Thyroxine and Tri-iodothyronine synthesis

The synthesis of Thyroxine (T4) and Tri-iodothyronine (T3) occurs in the thyroid gland. These hormones play a crucial role in regulating metabolic processes in the body. The synthesis of T4 and T3 involves the incorporation of iodine and an amino acid called tyrosine.

Synthesis steps:
1. Iodide uptake: Iodide ions (I-) are actively transported into the thyroid follicular cells from the bloodstream. This process is facilitated by a sodium-iodide symporter (NIS) located on the basolateral membrane of the follicular cells.

2. Oxidation of iodide: Once inside the follicular cells, iodide ions are oxidized to iodine (I2) by an enzyme called thyroid peroxidase (TPO). This step is critical for the subsequent incorporation of iodine into tyrosine.

3. Iodination of tyrosine: Within the thyroid follicular cells, iodine is covalently attached to the tyrosine residues of thyroglobulin, a large glycoprotein synthesized by the thyroid gland. Thyroglobulin contains numerous tyrosine residues that can potentially be iodinated.

4. Coupling of iodotyrosines: Two iodotyrosine molecules, one mono-iodotyrosine (MIT) and one di-iodotyrosine (DIT), are coupled together by TPO to form T4. Alternatively, coupling of one MIT with one DIT results in the formation of T3. These reactions occur within the thyroglobulin molecule.

5. Proteolysis and hormone release: When the thyroid gland is stimulated, thyroglobulin is taken up by the follicular cells through endocytosis. Lysosomal enzymes then break down thyroglobulin, releasing T4 and T3 into the bloodstream.

Role of tyrosine in synthesis:
Tyrosine, an amino acid, plays a crucial role in the synthesis of T4 and T3. It serves as the precursor for the iodinated tyrosine residues within thyroglobulin. The iodination of tyrosine residues is essential for the subsequent coupling reactions that lead to the formation of T4 and T3.

Therefore, the correct answer is option 'B' - Tyrosine.