Modeling of the 5'-deiodination of thyroxine by iodothyronine deiodinase: chemical corroboration of a selenenyl iodide intermediate

Extremely stable system of 1-haloselanyl-anthraquinones: experimental and theoretical investigations

Highly stable selanyl halides, 1-ATQSeX (X = I (1), Br (2) and Cl (3)), were prepared. The structures of 1, 2, 6 (1-ATQSeX: X = Me) and 7 (1-ATQBr) were determined. QC calculations were performed on 1-3, 4 (X = F), 5 (X = H), 6, 7 and 8 (X = SeATQ-1). The O⋯Se distances in 1-4 from the sum of the vdW radii of the atoms (Δr(Se, O1)) were less than -1 Å, in magnitude, which must be the driving force for the high stability. The O-*-Se interactions seem stronger in the order of 1 < 2 < 3 < 4. The intrinsic dynamic and static natures of O⋯Se and/or Se⋯X in 1-8 are elucidated by QTAIM dual functional analysis (QTAIM-DFA). The Se-*-I, Se-*-Br, Se-*-Cl and Se-*-F interactions in 1-4 are predicted to have the natures of covalent, TBP with CT, TBP with CT, and typical HB with covalency, respectively, whereas O-*-Ses in 1-4 are all predicted to have the nature of MC with CT. The Se-*-H, Se-*-CMe and Se-*-Se interactions in 5, 6 and 8 are all predicted to have the covalent nature, while O-*-Ses in 5, 6 and 8 are all predicted to have the nature of typical HB with no covalency. The E(2) values of 1-6 and 8 are calculated with NBO analysis, and correlate excellently with Δr(Se, O1), except for Se-*-F, for which E(2) is evaluated to be much larger. The E(2) values also correlate very well with Cii-1 for all Se-*-X in 1-4, although data from 5, 6 and 8 deviated from the correlation, where Cii is the diagonal element of the compliance (force) constant for the internal vibrations. The behaviour of the interactions is further examined based on the QTAIM-DFA parameters of θ and θp. The stabilizing effect is further confirmed by the calculations with the ν(CO) values analyzed carefully.

Highly Electrophilic Intermediates in the Bypass Mechanism of Glutathione Peroxidase: Synthesis, Reactivity, and Structures of Selenocysteine-Derived Cyclic Selenenyl Amides

Selenocysteine (Sec)-derived cyclic selenenyl amides, formed by the intramolecular cyclization of Sec selenenic acids (Sec-SeOHs), have been postulated to function as protective forms in the bypass mechanism of glutathione peroxidase (GPx). However, their chemical properties have not been experimentally elucidated in proteins or small-molecule systems. Recently, we reported the first nuclear magnetic resonance observation of Sec-SeOHs and their cyclization to the corresponding cyclic selenenyl amides by using selenopeptide model systems incorporated in a molecular cradle. Herein, we elucidate the structures and reactivities of Sec-derived cyclic selenenyl amides. The crystal structures and reactions toward a cysteine thiol or a 1,3-diketone-type chemical probe indicated the highly electrophilic character of cyclic selenenyl amides. This suggests that they can serve not only as protective forms to suppress the inactivation of Sec-SeOHs in GPx but also as highly electrophilic intermediates in the reactions of selenoproteins.

Modeling Selenoprotein Se-Nitrosation: Synthesis of a Se-Nitrososelenocysteine with Persistent Stability

The Se-nitrosation in selenoproteins such as glutathione peroxidase and thioredoxin reductase to produce Se-nitrososelenocysteines (Sec-SeNOs) has been proposed to play crucial roles in signaling processes mediated by reactive nitrogen species and nitrosative-stress responses, although chemical evidence for the formation of Sec-SeNOs has been elusive not only in proteins but also in small-molecule systems. Herein, we report the first synthesis of a Sec-SeNO by employing a selenocysteine model system that bears a protective molecular cradle. The Sec-SeNO was characterized using ¹H and ⁷⁷Se nuclear magnetic resonance as well as ultraviolet/visible spectroscopy and found to have persistent stability at room temperature in solution. The reaction processes involving the Sec-SeNO provide experimental information that serves as a chemical basis for elucidating the reaction mechanisms involving the SeNO species in biological functions, as well as in selenol-catalyzed NO generation from S-nitrosothiols.

A Simple Substitution on Thyroid Hormones Remarkably Alters the Regioselectivity of Deiodination by a Deiodinase Mimic

The regioselective deiodinations of L-thyroxine (T4) play key roles in the thyroid hormone homeostasis. These reactions are catalyzed by three isoforms of the selenoenzymes, iodothyronine deiodinases (Dio1, Dio2 and Dio3), which are highly homologous in nature. Dio1 mediates 5'- or 5-deiodinations of T4 to produce T3 and rT3, respectively. In contrast, Dio2 and Dio3 are selective to 5'- or 5-deiodination to produce T3 and rT3, respectively. Understanding of the regioselectivity of deiodination at the molecular level is important as abnormal levels of thyroid hormone have been implicated in various clinical conditions, such as hypoxia, myocardial infarction, neuronal ischemia and cancer. In this paper, we report that the electronic properties of the iodine atoms in thyroxine (T4) can be modulated through a simple substitution in the 4'-phenolic moiety. This leads to the change in the regioselectivity of deiodination by different small molecule mimics of Dio enzymes. By using this chemical approach, we also show that the substitution of a strong electron withdrawing group facilitates the removal of all four iodine atoms in the T4 derivative. Theoretical investigations on the hydrogen bonded adducts of T4 with imidazole indicate that the charge on the iodine atoms depend on the nature of hydrogen bond between the -OH group of T4 and the imidazole moiety. While the imidazole can act as either hydrogen bond acceptor (HBA) or hydrogen bond donor (HBD), the protonated imidazole acts exclusively as HBD in T4-imidazole complex. These studies support the earlier observations that the histidine residue at the active sites of the deiodinases play an important role not only in the substrate binding, but also in altering the regioselectivity of the deiodination reactions.

Modeling Type-1 Iodothyronine Deiodinase with Peptide-Based Aliphatic Diselenides: Potential Role of Highly Conserved His and Cys Residues as a General Acid Catalyst

Type-1 iodothyronine deiodinase (ID-1) catalyzes the reductive elimination of 5'-I and 5-I on the phenolic and tyrosyl rings of thyroxine (T4), respectively. Chemically verifying whether I atoms with different chemical properties undergo deiodination through a common mechanism is challenging. Herein, we report the modeling of ID-1 using aliphatic diselenide (Se-Se) and selenenylsulfide (Se-S) compounds. Mechanistic investigations of deiodination using the ID-1-like reagents suggested that the 5'-I and 5-I deiodinations proceed via the same mechanism through an unstable intermediate containing a Se⋅⋅⋅I halogen bond between a selenolate anion, reductively produced from Se-Se (or Se-S) in the compound, and an I atom in T4. Moreover, imidazolium and thiol groups, which may act as general acid catalysts, promoted the heterolytic cleavage of the C-I bond in the Se⋅⋅⋅I intermediate, which is the rate-determining step, by donating a proton to the C atom.

Deiodinases control local cellular and systemic thyroid hormone availability

Iodothyronine deiodinases (DIO) are a family of selenoproteins controlling systemic and local availability of the major thyroid hormone l-thyroxine (T4), a prohormone secreted by the thyroid gland. T4 is activated to the active 3,3'-5-triiodothyronine (T3) by two 5'-deiodinases, DIO1 and DIO2. DIO3, a 5-deiodinase selenoenzyme inactivates both the prohormone T4 and its active form T3. DIOs show species-specific different patterns of temporo-spatial expression, regulation and function and exhibit different mechanisms of reaction and inhibitor sensitivities. The main regulators of DIO expression and function are the thyroid hormone status, several growth factors, cytokines and altered pathophysiological conditions. Selenium (Se) status has a modest impact on DIO expression and translation. DIOs rank high in the priority of selenium supply to various selenoproteins; thus, their function is impaired only during severe selenium deficiency. DIO variants, polymorphisms, SNPs and rare mutations have been identified. Development of DIO isozyme selective drugs is ongoing. A first X-ray structure has been reported for DIO3. This review focusses on the biochemical characteristics and reaction mechanisms, the relationships between DIO selenoproteins and their importance for local and systemic provision of the active hormone T3. Nutritional, pharmacological, and environmental factors and inhibitors, such as endocrine disruptors, impact DIO functions.

Insights into the Mechanism of Human Deiodinase 1

The three isoenzymes of iodothyronine deiodinases (DIO1-3) are membrane-anchored homo-dimeric selenoproteins which share the thioredoxin-fold structure. Several questions regarding their catalytic mechanisms still remain open. Here, we addressed the roles of several cysteines which are conserved among deiodinase isoenzymes and asked whether they may contribute to dimerization and reduction of the oxidized enzyme with physiological reductants. We also asked whether amino acids previously identified in DIO3 play the same role in DIO1. Human DIO1 and 2 were recombinantly expressed in insect cells with selenocysteine replaced with cysteine (DIO1U126C) or in COS7 cells as selenoprotein. Enzyme activities were studied by radioactive deiodination assays with physiological reducing agents and recombinant proteins were characterized by mass spectrometry. Mutation of Cys124 in DIO1 prevented reduction by glutathione, while 20 mM dithiothreitol still regenerated the enzyme. Protein thiol reductants, thioredoxin and glutaredoxin, did not reduce DIO1U126C. Mass spectrometry demonstrated the formation of an intracellular disulfide between the side-chains of Cys124 and Cys(Sec)126. We conclude that the proximal Cys124 forms a selenenyl-sulfide with the catalytic Sec126 during catalysis, which is the substrate of the physiological reductant glutathione. Mutagenesis studies support the idea of a proton-relay pathway from solvent to substrate that is shared between DIO1 and DIO3.

Modeling of selenocysteine-derived reactive intermediates utilizing a nano-sized molecular cavity as a protective cradle

In the biological functions of selenoproteins, various highly reactive species formed by oxidative modification of selenocysteine residues have been postulated to play crucial roles. Representative examples of such species are selenocysteine selenenic acids (Sec-SeOHs) and selenocysteine selenenyl iodides (Sec-SeIs), which have been widely recognized as important intermediates in the catalytic cycle of glutathione peroxidase (GPx) and iodothyronine deiodinase, respectively. However, examples of even spectroscopic observation of Sec-SeOHs and Sec-SeIs in either protein or small-molecule model systems remain elusive so far, most likely due to their notorious instability. For the synthesis of small-molecule model compounds of these reactive species, it is essential to suppress their very facile bimolecular decomposition such as self-condensation and disproportionation. Here we outline a novel method for the synthesis of stable small-molecule model compounds of the selenocysteine-derived reactive species, in which a nano-sized molecular cavity is used as a protective cradle to accommodate the reactive selenocysteine unit. Stabilization by the molecular cradle led to the successful synthesis of Sec-SeOHs, which are stable in solution at low temperatures, and a Sec-SeI, which can be isolated as crystals. The catalytic cycle of GPx was investigated using the NMR-observable Sec-SeOH models, and all the chemical processes proposed for the catalytic cycle of GPx, including the bypass process from Sec-SeOH to the corresponding cyclic selenenyl amide, were experimentally confirmed. Detailed protocols for the syntheses of selenopeptide derivatives bearing the molecular cradle and for the spectroscopic monitoring of their reactions are provided.

Late-Stage Functionalization of the Periphery of Oligophenylene Dendrimers with Various Arene Units via Fourfold C-H Borylation

Late-stage functionalization of the periphery of oligophenylene dendrimers was efficiently achieved via site-selective C-H activation of a preconstructed, readily accessible dendron. By fourfold iridium-catalyzed C-H borylation followed by Suzuki-Miyaura cross-coupling, various arene units were introduced into the end points of the 1,3,5-phenylene-based hydrocarbon dendron. Coupling of the modified dendrons with a core unit, such as 2,6-dibromobenzoic acid derivatives, afforded the periphery-functionalized dendrimers that also have an endohedral functionality at the core position.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Isolable small-molecule cysteine sulfenic acid

A small-molecule cysteine sulfenic acid (Cys–SOH) with ‘shelf stability’ protected by a molecular cradle was synthesized by direct oxidation of a thiol with H₂O₂. Its crystal structure and biologically relevant reactivity were elucidated.

Halogen Bonding in Biomimetic Deiodination of Thyroid Hormones and their Metabolites and Dehalogenation of Halogenated Nucleosides

Thyroid hormones (THs) are key players in the endocrine system and play pivotal roles in carbohydrate and fat metabolism, protein synthesis, overall growth, and brain development. The thyroid gland predominantly produces thyroxine or 3,5,3',5'-tetraiodothyronine (T4) as a prohormone; three isoforms of a mammalian selenoenzyme-iodothyronine deiodinase (DIO1, DIO2 and DIO3)-catalyze the regioselective deiodination of T4 to produce biologically active and inactive metabolites. Whereas DIO1 catalyzes both 5- and 5'-deiodination of T4, DIO2 and DIO3 selectively mediate 5- and 5'-deiodination, respectively. In this review we discuss the regioselective deiodination of THs in the presence of organochalcogen compounds. Naphthalene-based compounds containing sulfur and/or selenium at the peri positions mediate regioselective 5-deiodination of THs, detailed mechanistic studies having revealed that the heterolytic cleavage of the C-I bond is facilitated by the formation of cooperative Se/S⋅⋅⋅I halogen bonds and Se/S⋅⋅⋅Se chalcogen bonds. We also discuss the biomimetic deiodination of several TH metabolites, including sulfated THs, iodothyronamines, and iodotyrosines. A brief discussion on the dehalogenation of halogenated nucleosides and nucleobases in the presence of organochalcogen compounds is also included.

Modeling Thioredoxin Reductase-Like Activity with Cyclic Selenenyl Sulfides: Participation of an NH⋅⋅⋅Se Hydrogen Bond through Stabilization of the Mixed Se-S Intermediate

At the redox-active center of thioredoxin reductase (TrxR), a selenenyl sulfide (Se-S) bond is formed between Cys497 and Sec498, which is activated into the thiolselenolate state ([SH,Se^- ]) by reacting with a nearby dithiol motif ([SH^Cys59 ,SH^Cys64 ]) present in the other subunit. This process is achieved through two reversible steps: an attack of a cysteinyl thiol of Cys59 at the Se atom of the Se-S bond and a subsequent attack of a remaining thiol at the S atom of the generated mixed Se-S intermediate. However, it is not clear how the kinetically unfavorable second step progresses smoothly in the catalytic cycle. A model study that used synthetic selenenyl sulfides, which mimic the active site structure of human TrxR comprising Cys497, Sec498, and His472, suggested that His472 can play a key role by forming a hydrogen bond with the Se atom of the mixed Se-S intermediate to facilitate the second step. In addition, the selenenyl sulfides exhibited a defensive ability against H₂ O₂ -induced oxidative stress in cultured cells, which suggests the possibility for medicinal applications to control the redox balance in cells.

The Colorful Diversity of Thyroid Hormone Metabolites

Since the discovery of L-thyroxine, the main secretory product of the thyroid gland, and its major metabolite T3, which exerts the majority of thyroid hormone action via ligand-dependent modulation of the function of T3 receptors in nuclei, mitochondria, and other subcellular compartments, various other T4-derived endogenous metabolites have been identified in blood and tissues of humans, animals, and early protochordates. This review addresses major historical milestones and experimental findings resulting in the discovery of the key enzymes of thyroid hormone metabolism, the three selenoprotein deiodinases, as well as the decarboxylases and amine oxidases involved in formation and degradation of recently identified endogenous thyroid hormone metabolites, i.e. 3-iodothyronamine and 3-thyroacetic acid. The concerted action of deiodinases 2 and 3 in regulation of local T3 availability is discussed. Special attention is given to the role of the thyromimetic "hot" metabolite 3,5-T2 and the "cool" 3-iodothyronamine, especially after administration of pharmacological doses of these endogenous thyroid hormone metabolites in various animal experimental models. In addition, available information on the biological roles of the two major acetic acid derivatives of thyroid hormones, i.e. Tetrac and Triac, as well as sulfated metabolites of thyroid hormones is reviewed. This review addresses the consequences of the existence of this broad spectrum of endogenous thyroid hormone metabolites, the "thyronome," beyond the classical thyroid hormone profile comprising T4, T3, and rT3 for appropriate analytical coverage and clinical diagnostics using mass spectrometry versus immunoassays for determination of total and free concentrations of thyroid hormone metabolites in blood and tissues.

Bacterial Tetrabromopyrrole Debrominase Shares a Reductive Dehalogenation Strategy with Human Thyroid Deiodinase

Enzymatic dehalogenation is an important and well-studied biological process in both the detoxification and catabolism of small molecules, many of which are anthropogenic in origin. However, dedicated dehalogenation reactions that replace a halogen atom with a hydrogen are rare in the biosynthesis of natural products. In fact, the debrominase Bmp8 is the only known example. It catalyzes the reductive debromination of the coral settlement cue and the potential human toxin 2,3,4,5-tetrabromopyrrole as part of the biosynthesis of the antibiotic pentabromopseudilin. Using a combination of protein crystallography, mutagenesis, and computational modeling, we propose a catalytic mechanism for Bmp8 that is reminiscent of that catalyzed by human deiodinases in the maintenance of thyroid hormones. The identification of the key catalytic residues enabled us to recognize divergent functional homologues of Bmp8. Characterization of one of these homologues demonstrated its debromination activity even though it is found in a completely distinct genomic context. This observation suggests that additional enzymes outside those associated with the tetrabromopyrrole biosynthetic pathway may be able to alter the lifetime of this compound in the environment.

Model study on trapping of protein selenenic acids by utilizing a stable synthetic congener

Model studies on the trapping reaction of protein selenenic acids were carried out with a stable primary-alkyl model compound.

Organoselenium in Nature

Selenium, among the naturally occurring elements, is nowadays considered the most relevant for the redox homeostasis of living systems. In this chapter, its role in plants, bacteria, and humans is scholarly discussed. Some plants have the possibility to accumulate this element, thus becoming a natural source for animals and humans, in which selenium is embedded in selenoproteins, as the 21^st amino acid, selenocysteine (l-Sec). The main classes of selenoenzymes (glutathione peroxidase, thioredoxin reductase, and iodothyronine deiodinases) are reported here and the molecular mechanism that characterizes their physiological action is discussed.

Novel thyroid hormone analogues, enzyme inhibitors and mimetics, and their action

Thyroid hormones (THs) play key roles in modulating the overall metabolism of the body, protein synthesis, fat metabolism, neuronal and bone growth, and cardiovascular as well as renal functions. In this review, we discuss on the thyroid hormone synthesis and activation, thyroid hormone receptors (TRs) and mechanism of action, applications of thyroid hormone analogues, particularly the compounds that are selective ligands for TRβ receptors, or enzyme inhibitors for the treatment of thyroidal disorders with a specific focus on thyroid peroxidase and iodothyronine deiodinases. We also discuss on the development of small-molecule deiodinase mimetics and their mechanism of deiodination, as these compounds have the potential to regulate the thyroid hormone levels.

Organoselenium compounds as mimics of selenoproteins and thiol modifier agents

Selenium is an essential trace element for animals and its role in the chemistry of life relies on a unique functional group: the selenol (-SeH) group. The selenol group participates in critical redox reactions. The antioxidant enzymes glutathione peroxidase (GPx) and thioredoxin reductase (TrxR) exemplify important selenoproteins. The selenol group shares several chemical properties with the thiol group (-SH), but it is much more reactive than the sulfur analogue. The substitution of S by Se has been exploited in organic synthesis for a long time, but in the last 4 decades the re-discovery of ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one) and the demonstration that it has antioxidant and therapeutic properties has renovated interest in the field. The ability of ebselen to mimic the reaction catalyzed by GPx has been viewed as the most important molecular mechanism of action of this class of compound. The term GPx-like or thiol peroxidase-like reaction was previously coined in the field and it is now accepted as the most important chemical attribute of organoselenium compounds. Here, we will critically review the literature on the capacity of organoselenium compounds to mimic selenoproteins (particularly GPx) and discuss some of the bottlenecks in the field. Although the GPx-like activity of organoselenium compounds contributes to their pharmacological effects, the superestimation of the GPx-like activity has to be questioned. The ability of these compounds to oxidize the thiol groups of proteins (the thiol modifier effects of organoselenium compounds) and to spare selenoproteins from inactivation by soft-electrophiles (MeHg⁺, Hg²⁺, Cd²⁺, etc.) might be more relevant for the explanation of their pharmacological effects than their GPx-like activity. In our view, the exploitation of the thiol modifier properties of organoselenium compounds can be harnessed more rationally than the use of low mass molecular structures to mimic the activity of high mass macromolecules that have been shaped by millions to billions of years of evolution.

The distribution and mechanism of iodotyrosine deiodinase defied expectations

Iodotyrosine deiodinase (IYD) is unusual for its reliance on flavin to promote reductive dehalogenation under aerobic conditions. As implied by the name, this enzyme was first discovered to catalyze iodide elimination from iodotyrosine for recycling iodide during synthesis of tetra- and triiodothyronine collectively known as thyroid hormone. However, IYD likely supports many more functions and has been shown to debrominate and dechlorinate bromo- and chlorotyrosines. A specificity for halotyrosines versus halophenols is well preserved from humans to bacteria. In all examples to date, the substrate zwitterion establishes polar contacts with both the protein and the isoalloxazine ring of flavin. Mechanistic data suggest dehalogenation is catalyzed by sequential one electron transfer steps from reduced flavin to substrate despite the initial expectations for a single two electron transfer mechanism. A purported flavin semiquinone intermediate is stabilized by hydrogen bonding between its N5 position and the side chain of a Thr. Mutation of this residue to Ala suppresses dehalogenation and enhances a nitroreductase activity that is reminiscent of other enzymes within the same structural superfamily.

Design of Radioiodinated Pharmaceuticals: Structural Features Affecting Metabolic Stability towards in Vivo Deiodination

Radioiodinated pharmaceuticals are convenient tracers for clinical and research investigations because of the relatively long half-lives of radioactive iodine isotopes (i.e., 123I, 124I, and 131I) and the ease of their chemical insertion. Their application in radionuclide imaging and therapy may, however, be hampered by poor in vivo stability of the C-I bond. After an overview of the use of iodine in biology and nuclear medicine, we present here a survey of the catabolic pathways for iodinated xenobiotics, including their biodistribution, accumulation, and biostability. We summarize successful rational improvements in the biostability and conclude with general guidelines for the design of stable radioiodinated pharmaceuticals. It appears to be necessary to consider the whole molecule, rather than the radioiodinated fragment alone. Iodine radionuclides are generally retained in vivo on sp2 carbon atoms in iodoarenes and iodovinyl moieties, but not in iodinated heterocycles or on sp3 carbon atoms. Iodoarene substituents also have an influence, with increased in vivo deiodination in the cases of iodophenols and iodoanilines, whereas methoxylation and difluorination improve biostability.

Modeling of the Bioactivation of an Organic Nitrate by a Thiol to Form a Thionitrate Intermediate

Thionitrates (R–SNO₂) have been proposed as key intermediates in the biotransformation of organic nitrates that have been used for the clinical treatment of angina pectoris for over 100 years. It has been proposed and widely accepted that a thiol would react with an organic nitrate to afford a thionitrate intermediate. However, there has been no example of an experimental demonstration of this elementary chemical process in organic systems. Herein, we report that aryl- and primary-alkyl-substituted thionitrates were successfully synthesized by the reaction of the corresponding lithium thiolates with organic nitrates by taking advantage of cavity-shaped substituents. The structure of a primary-alkyl-substituted thionitrate was unambiguously established by X-ray crystallographic analysis.

A DFT investigation of a bulky biomimetic model catalyzing the 5'-outer ring deiodination of thyroxine

This paper illustrates the outcomes of a density functional theory investigation aimed at unraveling mechanistic aspects of the 5'-outer ring deiodination process of thyroxine (T4) assisted by the sterically protected organoselenol compound BpqSeH. BpqSeH, which was previously synthesized and tested for its deiodinase activity, is able to afford the active hormone 3,5,3'-tetraiodothyronine (T3) by selective outer-ring deiodination of T4, and to protect the SeH moiety inside the nano-sized molecular cavity from further reactivity, allowing its isolation and characterization. Calculations were also performed including an imidazole ring that, mimicking a His residue in the active site of the original enzyme, plays an crucial role in deprotonating the selenol moiety. Both the suggested enol/keto tautomerization and the previously proven formation of an intermediate whose main characteristic is the presence of a Se⋯I⋯C halogen bond, were examined along the pathway leading to 5'-outer ring deiodination. The calculated potential energy surface showed that neither the pathway encompassing enol/keto tautomerism nor the formation of a halogen bond paving the way to C-I bond breaking and chalcogen-I bond forming is viable. The exergonic formation of the final selenenyl iodide product confirms the stabilization effect of the molecular cavity. Graphical Abstract Computed free energy profile describing the 5'-outer deiodination of thyroxine assisted by the steric hindered organoselenol BpqSH compound. The molecular electrostatic potential map reoported for the INT1 intermediate shows the non-covalent Se-I interaction, due to the attraction between charges of opposite sign, that weakens the C-I bond and prepares the formation of the new Se-I bond.

Synthesis of a Stable Primary-Alkyl-Substituted Selenenyl Iodide and Its Hydrolytic Conversion to the Corresponding Selenenic Acid

A primary-alkyl-substituted selenenyl iodide was successfully synthesized through oxidative iodination of a selenol with N-iodosuccinimide by taking advantage of a cavity-shaped steric protection group. The selenenyl iodide exhibited high thermal stability and remained unchanged upon heating at 100 °C for 3 h in [D₈]toluene. The selenenyl iodide was reduced to the corresponding selenol by treatment with dithiothreitol. Hydrolysis of the selenenyl iodide under alkaline conditions afforded the corresponding selenenic acid almost quantitatively, corroborating the chemical validity of the recent proposal that hydrolysis of a selenenyl iodide to a selenenic acid is potentially involved in the catalytic mechanism of an iodothyronine deiodinase.

New insights into the structure and mechanism of iodothyronine deiodinases

Iodothyronine deiodinases are a family of enzymes that remove specific iodine atoms from one of the two aromatic rings in thyroid hormones (THs). They thereby fine-tune local TH concentrations and cellular TH signaling. Deiodinases catalyze a remarkable biochemical reaction, i.e., the reductive elimination of a halogenide from an aromatic ring. In metazoans, deiodinases depend on the rare amino acid selenocysteine. The recent solution of the first experimental structure of a deiodinase catalytic domain allowed for a reappraisal of the many mechanistic and mutagenesis data that had been accumulated over more than 30 years. Hence, the structure generates new impetus for research directed at understanding catalytic mechanism, substrate specificity, and regulation of deiodinases. This review will focus on structural and mechanistic aspects of iodothyronine deiodinases and briefly compare these enzymes with dehalogenases, which catalyze related reactions. A general mechanism for the selenium-dependent deiodinase reaction will be described, which integrates the mouse deiodinase 3 crystal structure and biochemical studies. We will summarize further, sometimes isoform-specific molecular features of deiodinase catalysis and regulation, and we will then discuss available compounds for modulating deiodinase activity for therapeutic purposes.

Dynamic and static behavior of the E–E′ bonds (E, E′ = S and Se) in cystine and derivatives, elucidated by AIM dual functional analysis

AIM-DFA (AIM dual functional analysis) is applied to the E–E′ bonds (E, E′ = S and Se) in R-cystine (1), its derivatives and MeEE′Me. The nature of E–E′ is elucidated by (θ_p, κ_p: dynamic behavior) and (R, θ: static behavior), through AIM-DFA.

Crystal structure of mammalian selenocysteine-dependent iodothyronine deiodinase suggests a peroxiredoxin-like catalytic mechanism

Local levels of active thyroid hormone (3,3',5-triiodothyronine) are controlled by the action of activating and inactivating iodothyronine deiodinase enzymes. Deiodinases are selenocysteine-dependent membrane proteins catalyzing the reductive elimination of iodide from iodothyronines through a poorly understood mechanism. We solved the crystal structure of the catalytic domain of mouse deiodinase 3 (Dio3), which reveals a close structural similarity to atypical 2-Cys peroxiredoxin(s) (Prx). The structure suggests a route for proton transfer to the substrate during deiodination and a Prx-related mechanism for subsequent recycling of the transiently oxidized enzyme. The proposed mechanism is supported by biochemical experiments and is consistent with the effects of mutations of conserved amino acids on Dio3 activity. Thioredoxin and glutaredoxin reduce the oxidized Dio3 at physiological concentrations, and dimerization appears to activate the enzyme by displacing an autoinhibitory loop from the iodothyronine binding site. Deiodinases apparently evolved from the ubiquitous Prx scaffold, and their structure and catalytic mechanism reconcile a plethora of partly conflicting data reported for these enzymes.

An ebselen like catalyst with enhanced GPx activity via a selenol intermediate

Benzamide ring-substituted, quinine-derived ebselen analogue is synthesized which exists in selenol form upon addition of PhSH. It catalyses oxidation of PhSH with H₂O₂ faster (10³-fold) than ebselen.

The reactions of para-halo diaryl diselenides with halogens. A structural investigation of the CT compound (p-FC6H4)2Se2I2, and the first reported “RSeI3” compound, (p-ClC6H4)SeI·I2, which contains a covalent Se-I bond

The reactions of the diaryl-diselenides (p-FC(6)H(4))(2)Se(2) and (p-ClC(6)H(4))(2)Se(2) with diiodine have been investigated. Species of stoichiometry "RSeI" are formed when the ratio employed is 1:1. The solid-state structure of "(p-FC(6)H(4))SeI" has been determined, and shown to be a charge-transfer (CT) adduct, (p-FC(6)H(4))(2)Se(2)I(2), where the Se-Se bond is retained and the diiodine molecule interacts with only one of the selenium atoms. The Se-I bond in (p-FC(6)H(4))(2)Se(2)I(2) is 2.9835(12) Å, which is typical for a (10-I-2) Se-I-I CT system. When diiodine is reacted in a 3:1 ratio with (p-XC(6)H(4))(2)Se(2) (X = F, Cl) species of stoichiometry "RSeI(3)" are formed. The structure of "(p-ClC(6)H(4))SeI(3)" reveals that this is not a selenium(IV) compound, but is better represented as a selenium(II) CT adduct, (p-ClC(6)H(4))SeI·I(2). The Se-I bond to the diiodine molecule is typical in magnitude for a CT adduct, Se-I: 2.8672(5) Å, whereas the other Se-I bond is much shorter, Se-I: 2.5590(6) Å, and is a genuine example of a rarely observed covalent Se-I bond, which appears to be stabilised by a weak Se···I interaction from a neighbouring iodine atom. The reaction of (p-ClC(6)H(4))SeI with Ph(3)P results in the formation of a CT adduct, Ph(3)PSe(p-ClC(6)H(4))I, which has a T-shaped geometry at selenium (10-Se-3). By contrast, the reaction of (p-FC(6)H(4))SeI with Ph(3)P does not form an adduct, but results in the formation of Ph(3)PI(2) and (p-FC(6)H(4))(2)Se(2).

Structural isomerism in (p-XC6H4)SeCl3 and (p-XC6H4)SeBr3 (X = F, Cl) compounds. Co-crystallisation of cis- and trans-dimeric forms of (p-ClC6H4)SeCl2(μ-Cl)2(p-ClC6H4)SeCl2. A new structural modification for the "PhSeBr" reagent, Ph2Se2Br2, containing an elongated Se-Se bond

A series of di(para-halophenyl)diselenides, (p-XC(6)H(4))(2)Se(2) (X = F, Cl) have been reacted with three equivalents of SO(2)Cl(2) or Br(2), leading to the formation of selenium(iv) RSeX(3) compounds. The structures of (p-FC(6)H(4))SeX(3) (X = Cl, Br) have been determined, and both exhibit a dimeric RSeX(2)(μ-X)(2)RSeX(2) structure consisting of two "saw-horse" (p-FC(6)H(4))SeX(3) units linked by two halide bridges, with an overall square pyramidal geometry at selenium. In both structures all the selenium and halogen atoms are planar, with both aryl rings located on the same side of the Se(2)X(6) plane (cis-isomer). The structure of (p-ClC(6)H(4))SeCl(3) also adopts a planar dimeric structure, however both cis- and trans-dimeric molecules are co-crystallised in the unit cell. In contrast, the structure of (p-ClC(6)H(4))SeBr(3) adopts a folded cis-dimeric structure due to steric constraints. Secondary Se···X interactions to the "vacant" sixth coordination site at selenium are a feature of most of these structures, but are most prominent in the folded structure of (p-ClC(6)H(4))SeBr(3). A re-examination of the PhSeBr/PhSeBr(3) system resulted in the isolation of crystals of a second structural form of "PhSeBr". The structure of Ph(2)Se(2)Br(2) consists of two PhSeBr units linked by an elongated Se-Se bond of 2.832(4) Å, and longer secondary Se···Br interactions of 3.333(4) Å to form a chain structure. Further weak Se···Br and Br···Br interactions are present, which form loosely linked rippled sheets of selenium and bromine atoms, similar to the sheets observed for the tetrameric form, Ph(4)Se(4)Br(4).

Synthesis of functionalized macrocyclic derivatives of trioxabicyclo[3.3.0]nonadiene

C₇₂-Macrocyclic systems functionalized with nitroaryl and arylamino groups were synthesized from the bisdioxine diacid dichloride 1,3,5,7-tetra-tert-butyl-2,6,9-trioxabicyclo[3.3.1]nona-3,7-diene-4,8-dicarbonyl dichloride (3).

Regioselective deiodination of thyroxine by iodothyronine deiodinase mimics: an unusual mechanistic pathway involving cooperative chalcogen and halogen bonding

Iodothyronine deiodinases (IDs) are mammalian selenoenzymes that catalyze the conversion of thyroxine (T4) to 3,5,3'-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) by the outer- and inner-ring deiodination pathways, respectively. These enzymes also catalyze further deiodination of T3 and rT3 to produce a variety of di- and monoiodo derivatives. In this paper, the deiodinase activity of a series of peri-substituted naphthalenes having different amino groups is described. These compounds remove iodine selectively from the inner-ring of T4 and T3 to produce rT3 and 3,3'-diiodothyronine (3,3'-T2), respectively. The naphthyl-based compounds having two selenols in the peri-positions exhibit much higher deiodinase activity than those having two thiols or a thiol-selenol pair. Mechanistic investigations reveal that the formation of a halogen bond between the iodine and chalcogen (S or Se) and the peri-interaction between two chalcogen atoms (chalcogen bond) are important for the deiodination reactions. Although the formation of a halogen bond leads to elongation of the C-I bond, the chalcogen bond facilitates the transfer of more electron density to the C-I σ* orbitals, leading to a complete cleavage of the C-I bond. The higher activity of amino-substituted selenium compounds can be ascribed to the deprotonation of thiol/selenol moiety by the amino group, which not only increases the strength of halogen bond but also facilitates the chalcogen-chalcogen interactions.