serpin family A member 7, thyroxine-binding globulin
Thyroxine-binding globulin (TBG, also called T4-binding globulin) is a globulin that binds thyroid hormones in circulation. It is one of three transport proteins (along with transthyretin and serum albumin) responsible for carrying the thyroid hormones thyroxine (T4) and triiodothyronine (T3) in the bloodstream. TBG is synthesized primarily in the liver and is a serpin with no inhibitory function like many other members of this class of proteins. There are two forms of inherited thyroxine-binding globulin deficiency: the complete form (TBG-CD), which results in a total loss of thyroxine-binding globulin, and the partial form (TBG-PD), which reduces the amount of this protein or alters its structure. Neither of these conditions causes any problems with thyroid function, but it can be mistaken for more serious thyroid disorders, such as hypothyroidism. This family belongs to the clade A of the serpin superfamily, which includes the classical serine proteinase inhibitors, alpha-1-antitrypsin and alpha-1-antichymotrypsin, protein C inhibitor, kallistatin, and non-inhibitory serpins, like corticosteroid and thyroxin binding globulins. In general, SERine Proteinase INhibitors (serpins) exhibit conformational polymorphism shifting from native to cleaved, latent, delta, or polymorphic forms. Many serpins, such as antitrypsin and antichymotrypsin, function as serine protease inhibitors which regulate blood coagulation cascades. Non-inhibitory serpins perform many diverse functions such as chaperoning proteins or transporting hormones. Serpins are of medical interest because mutants have been associated with blood clotting disorders, emphysema, cirrhosis, and dementia. A classification based on evolutionary relatedness has resulted in the assignment of serpins to 16 clades designated A-P along with some orphans.
Comment:depending on the conformational state, the RC loop is surface accessible in the active form or buried and inserted as the central beta strand in the inactive form.
Structure:2CEO_A: Human thyroxine-binding globulin RCL in open flexible conformation
Comment:thyroxine-binding globulin differs from other serpins in having the upper half of its main beta-sheet fully opened, so its RCL can readily move in and out of the sheet to give an equilibrated binding and release of thyroxine
Jiyao W et al.(2023). "The conserved domain database in 2023.",