All of the following steps in synthesis of thyroid hormones is catalysed by thyroid peroxidase enzyme except
Iodide trapping is not catalysed by thyroid peroxidase. Oxidation, organification and coupling are catalysed by it. Steps in synthesis and secretion of thyroid hormone: Thyroglobulin synthesis by thyroid follicular cells Iodide trapping:Na+ - I- sympoer (NIS) actively transpos I- into the follicular cells. This is inhibited by thiocyanate and perchlorate anions and is stimulated by TSH. Iodide is transpoed into the lumen of the follicle by I -- Cl- transpoer called Pendrin. Oxidation of I- to I2 catalysed by the thyroid peroxidase (TPO). Organoification/ Iodination of tyrosine residues of thyroglobuli: Binding of I2 to tyrosine residues of thyroglobulin to form MIT (monoiodotyrosine) and DIT (Diiodotyrosine). Catalysed by TPO. Coupling reaction mediated by TPO: MIT+DIT - T3, DIT - T4. Storage and release: T3 and T4 are stored bound to thyroglobulin. When stimulated, they are cleaved from thyroglobulin and released into the circulation. Ref: Guyton and Hall 13th edition Pgno: 951
Coupling
Oxidation
Organification
Iodide trapping
Endocrinology