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== TTR functions ==
== Human TTR ==
 
=== Fonctions ===
 


Identified on 1942, Human transthyretin (TTR) ([[1dvq]]) is a transport protein encoded by the TTR gene, located on chromosome 18 <ref> Wallace MR, Naylor SL, Kluve-Beckerman B, Long GL, McDonald L, Shows TB, Benson MD, Localization of the human prealbumin gene to chromosome 18 [archive], Biochem Biophys Res Commun, 1985;129:753–758</ref>. It was originally called prealbumin as it runs faster than albumin ([[1bm0]]) during SDS-PAGE <ref> Seibert FB, Nelson JW. Electrophoretic study of the blood protein response in tuberculosis. J Biol Chem 1942; 143: 29–38. </ref>. After discovering its binding and transport ability to thyroid hormones, it was given the name of “thyroxine-binding prealbumin” (TBPA). Finally, its actual name refers to an additional carrier function: '''trans'''ports '''thyr'''oxine (T4) and '''retin'''ol (vitamin A).
Identified on 1942, Human transthyretin (TTR) ([[1dvq]]) is a transport protein encoded by the TTR gene, located on chromosome 18 <ref> Wallace MR, Naylor SL, Kluve-Beckerman B, Long GL, McDonald L, Shows TB, Benson MD, Localization of the human prealbumin gene to chromosome 18 [archive], Biochem Biophys Res Commun, 1985;129:753–758</ref>. It was originally called prealbumin as it runs faster than albumin ([[1bm0]]) during SDS-PAGE <ref> Seibert FB, Nelson JW. Electrophoretic study of the blood protein response in tuberculosis. J Biol Chem 1942; 143: 29–38. </ref>. After discovering its binding and transport ability to thyroid hormones, it was given the name of “thyroxine-binding prealbumin” (TBPA). Finally, its actual name refers to an additional carrier function: '''trans'''ports '''thyr'''oxine (T4) and '''retin'''ol (vitamin A).
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== Human TTR ==


=== Structure ===
=== Structure ===
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The TTR – ligand interaction provides kinetic stabilization the protein. The more the affinity is high, the more the ligand stabilizes the complex. The dissociation constants with T4 and retinol-binding protein (RBP) are respectively from 1,1.10-7 to 1,5.10-7 M <ref name = "Monaco"> Monaco, H., Rizzi, M., & Coda, A. (1995). Structure of a complex of two plasma proteins: transthyretin and retinol-binding protein. Science, 268(5213), 1039–1041. doi: http://dx.doi.org/10.1126/science.7754382</ref>.
The TTR – ligand interaction provides kinetic stabilization the protein. The more the affinity is high, the more the ligand stabilizes the complex. The dissociation constants with T4 and retinol-binding protein (RBP) are respectively from 1,1.10-7 to 1,5.10-7 M <ref name = "Monaco"> Monaco, H., Rizzi, M., & Coda, A. (1995). Structure of a complex of two plasma proteins: transthyretin and retinol-binding protein. Science, 268(5213), 1039–1041. doi: http://dx.doi.org/10.1126/science.7754382</ref>.


==== TTR-T4 complex ====
 
The crystal structure of this complex is orthorhombic <ref name="wo">Wojtczak, A., Cody, V., Luft, J. R., & Pangborn, W. (1996). Structures of Human Transthyretin Complexed with Thyroxine at 2.0 Å Resolution and 3’,5’-Dinitro-N-acetyl-L-thyronine at 2.2 Å Resolution. Acta Crystallographica Section D Biological Crystallography, 52(4), 758–765. doi:http://dx.doi.org/10.1107/s0907444996003046 </ref>.
Two hormone binding sites are located at the dimer–dimer region bind T4 with negative cooperativity. Under physiological conditions, the bound between the natural ligand and the tetramer can’t be broken down. Moreover, there is only one hormone bound per tetramer. The negative cooperativity mechanism explain the fact that the affinity constants (Ka) for the binding of the first and the second T4 changes, they are respectively 108 and 106 M-1 <ref name= "Klabunde">PMID: 10742177</ref>.
[[Image:t4.png|thumb|left|alt=Puzzle globe|Caption for the image|Structure of thyroxine (T4)|200px]]
[[Image:t4.png|thumb|left|alt=Puzzle globe|Caption for the image|Structure of thyroxine (T4)|200px]]


Two hormone binding sites are located at the dimer–dimer region bind T4 with negative cooperativity. Under physiological conditions, the bound between the natural ligand and the tetramer can’t be broken down. Moreover, there is only one hormone bound per tetramer. The negative cooperativity mechanism
[[Image:1ggl.jpg|thumb|left|alt=Puzzle globe|Caption for the image|Structure of retinol binding protein (RBP)|200px]]








For the TTR-T4 complex, the HBP play a key role. The HBP interact with the four iodine groups of the thyroxine. HBPs bind the iodine of the ligands in two different ways: 3 2’ 1 1’ or 3’ 2 1 1’ with prime indicating the HBP symmetry <ref name= "Klabunde">PMID: 10742177</ref>.
A significant contribution of T4 binding to TTR comes from charged groups near the periphery of the binding site. Glu 54 and Lys15 are located near the HBP-1 pocket allowing potential electrostatic interactions with the ligands <ref name= "Klabunde">PMID: 10742177</ref>.




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Thyroxine is bound deep in the cleft of the channel surface between the side chains of residues Leu17, Alal8 and Leull0, with interactions of its phenolic ring with Ser117 and Thr119 in the P2 pocket which has a more nucleophilic character than the P1 pocket, and with its alanyl moiety interacting with Glu54 and Lysl5 near the channel entrance.
T 4 interactions with TTR side chains shows that it can make good hydrogen-bonding contacts with Lysl5 and Glu54.
The P3 pocket forms close contacts between iodine and Leu110 backbone N atom, while the shortest contacts for 3'-I are formed with the carbonyl of Alal09. These contacts for the low-occupancy model of the hormone are shifted toward the tetramer center and toward the carbonyl and hydroxyl of Ser117, as well as Alal09.


==== TTR-RBP complex ====
TTR is a specific carrier of retinol-binding protein (RBP). RBPs have a molecular mass of 21 kDa. They are composed of an eight-stranded β-barrel and a C-terminal α-helix.  
TTR is a specific carrier of retinol-binding protein (RBP). RBPs have a molecular mass of 21 kDa. They are composed of an eight-stranded β-barrel and a C-terminal α-helix.  
One tetramer of TTR can bind two molecules of RBP in vitro (1:2 stoichiometry). However, when we isolate the TTR-RBP complex from the plasma (in vivo) we find a 1:1 stoichiometry <ref name= "Naylor"> "Naylor, H. M., & Newcomer, M. E. (1999). The Structure of Human Retinol-Binding Protein (RBP) with Its Carrier Protein Transthyretin Reveals an Interaction with the Carboxy Terminus of RBP†,‡. Biochemistry, 38(9), 2647–2653. doi:http://dx.doi.org/10.1021/bi982291i"</ref> . The β-barrel entrance loop involved in A-B strands binding (amino acids from 31 to 38, hairpin1) is also implicated in the TTR-RBP interaction.  
One tetramer of TTR can bind two molecules of RBP in vitro (1:2 stoichiometry). However, when we isolate the TTR-RBP complex from the plasma (in vivo) we find a 1:1 stoichiometry <ref name= "Naylor"> "Naylor, H. M., & Newcomer, M. E. (1999). The Structure of Human Retinol-Binding Protein (RBP) with Its Carrier Protein Transthyretin Reveals an Interaction with the Carboxy Terminus of RBP†,‡. Biochemistry, 38(9), 2647–2653. doi:http://dx.doi.org/10.1021/bi982291i"</ref> .  
RBP-TTR complex stale at high ionic strength and dissociate at low ionic strength <ref name ="Zanetti">Zanotti, G., Ottonello, S., Berni, R., & Monaco, H. L. (1993). Crystal Structure of the Trigonal Form of Human Plasma Retinol-binding Protein at 2·5 Å Resolution. Journal of Molecular Biology, 230(2), 613–624. doi: http://dx.doi.org/10.1006/jmbi.1993.1173 </ref>. It is explained by the presence of a hydrophobic surface in the contact region, represented by hairpin 1,2,3 (include Leu35, 63, 64 and 67). Trp67 (close to hairpin1) seems to be involved in the binding<ref name="Zanetti"/>. The dissociation constant of this complex is around 0.4 µM <ref name= "Naylor"/>.
 
 
 
 
 
 
 


[[Image:images.png|thumb|left|alt=Puzzle globe|Caption for the image|Human RBP : Human TTR structure in "opposite dimer" model|200px]]
RBP has two polypeptide chains, E and F, which are bound to “opposite dimers” of TTR, the main TTR-RBP contact region is made between TTR-D / RBP-E and TTR-B / RBP-F. Thus, we observe an asymmetry in RBP-TTR relationship: RBP-E has more extensive interactions with TTR than molecule RBP-F<ref name= "Naylor"/>.
The specific protein – protein recognition of this complex is classified in the three-dimensional docking model. The recognition site between RBP and TTR implies the positioning of the open end on the RBP β-barrel. For each interface 21 amino acids are involved from both proteins. At the periphery of the site there is charged amino acids. Half of the amino acid side chains are hydrophobic or aromatic in this area. Then, at the center of the site, we find hydrophobic amino acids, Leu and Ile are the predominant amino acids.  RBPs present two different complementary surfaces to a dimeric surface. Indeed, at the core of the RBP-E interface, we find Ile84 from TTR-A and TTR-D, as well as Val20 and Ala81. Moreover, Trp67, Phe96, and Leu63 and 97 from RBP are surrounded by Val20, Leu82, and Ile84 from TTR-A and D.
All mammalian RBP’s have a carboxy-terminal extension of eight amino acids<ref name= "Naylor"/>. This region interacts with TTR and the carboxy-extension of RBP-E is deeply located in the RBP-TTR interface. Indeed, the two terminal Leu182 and Leu183 are embedded in a hydrophobic region which includes Leu82 from TTR-A monomer and Val69 of RBP-E. Moreover, the terminal carboxylate group of RBP-E extension adopts a position to be neutralized by Arg21 of TTR-A. This interaction allows to bury more than 40% of the surface area, compared to the area buried without the carboxy terminal group of RBP-E<ref name= "Naylor"/>.


== Disease ==
== Disease ==
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