TBG protein also known as THBG protein or thyroxine-binding globulin is a member of serpin family of proteins. The latter is a superfamily of proteins with similar structures that were initially identified for their protease inhibitory activity. However, unlike many other members of this class of proteins, TBG protein has no inhibitory activity. This protein is mainly synthetized in the liver and is released in the bloodstream as a 54 kDa acidic glycoprotein made of a single polypeptide chain of 395 amino acids and four hetero saccharide units. THBG protein is encoded by a single gene copy located on Xq22 which consists of five exons spanning 5.5 kbp.
Since TBG is a protein belonging to serine family of proteins, it main structural element is the reactive center loop (RCL). Upon cleavage by proteases, the RCl element is inserted into the protein core adopting a ß-strand conformation. This is known as the relaxed transition S-to-R. After S-to-R transition TBG protein affinity for T4 decreases. Indeed, proteolytic cleavage of TBG appears to be a mechanism for site specific release of T4 independently or homeostatic control.
TBG protein binds to thyroid hormones thyroxine (T4) and triiodothyronine (T3). THBG protein, along with transthyretin and serum albumin, carries T4 and T3 in the bloodstream. However, among the different thyroid hormone carriers mentioned, TBG protein has the highest activity for T3 and T4 but is present at the lowest concentration. Unlike albumin and transthyretin, TBG has a single binding site.
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