YR- It was an added ingredient
Sorry it took so long
If this diagram doesnt give you a headache........ then you are one cool puppy
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Thyroxine
The thyroid gland is a specialized organ that releases the growth hormone thyroxine. Thyroxine (C01829) is synthesized from the aromatic amino acid tyrosine by multiple iodination, rearrangement and hydrolysis steps. The process occurs on the specialized protein thyroglobin using the protein's tyrosine residues as substrate. Thyroglobin contains about 140 tyrosine residues.
Tyrosine residues are iodinated at position 3 and 5 of the benzene ring to form the intermediates 3-iodo-L-tyrosine and 3,5-diiodo-L-tyrosine. The enzyme catalyzing the reaction is called iodoperoxidase (EC 1.11.1.8). Two diiodo-tyrosine residues are linked through an ether bond forming thyroxine, which at this point is still part of thyroglobin. During the rearrangement, the mechanism of which is not fully elucidated, epoxide intermediates are likely to form. Note that all reaction steps are catalyzed by the thyroid specific peroxidase.
Tyrosine residues next to each other on the surface of thyroglobin are first iodinated and then covalently linked through a ether bridge. The rearrangement reaction involves the formation of possibly epoxide intermediates. The exact reaction mechanism is not fully understood.
Research on non-protein bound tyrosine yields reactive epoxide intermediates with structures that may resemble the intermediates found during thyroxine synthesis on thyroglobin.
Thyroglobin linked thyroxine units are not physiologically active. Thyroglobin acts as a thyroxine storage device and the proteolysis acts as control mechanism of hormone activation. Thyroglobin is proteolytically degraded (peptide bond hydrolysis) and free thyroxine units are released. This free thyroxine is subsequently secreted into the blood plasma where it is bound to plasma proteins like globins, pre-albumin and albumin.
Thyroxine enters target cells where it controls gene expression by binding to nuclear receptor proteins (transcription factors in eukaryotic cell nuclei). It acts as a morphogen by controlling the growth and differentiation of cells. For example, it has been identified as the hormone controlling metamorphosis of tadpoles to frogs.
