title: Modelling of Thyroid Peroxidase Reveals Insights into Its Enzyme Function and Autoantigenicity.
abstract: Thyroid peroxidase (TPO) catalyses the biosynthesis of thyroid hormones and is a major autoantigen in Hashimoto's disease--the most common organ-specific autoimmune disease. Epitope mapping studies have shown that the autoimmune response to TPO is directed mainly at two surface regions on the molecule: immunodominant regions A and B (IDR-A, and IDR-B). TPO has been a major target for structural studies for over 20 years; however, to date, the structure of TPO remains to be determined. We have used a molecular modelling approach to investigate plausible modes of TPO structure and dimer organisation. Sequence features of the C-terminus are consistent with a coiled-coil dimerization motif that most likely anchors the TPO dimer in the apical membrane of thyroid follicular cells. Two contrasting models of TPO were produced, differing in the orientation and exposure of their active sites relative to the membrane. Both models are equally plausible based upon the known enzymatic function of TPO. The "trans" model places IDR-B on the membrane-facing side of the myeloperoxidase (MPO)-like domain, potentially hindering access of autoantibodies, necessitating considerable conformational change, and perhaps even dissociation of the dimer into monomers. IDR-A spans MPO- and CCP-like domains and is relatively fragmented compared to IDR-B, therefore most likely requiring domain rearrangements in order to coalesce into one compact epitope. Less epitope fragmentation and higher solvent accessibility of the "cis" model favours it slightly over the "trans" model. Here, IDR-B clusters towards the surface of the MPO-like domain facing the thyroid follicular lumen preventing steric hindrance of autoantibodies. However, conformational rearrangements may still be necessary to allow full engagement with autoantibodies, with IDR-B on both models being close to the dimer interface. Taken together, the modelling highlights the need to consider the oligomeric state of TPO, its conformational properties, and its proximity to the membrane, when interpreting epitope-mapping data.
authors.affiliation: Biomedicine Discovery Institute and Department of Biochemistry and Molecular Biology, Monash University, Clayton, Australia.
authors.forename: Sarah N
authors.initials: SN
authors.lastname: Le
authors.affiliation: Biomedicine Discovery Institute and Department of Biochemistry and Molecular Biology, Monash University, Clayton, Australia.
authors.forename: Benjamin T
authors.initials: BT
authors.lastname: Porebski
authors.affiliation: Biomedicine Discovery Institute and Department of Biochemistry and Molecular Biology, Monash University, Clayton, Australia.
authors.forename: Julia
authors.initials: J
authors.lastname: McCoey
authors.affiliation: Biomedicine Discovery Institute and Department of Biochemistry and Molecular Biology, Monash University, Clayton, Australia.
authors.forename: James
authors.initials: J
authors.lastname: Fodor
authors.affiliation: Biomedicine Discovery Institute and Department of Biochemistry and Molecular Biology, Monash University, Clayton, Australia.
authors.forename: Blake
authors.initials: B
authors.lastname: Riley
authors.affiliation: The Centre of Postgraduate Medical Education, Department of Biochemistry and Molecular Biology, Warsaw, Poland.
authors.forename: Marlena
authors.initials: M
authors.lastname: Godlewska
authors.affiliation: Institute of Biochemistry and Biophysics PAS, Department of Genetics, Warsaw, Poland.
authors.forename: Monika
authors.initials: M
authors.lastname: Góra
authors.affiliation: The Centre of Postgraduate Medical Education, Department of Biochemistry and Molecular Biology, Warsaw, Poland.
authors.forename: Barbara
authors.initials: B
authors.lastname: Czarnocka
authors.affiliation: King's College London School of Medicine, Division of Diabetes and Nutrition Sciences, London, United Kingdom.
authors.forename: J Paul
authors.initials: JP
authors.lastname: Banga
authors.affiliation: Biomedicine Discovery Institute and Department of Biochemistry and Molecular Biology, Monash University, Clayton, Australia.
authors.forename: David E
authors.initials: DE
authors.lastname: Hoke
authors.affiliation: Biomedicine Discovery Institute and Department of Biochemistry and Molecular Biology, Monash University, Clayton, Australia.
authors.forename: Itamar
authors.initials: I
authors.lastname: Kass
authors.affiliation: Biomedicine Discovery Institute and Department of Biochemistry and Molecular Biology, Monash University, Clayton, Australia.
authors.forename: Ashley M
authors.initials: AM
authors.lastname: Buckle
mesh_terms: D000595:Amino Acid Sequence; D001324:Autoantigens / Q000737:chemistry / Q000276:immunology* / Q000378:metabolism*; D002462:Cell Membrane / Q000201:enzymology; D004795:Enzyme Stability; D005110:Extracellular Space / Q000201:enzymology; D006801:Humans; D007453:Iodide Peroxidase / Q000737:chemistry / Q000276:immunology* / Q000378:metabolism*; D033862:Iron-Binding Proteins / Q000737:chemistry / Q000276:immunology* / Q000378:metabolism*; D056004:Molecular Dynamics Simulation; D008969:Molecular Sequence Data; D055503:Protein Multimerization; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D013816:Thermodynamics
chemical_list: D001324:Autoantigens; D033862:Iron-Binding Proteins; C056658:TPO protein, human; D007453:Iodide Peroxidase