title: 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 Thr(342) of the reactive loop and Tyr(241) 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 Lys(243), which is, in turn, linked to the s4/5B loop, flanking the thyroxine-binding site, by Arg(378). 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.
authors.affiliation: Department of Biochemistry, Nanjing University, Nanjing, China.
authors.forename: Xiaoqiang
authors.initials: X
authors.lastname: Qi
authors.forename: François
authors.initials: F
authors.lastname: Loiseau
authors.forename: Wee Lee
authors.initials: WL
authors.lastname: Chan
authors.forename: Yahui
authors.initials: Y
authors.lastname: Yan
authors.forename: Zhenquan
authors.initials: Z
authors.lastname: Wei
authors.forename: Lech-Gustav
authors.initials: LG
authors.lastname: Milroy
authors.forename: Rebecca M
authors.initials: RM
authors.lastname: Myers
authors.forename: Steven V
authors.initials: SV
authors.lastname: Ley
authors.forename: Randy J
authors.initials: RJ
authors.lastname: Read
authors.forename: Robin W
authors.initials: RW
authors.lastname: Carrell
authors.forename: Aiwu
authors.initials: A
authors.lastname: Zhou
mesh_terms: D000305:Adrenal Cortex Hormones / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D000494:Allosteric Regulation / Q000502:physiology; D001665:Binding Sites; D006801:Humans; D017433:Protein Structure, Secondary; D013974:Thyroxine / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D056825:Thyroxine-Binding Globulin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D014156:Transcortin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
chemical_list: D000305:Adrenal Cortex Hormones; C534935:SERPINA6 protein, human; D056825:Thyroxine-Binding Globulin; D014156:Transcortin; D013974:Thyroxine