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

Abstract

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.hyroid hormones regulate mammalian development as well as energy expenditure and metabolism in the adult (1). The active, nuclear receptor-binding form of thyroid hormone is 3,3′,5-triiodothyronine (T 3 ). The thyroid gland mainly releases thyroxine [3,3′,5,5′-tetraiodothyronine (T 4 ); Fig. 1A], and T 3 is formed and degraded through elimination of iodine atoms from the 5′-and 5-positions, respectively (2) (Fig. S1). Three types of deiodinase enzymes are involved in activation and inactivation of thyroid hormones (Fig. S1). They form a family of transmembrane enzymes with homologous catalytic domains (2). Type II deiodinase (Dio2) catalyzes the activating (outer ring) 5′-deiodination of the prohormone T 4 to T 3 , whereas type III deiodinase (Dio3) catalyzes inactivating (inner ring) 5-deiodination (2). Type I deiodinase (Dio1) is capable of both types of reactions (3). Cells targeted by the hormone can thereby fine-tune intracellular T 3 levels through deiodinase expression according to their needs during development or upon metabolic challenges. Dio1, Dio2, and Dio3 share a conserved amino acid sequence and a selenocysteine residue essential for efficient deiodination, although nonmammalian cysteine-deiodinases have been found (4, 5). Although most of the characterized selenoproteins act as peroxidases or protein reductases, deiodinases are the only selenoenzymes known to catalyze halogen eliminations from aromatic rings. Despite a large body of experimental data, there is no coherent model for deiodinase catalysis that incorporates the bulk of available data. Results and DiscussionTo explore the unique deiodinase catalytic mechanism, we determined the crystal structure of the cytoplasmic catalytic domain of Mus musculus Dio3 (Dio3 cat ) with Sec 170 replaced by cysteine ( Fig. 1 B and C and Table 1). The structu...

New insights into the structure and mechanism of iodothyronine deiodinases

“…The lack of molecular insight likely contributes to the paucity of small molecule modulators for deiodinases, which could be potential drugs for the treatment of some forms of hyperthyroidism, hypothyroidism, and cancer (Manna et al 2013, Ciavardelli et al 2014). We will review the reaction mechanism, regulation and molecular structure of deiodinases based on the recently determined crystal structure of mouse Dio3 (mDio3; Schweizer et al 2014a). We will shortly discuss modulatory compounds and emerging drug development opportunities.…”

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

“…The selenenyl iodide 2 was reduced to the corresponding selenol 1 by dithiothreitol (Scheme 3), as was in the case with the aromatic counterpart, BpqSeI (3) [7]. Molecules 2015, 20, page-page has been proposed to be potentially involved in the second half-reaction of ID-3 [14], but no chemical evidence has been available for this reaction process. …”

“…Molecules 2015, 20, page-page has been proposed to be potentially involved in the second half-reaction of ID-3 [14], but no chemical evidence has been available for this reaction process. …”

“…This corroborates the possible formation of sulfenic acids by similar transformations in biological systems [20,21]. As for the selenium counterpart, it has recently been proposed that related hydrolytic formation of a selenenic acid is potentially involved in the catalytic mechanism of ID-3; Steegborn et al suggested that a selenenyl iodide intermediate generated in the active site of the enzyme after deiodination of an iodothyronine substrate might be hydrolyzed to afford a selenenic acid, which is then reduced to a selenol via a selenenyl sulfide [14]. However, there has been no chemical evidence for the hydrolytic conversion of a selenenyl iodide to a selenenic acid.…”

“…Despite being a primary-alkyl group, this substituent was found to be effective for the stabilization of reactive species such as a sulfenic acid (X = SOH) [12], a sulfenyl iodide (X = SI) [11], and a selenenic acid (X = SeOH) [13]. Here we report the synthesis of a stable primary-alkyl-substituted selenenyl iodide by utilizing this molecular cavity and its hydrolytic conversion to the corresponding selenenic acid; hydrolysis of a selenenyl iodide to a selenenic acid has been proposed to be potentially involved in the second half-reaction of ID-3 [14], but no chemical evidence has been available for this reaction process.…”