Transthyretin (TTR) is a 55 kDa protein responsible for the transport of thyroid hormones and retinol in vertebrates. TTR distributes the two thyroid hormones T3 (3,5,3'-triiodo-L-thyronine) and T4 (Thyroxin, or 3,5,3',5'-tetraiodo-L-thyronine), as well as retinol (vitamin A) through the formation of a macromolecular complex that includes each of these as well as retinol-binding protein. Misfolded forms of TTR are implicated in the amyloid diseases familial amyloidotic polyneuropathy and senile systemic amyloidosis. TTR forms a homotetramer with each subunit consisting of eight beta-strands arranged in two sheets and a short alpha-helix. The central channel of the tetramer contains two independent binding sites, each located between a pair of subunits, which differ in their ligand binding affinity. A negative cooperativity has been observed for the binding of T4 and other TTR ligands. A fraction of plasma TTR is carried in high density lipoproteins by binding to apolipoprotein AI (apoA-I). TTR is able to proteolytically process apoA-I by cleaving its C-terminus; therefore TTR has protease activity in addition to its function in protein transport.
Comment:The homotetrameric thyretin structure contains two independent binding sites within the central core of the tetramer, each formed by a pair of monomers.
Structure:1IE4_A; Rattus norvegicus transthyretin binds two molecules of thyroxine in two independent binding sites, each formed by a pair of subunits. Contacts determined at 3.5 Angstroms.
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