Thyroxine binding to type III iodothyronine deiodinase.

Iodothyronine deiodinases (Dios) are important selenoproteins that control the concentration of the active thyroid hormone (TH) triiodothyronine through regioselective deiodination. The X-ray structure of a truncated monomer of Type III Dio (Dio3), which deiodinates TH inner rings through a selenocysteine (Sec) residue, revealed a thioredoxin-fold catalytic domain supplemented with an unstructured Ω-loop. Loop dynamics are driven by interactions of the conserved Trp207 with solvent in multi-microsecond molecular dynamics simulations of the Dio3 thioredoxin(Trx)-fold domain. Hydrogen bonding interactions of Glu200 with residues conserved across the Dio family anchor the loop's N-terminus to the active site Ser-Cys-Thr-Sec sequence. A key long-lived loop conformation coincides with the opening of a cryptic pocket that accommodates thyroxine (T4) through an I⋯Se halogen bond to Sec170 and the amino acid group with a polar cleft. The Dio3-T4 complex is stabilized by an I⋯O halogen bond between an outer ring iodine and Asp211, consistent with Dio3 selectivity for inner ring deiodination. Non-conservation of residues, such as Asp211, in other Dio types in the flexible portion of the loop sequence suggests a mechanism for regioselectivity through Dio type-specific loop conformations. Cys168 is proposed to attack the selenenyl iodide intermediate to regenerate Dio3 based upon structural comparison with related Trx-fold proteins.

Chalcogen bonding interactions between reducible sulfur and selenium compounds and models of zinc finger proteins.

Reducible sulfur and selenium (r-S/Se) compounds, defined as sulfur and selenium compounds not in the lowest -2 oxidation state (e.g., -1 to +6), release Zn(2+) from zinc-sulfur proteins such as zinc fingers (ZFs) and metallothionein. A series of density functional theory calculations was performed on donor-acceptor complexes between r-S/Se compounds and models of the Cys2His2, Cys3His and Cys4 ZF sites. These S⋯S/Se chalcogen bonding interactions consist of the donation of electron density from a S lone pair on the ZF model to a S/Se-X antibonding molecular orbital of the r-S/Se compound. The strength of the interaction was shown to be dependent upon the Lewis basicity of the ZF model (Cys4>Cys3His>Cys2His2) and the Lewis acidity of the r-S/Se compound as measured by the energy of the S/Se-X antibonding orbital. Interactions with the softer r-Se compounds were stronger than the r-S compounds, consistent with the greater reactivity of the former with ZF proteins.

Sulfur and selenium antioxidants: challenging radical scavenging mechanisms and developing structure-activity relationships based on metal binding.

Because sulfur and selenium antioxidants can prevent oxidative damage, numerous animal and clinical trials have investigated the ability of these compounds to prevent the oxidative stress that is an underlying cause of cardiovascular disease, Alzheimer's disease, and cancer, among others. One of the most common sources of oxidative damage is metal-generated hydroxyl radical; however, very little research has focused on determining the metal-binding abilities and structural attributes that affect oxidative damage prevention by sulfur and selenium compounds. In this review, we describe our ongoing investigations into sulfur and selenium antioxidant prevention of iron- and copper-mediated oxidative DNA damage. We determined that many sulfur and selenium compounds inhibit Cu(I)-mediated DNA damage and that DNA damage prevention varies dramatically when Fe(II) is used in place of Cu(I) to generate hydroxyl radical. Oxidation potentials of the sulfur or selenium compounds do not correlate with their ability to prevent DNA damage, highlighting the importance of metal coordination rather than reactive oxygen species scavenging as an antioxidant mechanism. Additional gel electrophoresis, mass spectrometry, and UV-visible studies confirmed sulfur and selenium antioxidant binding to Cu(I) and Fe(II). Ultimately, our studies established that both the hydroxyl-radical-generating metal ion and the chemical environment of the sulfur or selenium significantly affect DNA damage prevention and that metal coordination is an essential mechanism for these antioxidants.

Redox active motifs in selenoproteins.

Selenoproteins use the rare amino acid selenocysteine (Sec) to act as the first line of defense against oxidants, which are linked to aging, cancer, and neurodegenerative diseases. Many selenoproteins are oxidoreductases in which the reactive Sec is connected to a neighboring Cys and able to form a ring. These Sec-containing redox motifs govern much of the reactivity of selenoproteins. To study their fundamental properties, we have used (77)Se NMR spectroscopy in concert with theoretical calculations to determine the conformational preferences and mobility of representative motifs. This use of (77)Se as a probe enables the direct recording of the properties of Sec as its environment is systematically changed. We find that all motifs have several ring conformations in their oxidized state. These ring structures are most likely stabilized by weak, nonbonding interactions between the selenium and the amide carbon. To examine how the presence of selenium and ring geometric strain governs the motifs' reactivity, we measured the redox potentials of Sec-containing motifs and their corresponding Cys-only variants. The comparisons reveal that for C-terminal motifs the redox potentials increased between 20-25 mV when the selenenylsulfide bond was changed to a disulfide bond. Changes of similar magnitude arose when we varied ring size or the motifs' flanking residues. This suggests that the presence of Sec is not tied to unusually low redox potentials. The unique roles of selenoproteins in human health and their chemical reactivities may therefore not necessarily be explained by lower redox potentials, as has often been claimed.

Oxidation of biologically relevant chalcogenones and their Cu(I) complexes: insight into selenium and sulfur antioxidant activity.

Hydroxyl radical damage to DNA causes disease, and sulfur and selenium antioxidant coordination to hydroxyl-radical-generating Cu(+) is one mechanism for their observed DNA damage prevention. To determine how copper binding results in antioxidant activity, biologically relevant selone and thione ligands and Cu(+) complexes of the formula [Tpm*Cu(L)](+) [Tpm* = tris(3,5-dimethylpyrazolyl)methane; L = N,N'-dimethylimidazole selone or thione] were treated with H2O2 and the products analyzed by (1)H, (13)C{(1)H}, and (77)Se{(1)H} NMR spectroscopy, mass spectrometry, and X-ray crystallography. Upon H2O2 treatment, selone and thione binding to Cu(+) prevents oxidation to Cu(2+); instead, the chalcogenone ligand is oxidized. Thus, copper coordination by sulfur and selenium compounds can provide targeted sacrificial antioxidant activity.

Interactions of Cu(I) with selenium-containing amino acids determined by NMR, XAS, and DFT studies.

Cu(I) coordination by organoselenium compounds was recently reported as a mechanism for their prevention of copper-mediated DNA damage. To establish whether direct Se-Cu coordination may be involved in selenium antioxidant activity, Cu(I) coordination of the selenoamino acids methyl-Se-cysteine (MeSeCys) and selenomethionine (SeMet) was investigated. NMR results in D(2)O indicate that Cu(I) binds to the Se atom of both MeSeCys and SeMet as well as the carboxylic acid oxygen atom(s) or amine nitrogen atoms. X-ray absorption spectroscopy (XAS) and density functional theory (DFT) results confirm Se-Cu coordination, with the identification of a 2.4 Å Se-Cu vector in both the Se- and Cu-EXAFS data. XAS studies also show Cu(I) in an unusual three-coordinate environment with the additional two ligands arising from O/N (2.0 Å). DFT models of 1:1 Cu-selenoamino acid complexes suggest that both selenoamino acids coordinate Cu(I) through the selenium and amino groups, with the third ligand assumed to be water. These compounds represent the first structurally characterized copper(I) complexes with sulfur- or selenium-containing amino acids.

Is halogen bonding the basis for iodothyronine deiodinase activity?

Density functional theory studies of S...X and Se...X (X = Br, I) halogen-bonding interactions are used to interpret the selection of selenium and iodine for thyroid hormone signaling. A new mechanism for dehalogenation in terms of halogen bonding is proposed. The activation barriers for deiodination of an aromatic iodide by MeSeH and MeSH (17.6 and 19.8 kcal/mol) are consistent with the relative rates of deiodination by iodothyronine deiodinase and its cysteine mutant.

DFT study of the glutathione peroxidase-like activity of phenylselenol incorporating solvent-assisted proton exchange.

Modeling of the glutathione peroxidase-like activity of phenylselenol has been accomplished using density-functional theory and solvent-assisted proton exchange (SAPE). SAPE is a modeling technique intended to mimic solvent participation in proton transfer associated with chemical reaction. Within this method, explicit water molecules incorporated into the gas-phase model allow relay of a proton through the water molecules from the site of protonation in the reactant to that in the product. The activation barriers obtained by SAPE for the three steps of the GPx-like mechanism of PhSeH fall within the limits expected for a catalytic system at physiological temperatures (DeltaG(1)++ = 19.1 kcal/mol; DeltaG(2)++= 6.6 kcal/mol; G(3)++ = 21.7 kcal/mol) and are significantly lower than studies which require direct proton transfer. The size of the SAPE network is also considered for the model of the reduction of the selenenic acid, step 2 of the GPx-like cycle. Use of a four-water network better accommodates the reaction pathway and reduces the activation barrier by 5 kcal/mol over the two-water model.

Activation energies of selenoxide elimination from Se-substituted selenocysteine.

Transition states for selenoxide elimination have been determined for a series of Se-substituted selenocysteine (RSeCys) derivatives that have potential use in the prevention and treatment of cancer, either directly or in conjunction with cisplatin (to reduce its nephrotoxic effects). Reduced activation barriers vs R=Me and R=Ph are found when the alkyl chain length is increased or when activating groups are para to the selenide. Ortho substitution of Lewis bases stabilizes the transition state by directly donating electron density to the selenoxide. The results suggest that RSeCys derivatives incorporating the properties of glutathione peroxidase mimics will, upon oxidation, rapidly eliminate selenenic acid, a precursor to chemopreventative selenols.

The theoretical 77Se chemical shift as a probe of selenium state in selenoproteins and their mimics.

Theoretical 77Se chemical shifts of a series of simple organoselenium compounds are compared to known NMR data for various derivatives of selenoenzymes and selenoamino acids. Since the theoretical data only differs from the biochemical data set by an overall approximately 15-30 ppm downfield shift, simple theoretical model studies are suggested as an additional tool for the interpretation of selenoenzyme spectra. Further studies demonstrate that model systems can be extended to incorporate the effects of intramolecular interactions (such as Se.N bonds).