The thyroid gland depends on which enzyme to permit iodination of tyrosyl residues to make MIT and DIT?

The thyroid gland depends on which enzyme to permit iodination of tyrosyl residues to make MIT and DIT?

• A. Thyroid peroxidase (TPO)
• B. Thyroglobulin
• C. Deiodinase
• D. TSH

Answer: (A)

Thyroid peroxidase is the enzyme that carries out the iodination (organification) of tyrosyl residues on thyroglobulin, attaching iodine to form MIT and DIT. It oxidizes iodide to iodine and then uses that iodine to iodinate the tyrosine rings, producing MIT (one iodine) and DIT (two iodines). These then couple to form the thyroid hormones T3 and T4. Without this enzyme, iodination cannot occur and hormone synthesis stalls. Thyroglobulin provides the substrate scaffold but is not the catalytic enzyme. Deiodinase works later to remove iodine from thyroid hormones in peripheral tissues, not in the initial iodination step. TSH stimulates thyroid activity and enzyme production but is not the enzymatic catalyst itself.

Thyroid peroxidase is the enzyme that carries out the iodination (organification) of tyrosyl residues on thyroglobulin, attaching iodine to form MIT and DIT. It oxidizes iodide to iodine and then uses that iodine to iodinate the tyrosine rings, producing MIT (one iodine) and DIT (two iodines). These then couple to form the thyroid hormones T3 and T4. Without this enzyme, iodination cannot occur and hormone synthesis stalls. Thyroglobulin provides the substrate scaffold but is not the catalytic enzyme. Deiodinase works later to remove iodine from thyroid hormones in peripheral tissues, not in the initial iodination step. TSH stimulates thyroid activity and enzyme production but is not the enzymatic catalyst itself.