A Chemical Model for the Inner-Ring Deiodination of Thyroxine by Iodothyronine Deiodinase

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.

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.

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.

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.