Werner & Ingbar's The Thyroid: A Fundamental & Clinical Text, 9th Edition

9.Thyroid Hormone Structure–Function Relationships

Vivian Cody

Studies of the thyroid hormones continue to provide new insights into the molecular events that control their bio-synthesis, transport, and mechanism of action. Thyroid hormones are protein bound during transport in the general circulation, at the cell membrane, and at the cell nucleus. Therefore, their molecular interactions with proteins are of paramount interest. Most recently, emphasis has been focused on thyroid hormone protein interactions of transthyretin (TTR), its retinol-binding protein complex, thyroxine-binding globulin (TBG), and the hormone-binding domain of the thyroid nuclear receptor.

The major product of the thyroid gland, thyroxine (T4) (3′,5′,3,5-tetraiodo-L-thyronine), was first isolated by Kendall in 1914 (Fig. 9.1) (1). Its correct composition, however, was not established until 1927, by Harington and Barger (2). Although thyroid hormones elicit a multitude of biologic responses, the specific nature of their actions remains unclear. The impetus to synthesize and test many analogues stemmed from attempts to define those features essential for T4-like activity (3,4). A number of hypotheses have been proposed to relate the various structural features of thyroid hormones to the expression of their biologic effects (Table 9.1). Among the proposals suggested, the following are considered of key importance: (a) the unique role of iodine, for both its steric and its electronic properties; (b) the diphenyl ether linkage in controlling conformation; and (c) the 4′-hydroxyl and side-chain composition for receptor binding. Investigation of these hypotheses has been carried out using techniques such as nuclear magnetic resonance (NMR), spectroscopy, and X-ray crystallography to elucidate their structure–activity relationships.

FIGURE 9.1. Thyroxine with torsion angles and molecular components. (From Cody V. Thyroid hormone interactions: molecular conformation, protein binding, and hormone action. Endocr Rev 1980;1:140, with permission.)

TABLE 9.1. RELATIVE BINDING AFFINITIES AND BIOLOGIC POTENCIES OF SELECTED THYROID HORMONE ANALOGS


Compound

Thyroxine-binding Globulin (%)

Transthyretin (%)

Membrane (%)

Nuclear (%)

Potencya(%)


T3

100.0

39.3

96.6

12.5

18.1

D-T4

4.0

0.95

63.0

3.0

T4-propionic acid

3.6

76.4

23.0

3.0

T4-acetic acid

1.7

100.0

30.3

0.25

S-bridge T4

63.0

CH2-bridge T4

35.0

2.6

T3

9.0

1.4

100.0

100.0

100.0

rT3

38.0

3.1

67.5

0.1

< 0.1


aPotency is percentage of T3 (triiodothyronine; 3′,3,5-triiodothyronine).

T4, thyroxine; D-T4, detiorotomes; S-bridge T4, s replaces ether O between rings in T4; CH2-bridge T4, CH2 replaces ether O between rings in T4; rT3, reverse triiodothyronine.

From Cody V. Thyroid–hormone interactions: molecular conformation, protein binding, and hormone action. Endocr Rev 1980;1:140, with permission.

For example, it is well known that hypothyroidism is accompanied by high serum levels of low-density lipoprotein cholesterol and, potentially, an increased risk of atherosclerosis, whereas thyrotoxicosis is associated with decreased cholesterol levels. Thyroid hormones could not be used therapeutically to lower serum cholesterol because of their potential to induce cardiac side effects. Thus, modification of the thyroid hormone molecule was carried out to produce analogues that could differentiate between liver-selective actions and cardiotoxic effects (4). These studies revealed that the introduction of specific arylmethyl groups at the 3′-position of triiodothyronine (T3) resulted in analogues that are liver-selective, cardiac-sparing thyromimetic compounds.

More recent efforts have extended this concept to the design of thyromimetics with tissue-selective actions. For example, thyromimetics have been synthesized to be selective for the β-form of the thyroid hormone receptor (THRβ) (5), while other analogues elicit a T3 response from mutant forms of the thyroid receptor (TRβ R320C) (6). These analogues suggest that such “pharmacological rescue” agents have potential for the treatment of thyroid disease. Another example of the design of thyromimetics that could overcome mutational diseases is that involving treatment of amyloid formation of TTR. Rational design efforts have identified several structurally diverse molecules that can bind and stabilize TTR under fibrillogenic conditions (7,8).

STRUCTURE AND STEREOCHEMICAL CHARACTERISTICS

The thyronine nucleus constitutes the basic structural unit of thyroid hormones. By varying the degree of iodination, all known thyroid hormone structures can be derived (Fig. 9.2). Depending on the reference point, the molecule can be described as a substituted alanine amino acid or, in terms of the diphenyl ether moiety, with substituents in the 4′- and 1- positions (Fig. 9.1).

FIGURE 9.2. Spatial representation of thyroxine deiodination cascade. (From Auf'mkolk M, Koehrle J, Hesch RD, et al. Inhibition of rat liver iodothyronine deiodinase: interactions of aurone with the iodothyronine deiodinase binding site. J Biol Chem 1986;261:11623, with permission.)

The thyroid literature does not conform to the standard International Union of Pure and Applied Chemistry nomenclature, but uses instead the substituted diphenyl ether system. Because the thyronine nucleus has five single bonds (Fig. 9.1), rotation about these bonds results in a number of conformations, many of which have been observed in the solid state (9,10,11). This flexibility, quantitatively described by the magnitudes of these rotations, is controlled by (a) ring-substitution effects, (b) ether bridge substitutions, (c) side-chain composition, and (d) hydrogen-bonding effects.

A major structural feature of the thyroid hormones is that the two phenyl rings are joined by a bridging ether oxygen with an angle of 120 degrees. Early in the study of the thyroid hormones, it became evident that these substances possessed special stereochemical characteristics and that their three-dimensional features must be considered. Examination of space-filling models of the thyroid hormones revealed that the bulky orthoiodine atoms near the ether bridge caused the two aromatic ring systems to be nearly perpendicular, and that these bulky groups hindered rotation about the phenyl ether bonds. Structural data for the thyroid hormones showed that the minimal steric interaction between the orthotyrosyl iodine atoms and the phenolic orthohydrogen atoms was maintained when one ring is coplanar with, and the other perpendicular to, the plane of the two ether bonds (9). This gave rise to a skewed conformation of the hormone and the concept of preferred, if not somewhat rigid, orientations of the molecule (Fig. 9.3). These stereochemical properties also revealed that the 3′-iodine of the hormone 3,5,3′-triiodo-thyronine (T3) possessed two positional isomers—distal and proximal—depending on whether the phenolic ring iodine atom was oriented away from or near the tyrosyl ring, respectively. Activity measurements of rigid analogues revealed that a distal T3 conformation was the more active analogue.

FIGURE 9.3. Deiodination products of thyroxine to skewed triiodothyronine (T3) and antiskewed reverse(3′,5′,3-)triiodothyronine (rT3). (From Cody V, Koehrle J, Auf'mkolk M, Hesch RD. Structure-activity relationships of flavonoid deiodinase inhibitors and enzyme active site models. In: Cody V, Middleton E Jr, Harborne JB, eds. Plant flavonoids in biology and medicine: biochemical, pharmaceutical and structure-activity relationships. New York: Alan R. Liss, 1986:373, with permission.)

Crystallographic analysis of thyroid hormones shows that a skewed diphenyl ether conformation is observed in the structures of all 3,5-disubstituted hormone analogues (9,10,11). As mentioned earlier, the bulk of the tyrosyl ring substituents forces the diphenyl ether to adopt a skewed conformation (i.e., φ = 0 degrees, φ′ = 90 degrees), whereas removal of one of these substituents releases this constraint, permitting an antiskewed conformation (i.e., the tyrosyl ring is perpendicular to, and bisecting, the phenolic ring φ = 90 degrees and φ′ = 0 degrees), as observed in the crystal structure of 3′,5′,3-triiodothyronine (rT3) (12) (Fig. 9.3). T4 cannot adopt an antiskewed conformation because the bulk of the tyrosyl ring iodine atom would be placed into the electron density of the phenolic ring. Therefore, the active hormones T4 and T3, as well as 3,5-T2, can adopt only a skewed conformation, whereas those with at least one tyrosyl ring iodine can have either a skewed or antiskewed conformation (13).

All the thyroid hormone–binding proteins have a requirement for a diphenyl ether moiety, because those derivatives of T4 that contain only one ring or have a biphenyl connection do not possess activity or binding affinity. The need for the bridging atom to be oxygen is not absolute. Studies of various oxygen bridge–substituted analogues (C-X-C, where X = CH2, S) showed that the sulfur- and methylene-bridged analogues had substantial binding affinity and activity. Thus, one role of the oxygen bridge is to maintain the appropriate relative orientation of the iodophenyl substituents (14).

The conformational space available to the thyroid hormone side chain is defined by three torsion angles: χ1, χ2, and ψ (Fig. 9.1). From the set of all accessible conformations, only specific subsets are predicted to be favored energetically, and most of these have been observed (9,11,15). These features play a role in differentiating the binding affinity of hormone analogues for their various hormone-binding proteins. For example, T4 has the strongest binding affinity for TBG, whereas the metabolite tetraiodothyroacetic acid has the strongest binding affinity for TTR (see Table 9.1).

Under normal physiologic conditions, the thyroid hormone amino acid is a zwitterion; the amine has a net positive charge, and the carboxylic oxygen atoms have a net negative charge. As a result of the differences in pKa of the 4′-OH in T3 and T4 (8.47 and 6.73, respectively), the 4′-OH of T4 is about 80% ionized at physiologic pH, whereas in T3 it is about 10% ionized. Because many protein-substrate interactions are by way of receptor interactions, it is important to understand the nature of the hydrogen bonding in these structures. A study of the hydrogen bonding observed in the crystal structures of thyroid hormones shows that there is a high degree of directional specificity in the location of the hydrogen bond donors and acceptors (10). One unique property of iodine is its polarizability, which is reflected in its predisposition to form short intermolecular contact distances of the type I… I and I… O in the crystal lattice (16). The propensity for iodine to form such short intermolecular contacts may explain hormone-binding selectivity to the various T4-binding proteins.

PROTEIN-BINDING INTERACTIONS

Sequence analysis showed that TTRs from various species are more than 86% identical to those from humans (17). In the case of rat TTR, there are 22 residues that differ from the human sequence, most of which are distal from the ligand-binding domain. Structure–activity data show that thyroid hormone analogues have different binding affinities for TTR, depending on their substituent patterns (Table 9.1). In addition, many pharmacologic agents and natural products, such as plant flavonoids, nonsteroidal analgesic drugs, inotropic bipyridines, and organohalogen environmental agents, are strong competitors for T4binding to TTR, with binding affinities much greater than T4. For example, structure–activity correlations indicated that certain plant flavonoids that exhibit strong TTR binding can also exhibit various antihormonal properties, including inhibition of the enzyme iodothyronine deiodinase (13,18). These data further revealed that aurones are the most potent inhibitors of enzyme activity and strong competitors for T4 binding to TTR (18). Computer graphic modeling of the binding interactions with TTR suggested that the best structural homology between thyroid hormones and flavonoids involved the phenolic ring of both classes of compound (19,20). Studies of T4 displacement from TTR further revealed that a synthetic plant flavonoid, EMD 21388(3-methyl-4′,6-dihydroxy-3′,5′-dibromo-flavone), is the strongest competitor for T4 binding to human TTR (21,22), and showed that this T4 antagonist alters the circulating total and percentage of free thyroid hormones and serum thyrotropin concentrations (23).

To verify this structure–activity data, the crystal structures of a number of ligand complexes with human or rat TTR have been studied (7,8,24,25,26,27,28,29,30,31,32,33,34,35,36,37). Most crystals of human TTR ligand complexes are isomorphous with the orthorhombic P21212 cell reported previously for native TTR (38,39) and have two independent monomers in the asymmetric unit of the crystal lattice. The presence of a crystallographic twofold axis through the center of the binding domains requires that the ligand either possess molecular twofold symmetry or have a 50% statistical disorder. Because T4 does not possess twofold symmetry, it must occupy this site with a statistical disorder. On the other hand, crystals of rat TTR crystallize in a tetragonal space group P43212, with a complete tetramer in the asymmetric unit of the lattice (40). Data for the rat TTR-T4 complex (30) revealed electron density in the binding domain and provide the first opportunity to examine the hormone-binding interactions in a unique environment.

LIGAND-BINDING MODES

Structural data for the hTTR-T4 complex (39) revealed that T4 binds in a “forward” mode, with its 4′-OH buried deep within the channel running through the tetrameric protein, and has its amino acid side-chain near the channel entrance interacting with Lys-15 and Glu-54. Recently, the TTR-T4 complex was determined to be a cocrystallized hormone complex (28). These data showed that T4 binds deeper in the channel and displaces the bound water observed in the crystals soaked with T4 (39). Although the orientation is similar, the hormone is rotated such that it shares common binding sites for the 3- and 3′-iodine atoms. These data verify that T4 binding does not affect the main chain conformation significantly but results in local rearrangements of residue side-chains in the binding channel.

The orientation of the weak binding metabolite 3,3′-T2 (26) differs significantly from that of T4. As shown in Fig. 9.4, it binds deeper in the channel than T4, and in this orientation, 3,3′-T2 occupies the binding domain in a completely different manner from T4. The binding affinity of 3,3′-T2, which is 100-fold lower than that of T4, reflects the lack of the second pair of iodine atoms interacting in the channel.

FIGURE 9.4. Position of thyroxine (light line) and 3,3′-T2 (thick line) bound in their respective transthyretin complexes. The one-letter code symbols label the residues forming the binding site. (From Wojtczak A, Luft JR, Cody V. Mechanism of molecular recognition: crystal structure of 3,3′diiodo-L-thyronine human transthyretin complex and mutant interactions. J Biol Chem 1992;267:353, with permission.)

The thyroid hormone metabolite T4 acetic acid (Table 9.1), a tight binder of TTR, shows multiple binding modes in structures with human and rat TTR (24,30). These data revealed the hormone metabolite binds with a mixed population of both “forward” and “reverse” orientations in one binding domain, and two, alternate “forward” binding positions in the second domain. These added intermolecular interactions may reflect the tight binding affinity of this metabolite for TTR.

Structural results show that flavones bind to TTR in a manner different from T4. Data for the structure of the hTTR–EMD21388 complex revealed that bromoflavone binds deeper in the channel than T4; the bromine atoms occupy symmetric sites in both a “forward” mode (Fig. 9.5) and “reverse” mode, with the bromophenolic ring near the channel entrance in TTR (24,32). A bromoaurone analogue binds in a similar manner (25). The observation of two alternative binding orientations for EMD 21388 may explain its greater binding affinity for TTR (22). Similar results have been observed for the organohalogens, tri- and pentabromophenol, that were observed to bind exclusively in the “reverse” mode (35). Solution NMR data on TTR also indicated that thyroid hormones may bind in more than one mode (41).

FIGURE 9.5. Representation of EMD 21388 in the “reverse” binding mode of human transthyretin. The close contact between the flavone 4′-OH and NZ of Lys-15 (lysine) near the channel entrance is shown. (From Cody V, Wojtczak A, Ciszak E, et al. Differences in inhibitor and substrate binding in transthyretin crystal complexes. In: Gordon A, Gross J, Hennemann G, eds. Progress in thyroid research. Rotterdam: Balema, 1991:793, with permission.)

Biochemical data for the competitive inhibition of T4 binding by bipyridine inotropic agents revealed that the potent cardioactive positive inotrope milrinone, a 2-methyl-5-cyano-bipyridine, has a binding affinity of 59% that of T4, whereas its less potent parent inotrope, amrinone, the 5-amino-bipyridine analogue, was a weak competitor (42,43). Structural results for the TTR-milrinone complex showed that the 5-cyano group binds in the same site as the 5′-iodo group of the hormone (27). Modeling amrinone binding in the milrinone site revealed that the 5-amino group cannot participate in the same interactions as the 5-cyano group, thereby weakening its binding affinity.

TRANSTHYRETIN VARIANTS AND AMYLOIDIC DISEASE

Increased efforts have been made to understand the mechanisms underlying TTR tetramer stability and their relationship to the formation of amyloid fibrils that characterize amyloid diseases, including familial amyloid polyneuropathy (FAP), senile systemic amyloidosis, and Alzheimer's disease (44,45,46,47,48,49,50,51,52,53,54,55). Recent data showed that wild-type and variant TTR form amyloid fibrils that are the causative agents in FAP and senile systemic amyloidosis diseases. To date, more than 70 single amino acid variants of the 127 residue monomer of TTR have been implicated in FAP disease (44); however, structural studies of TTR variants have failed to identify major differences that could explain their amyloidogenicity (46,47,48,49). Several hypotheses have been proposed based on monomeric or dimeric amyloidogenic intermediates to explain fibril formation from TTR monomers. One model proposes head-to-tail polymerization of monomeric intermediates (44,53). Another model is based on the formation of linear aggregates of TTR molecules, each linked by a pair of disulfide bonds involving Cys-10 (46,48,55). In this case, the intermediate is a dimer. A third model is based on data from two engineered amyloidogenic mutants and requires dimers that are associated by antiparallel organization of the F and H strands of the native protein. This model requires the destabilization of the tetramer prior to fibril formation (44,45). Yet another model invokes proteolytic cleavage as the initiation step in fibril formation (45,52).

Although the mechanism of tetramer stabilization is still unclear, it has been shown that ligand binding stabilizes tetramer formation in all variant species (44,50). Therefore, one means of intervention for disease treatment could involve binding of nonthyromimetic analogues that can stabilize the TTR tetramer and possibly delay the onset of fibril formation. To this end, numerous compounds have been screened (5,6,7,8,50). Structural data for the TTR complex with flufenemic acid showed that it mediates intersubunit hydrophobic interactions and intersubunit hydrogen bonds that stabilize the normal tetrameric fold (50). Data for the TTR complex with another analgesic analogue, VCP-6, which has 628% the affinity of T4, showed that the molecule forms strong hydrogen bond interactions of its 2-carboxylate with Lys-15 and with the 3,5-dichloro atoms in symmetric hydrophobic pockets near the tetramer interface (Fig. 9.6) (36,37). These results suggest that the strategy of stabilization by strong competitors may prove fruitful. Comparison of the environment near the 22 residues in the rat sequence that differ from those in humans also permit evaluation of their influence on hormone-binding interactions and tetramer assembly.

FIGURE 9.6. Representation of VCP-6 bound to human transthyretin (TTR). The close contact between the 2-carboxylate and NZ of Lys-15 near the channel entrance is shown. Different electron density (Fo-Fc, 3σ) shows position of 3′,5′-chlorine atoms near the center of the tetramer. Light lines are a twofold symmetry-related monomer that makes up binding domain A. Hydrogen bonding to Lys-15 and close contacts of chlorine atoms in hydrophobic pockets result in tight binding affinity of VCP-6 to TTR.

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