Thyroxine-binding globulin

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Thyroxine-binding globulin
Thyroxine-binding globulin
Calotte model of the thyroxine-binding globulin (blue) with bound thyroxine (pink) according to PDB  2CEO
Properties of human protein
Mass / length primary structure 395 amino acids
Identifier
Gene names SERPINA7  ; TBG
External IDs
Inhibitor classification
MEROPS I04.955
Occurrence
Parent taxon Mammals

Thyroxine-binding globulin ( TBG ) is a specific transport protein for the thyroid hormones L-thyroxine (T4) and triiodothyronine (T3), which occurs in mammals . Humans produce TBG in the liver , from where it is released into the blood. Mutations in SERPINA7 - gene can cause hereditary TBG deficiency.

Transport of thyroid hormones

L-thyroxine (T4) is more than 99.9% bound to transport proteins in human blood serum , only about 0.03% is free hormone (fT4). Triiodothyronine (T3) is also bound to over 99%, around 0.3% is free (fT3).

TBG is the transport protein with the highest affinity for the thyroid hormones. Other proteins that can bind T4 and T3 with a lower affinity are transthyretin (TTR, prealbumin) and, unspecifically, albumin . This makes up a smaller proportion. SHBG ( sex hormone binding globulin ) is also able to bind the thyroid hormones.

The primary structure of TBG consists of 395 amino acids. TBG has a molecular mass of about 54  kDa . It can bind one molecule of thyroid hormone per molecule. Its serum concentration in humans is about 260 n mol / l (15 µg / ml). Since protein binding prevents rapid renal elimination , the biological half-life of T4 is about 5 to 8 days, for T3 only about 19 hours, as it has a 10 to 20-fold lower affinity for TBG and does not bind to transthyretin at all . The biologically inactive rT3 has an even lower binding to the serum proteins and therefore only a serum half-life of about four hours.

The total concentration of T4 and T3 - but not the concentration of the biologically active free hormones - essentially depends on the concentration and composition of the binding proteins.

Influencing factors

The serum concentration of TBG is influenced by the following factors:

TBG increased TBG humbled
pregnancy
Contraceptives , estrogen preparations
Drugs, e.g. B. Drugs, e.g. B.
Hunger severe catabolic states
acute hepatitis nephrotic syndrome
compensated cirrhosis of the liver decompensated cirrhosis of the liver
acute intermittent porphyria active acromegaly
genetically conditioned genetically conditioned

swell

  1. ↑ Homologues at OMA
  2. ↑ a b UniProt P05543

literature

  • Hotze, Lothar-Andreas; Schumm-Draeger, Petra-Maria; Thyroid Diseases: Diagnosis and Therapy; 5th edition Berlin 2003; ISBN 3-88040-002-4

See also