the GPx activity by reducing the barrier to the formation of the active specie selenol. 37nother important class of organoselenium compounds with enhanced GPx like activity is the diselenides.In 1994, Tomoda et al. 38 proposed a catalytic cycle to explain their pronounced ability to reduce peroxides at the expense of thiols. 38They postulated that the higher activity of diselenides as GPx mimics compared to ebselen is due to the formation of two active species in the catalytic cycle, the selenol 50 and the selenenyl sulfide 51 formed in the reaction of the diselenide 49 and a thiol, Scheme 4.
The mechanism of action of diselenides is depicted in Scheme 5.The active specie selenol 52 reacts with one equivalent of H 2 O 2 to produce water and selenenic acid 53.Reaction of selenenic acid with one equivalent of thiol produces water and selenenyl sulfide 54.Finally, 54 reacts with another equivalent of thiol to form disulfide and 52 to restart the catalytic cycle.
The enhanced GPx like activity of diselenides 52 carrying coordinating heteroatoms such as oxygen or nitrogen is ascribed to the stabilization of selenium by nonbonding interactions like Se•••N or Se•••O.These interactions play an important role modulating the activity of diselenides by preventing further oxidation by H 2 O 2 of selenium in the selenenic acid 53 to seleninic acid which is less reactive than the specie 53.
Selenides and tellurides have a similar mechanism of action, which is distinct to that of ebselen derivatives and diselenides, Scheme 6. Selenides and tellurides 55 are readily oxidized by peroxides to the corresponding selenoxides and telluroxides 56.These species are then reduced by thiol to regenerate the parent selenide or telluride through the specie 57. 39This facile redox cycle allows their use as peroxide decomposers.Detty and coauthors 34 have demonstrated that the rate-limiting step in thiol peroxidase like activity of tellurides is the oxidation of tellurides to the respective telluroxide rather than the reduction process of the intermediate 57. 34imilarly, Iwaoka and Tomoda 38 found identical behavior for selenides, following the progress of the reaction by a 77 Se NMR study, they ascribed that the rate-determining  step in the catalytic cycle is the oxidation process from selenide 55 to the selenoxide 56.

Conclusions
In the past decade the development of new organochalcogen compounds designed to mimic the activity of the selenoenzyme glutathione peroxidase has been a subject of intensive research.Diverse classes of organoselenides and tellurides were efficiently prepared and showed pronounced GPx like activity.Recent progress has also been made in the elucidation of their mechanism of action.Nonetheless, further improvements in the field are still an exciting challenge.A deeper understanding of the detailed mechanisms of action, the toxicity of these compounds in vivo trials and also the design of water soluble catalysts, are perhaps, the most promising frontiers in this subject.