Allosteric Modulation of Hormone Release from Thyroxine and Corticosteroid-binding Globulins

Abstract

The release of hormones from thyroxine-binding globulin (TBG) and corticosteroid-binding globulin (CBG) is regulated by movement of the reactive center loop in and out of the β-sheet A of the molecule. To investigate how these changes are transmitted to the hormone-binding site, we developed a sensitive assay using a synthesized thyroxine fluorophore and solved the crystal structures of reactive loop cleaved TBG together with its complexes with thyroxine, the thyroxine fluorophores, furosemide, and mefenamic acid. Cleavage of the reactive loop results in its complete insertion into the β-sheet A and a substantial but incomplete decrease in binding affinity in both TBG and CBG. We show here that the direct interaction between residue Thr342 of the reactive loop and Tyr241 of the hormone binding site contributes to thyroxine binding and release following reactive loop insertion. However, a much larger effect occurs allosterically due to stretching of the connecting loop to the top of the D helix (hD), as confirmed in TBG with shortening of the loop by three residues, making it insensitive to the S-to-R transition. The transmission of the changes in the hD loop to the binding pocket is seen to involve coherent movements in the s2/3B loop linked to the hD loop by Lys243, which is, in turn, linked to the s4/5B loop, flanking the thyroxine-binding site, by Arg378. Overall, the coordinated movements of the reactive loop, hD, and the hormone binding site allow the allosteric regulation of hormone release, as with the modulation demonstrated here in response to changes in temperature.

Binding interactions of perfluoroalkyl substances with thyroid hormone transport proteins and potential toxicological implications

“…5B, T4 docked into TBG with the correct orientation in which the carboxylate substituent forms a hydrogen bond with R378 and the OH-phenyl ring directed towards R381 on the other side of the ligand binding pocket. The docking results are consistent with the crystal structure (Qi et al, 2011). Unlike T4 forming a hydrogen bond with R378, the PFASs fit into the binding pocket of TBG with their carboxylate or sulfonate end groups forming a hydrogen bond with R381.…”

“…According to the results of several crystal structure of the TTR-ligands complex, K15 forms a crucial hydrogen bond interaction with T4 (Hamilton et al, 1993), flufenamic acid (Qi et al, 2011) and 4-hydroxy-chalcone (Polsinelli et al, 2016). To characterize the role of K15 in defining TTR binding towards PFASs, we compared the binding affinities of PFASs towards the wild-type TTR and TTRmutK15G mutant.…”

Corticosteroid-Binding Globulin Gene Mutations and Chronic Fatigue/Pain Syndromes: An Overview of Current Evidence

“…A significant excess in CFS relative risk among first (2.70, 95% CI: 1.56-4.66), second (2.34, 95% CI: 1.31-4. 19), and third degree relatives (1.93, 95% CI: 1.21-3.07) was observed.…”

“…the serpin superfamily 19 . The RCL of CBG is cleaved by human leukocyte elastase (HLE) at sites of inflammation 10,20 rather than by inhibiting proteinases 18 .…”

Heparin Binds Lamprey Angiotensinogen and Promotes Thrombin Inhibition through a Template Mechanism

“…Fluorescence of TNS was collected at emission wavelength of 430 nm with an excitation wavelength of 340 nm during l-ANT titration. Dissociation constants were calculated by Equation 1 (17,18,49).…”