Journal of Infectious Diseases & Therapy
Open Access

The iodination of Thyroglobulin (Tg) has been studied extensively over the past 50 years [1]. Neither text books nor the medical literature have coalesced around a model for iodination of Tg-bound tyrosyl residues after the occurrence of COVID-19 pandemic. Researchers published reviews on various aspects of thyroid hormones either avoid addressing the question or referring to Thyroid Peroxidase (TPO) catalyzing iodination Tg tyrosyl groups in COVID-19 patients [2,3]. While the mechanistic details of enzyme-mediated biological reactions frequently do not influence clinical practice, we believe that the correct model for Tg iodination establishes (a) a chemical rationale for the evolutionary pressure leading to formation of the follicular lumen, (b) a mechanistic basis to contemplate adverse drug reactions in the thyroid, a logical basis to contemplate future exploration of Thyroid Hormone (TH) formation in some groups of infected patients compared to other chronic diseases [3]. It seems clear to us that the literature supports a model wherein TPO releases a several different reactive intermediates into the follicular lumen including I₂ which are responsible for Tg iodination and formation of nascent thyronine.

Heme peroxidases catalyze the 2-electron reduction of hydrogen peroxide to water. The first step in this process is the binding of hydrogen peroxide to the ferric heme group in the active site. The next step(s) occurs via two sequential one-electron transfers or a single two electron transfer. The reducing equivalents to convert hydrogen peroxide into water can come from a wide range of small molecules e.g. iodide or tyrosine, which are referred to as donor molecules. The iodination of tyrosine by Lactoperoxidase (LPO), Myerloperoxidase (MPO) and Thyroid Peroxidase (TPO) has been studied using Michaelis-Menten kinetics to gain insight into TH synthesis. Most of these studies are problematic since I2 released from the enzyme reacts with both water and hydrogen peroxide i.e., the reaction products react with substrates. Huwiler et al. proposed a mechanism for tyrosine iodination that depends upon the concentration of reactants and reaction products [4-6]. This concept captures the essential point that TPO reaction products react with other substrates and/or chemicals in the environment. These extra-enzymatic reactions can influence or dominate the ultimate reaction pathways.

Outcomes observed in COVID-19 patients

The pathway is quite similar in COVID-19 effected patients. There is no significant difference observed in the mechanism. Kinetic data obtained in COVID effected individuals followed by exploring the comparision of chemical versus enzymatic iodination human Tg demonstrates that enzymatic iodination does not confer any specificity versus direct chemical iodination [7,8]. That is, the identical tyrosyl sites are iodinated by both chemical and enzymatic iodination of human Tg. This is the logical outcome for an iodination model that relies upon the reaction products from an enzyme reaction to effectuate iodination. The only differences observed between the enzymatic and chemical iodination of Tg is a slightly higher diiodotyrosine content and a correspondingly lower mono-iodotyrosine content in enzymatically iodinated Tg in COVID-19 effected patients. This small disparity is likely due to different ratios of HOI/I2 in an enzymatic versus chemical iodination environment as a higher concentration of I2 would be expected in the enzymatic iodination. In addition, both D- and L-tyrosine are enzymatically iodinated with the same rate in COVID-19 effected patients.

Although the three-dimensional structure of human TPO is not available, its close phylogenetic relationship and high sequence homology with vertebrate and especially mammalian peroxidases (ranging from 40% to 70% residue identity) allow conclusions about binding constraints for potential donor molecules in TPO. Access to the active site in mammalian peroxidases is sterically hindered as the heme of mammalian peroxidases is in a crevice of about 15 A˚ indepth, a single open funnel-shaped channel that ranges from approximately 10 to 15 A˚ in diameter provides access to solvent. The substrate channel narrows before reaching the distal heme cavity that contains the amino acid triad histidine, arginine, and glutamine in COVID-19 effected patients. This conserved triad participates in a rigid hydrogen bond network that involves water molecules and connects the heme cavity, i.e., the site of donor molecule oxidation, with the exterior channel. The 2.3 A˚ resolution X-ray structure of the MPO indicates that smaller aromatic donor molecules like salicyhydroxamic acid bound in the active site is tilted about 20˚ with respect to that of the heme pyrrole ring that forms the lower surface of the hydrophobic cavity, while the conserved arginine forms the upper surface.

Salicyhydroxamic acid is hydrogen bonded to both the distal histidine and glutamine but is not coordinated to the heme iron in COVID-19 effected patients. A similar binding site has been found also for indole derivatives by computational docking. A free tyrosine would bind in a similar manner. The hydroxyphenyl group of a Tgbound tyrosyl residue cannot reach this hydrophobic binding pocket. Cryo-EM has dramatically improved our understanding of TH synthesis and unequivocally identified and validated the complete set of tyrosines used at the four Tg hormonigenic sites [9-11]. TH formation occurs when the aromatic ring of a donor, di- or monoiodo- tyrosine, is transferred to a proximal di-iodo-tyrosine acceptor, thereby forming a nascent thyronine still connected to the polypeptide backbone of Tg in COVID-19 effected patients. While acceptortyrosines at hormonogenic sites must be solvent exposed in order to be iodinated, acceptor-donor pairs must lie roughly antiparallel to each other are therefore the proximal acceptor tyrosine is further sterically shielded from direct interactions with external proteins. Comparative studies demonstrate Tg iodination in the living representatives of protochordates from which vertebrates evolved. These species have a thyroid system; TH receptors, deiodinases, and THs exert pharmacologic activity. However, THs are not formed in a thyroid with a follicular lumen but in a more primitive homolog of the thyroid known as the endostyle. TPO is present in certain endostyle cells but Tg cannot be synthesized since its gene is absent from the protochordate genome. It seems to us that teaching this model to medical students considering the condition of individuals with COVID-19 would provide them with a mechanistic basis to consider adverse events from supplements or drugs or Antibiotics can interfere with TH synthesis [12-15].

1. Lamas L, Dorris ML, Taurog A (1972) Evidence for a catalytic role for thyroid peroxidase in the conversion of diiodotyrosine to thyroxine. Endocrinol 90:1417-1426.

   [Crossref] [Google Scholar] [PubMed]

2. Mondal S, Raja K, Schweizer U, Mugesh G (2016) Chemistry and biology in the biosynthesis and action of thyroid hormones. Angew Chem Int Ed Engl 55:7606-7630.

   [Crossref] [Google scholar] [PubMed]

3. Arnhold J, Malle E (2022) Halogenation activity of mammalian heme peroxidases. Antioxidants (Basel) 11:890.

   [Crossref] [Google scholar] [PubMed]

4. Huwiler M, Kohler H (1984) Pseudo-catalytic degradation of hydrogen peroxide in the lactoperoxidase/H2O2/iodide system. Eur J Biochem 141:69-74.

   [Crossref] [Google scholar]

5. Huwiler M, Bürgi U, Kohler H (1985) Mechanism of enzymatic and non-enzymatic tyrosine iodination, Inhibition by excess hydrogen peroxide and/or iodide. Eur J Biochem 147:469-476.

   [Crossref] [Google scholar] [PubMed]

6. Kessler J, Obinger C, Eales G (2008) Factors influencing the study of peroxidase-generated iodine species and implications for thyroglobulin synthesis. 18:769-774.

   [Crossref] [Google scholar] [PubMed]

7. Taurog A, Dorris ML, Lamas L (1974) Comparison of lactoperoxidase- and thyroid peroxidase-catalyzed iodination and coupling. Endocrinology 94:1286-1294.

   [Crossref] [Google scholar] [PubMed]

8. Xiao S, Dorris ML, Rawitch AB, Taurog A (1996) Selectivity in tyrosyl iodination sites in human thyroglobulin. Arch Biochem Biophys 334:284-94.

   [Crossref] [Google scholar] [PubMed]

9. Coscia F, Taler-Verčič A (2021) Cryo-EM: A new dawn in thyroid biology. Mol Cel Endocrinol 531:111309.

   [Crossref] [Google scholar] [PubMed]

10. Tosatto L, Coscia F (2022) A glance at post-translational modifications of human thyroglobulin: Potential impact on function and pathogenesis. Eur Thyroid J 11.

    [Crossref] [Google scholar] [PubMed]

11. Coscia F, Taler-Verčič A, Chang VT, Sinn L, O'Reilly FJ, et al. (2020) The structure of human thyroglobulin. Nature 578:627-630

    [Crossref] [Google scholar] [PubMed]

12. Habza-Kowalska E, Kaczor AA, Żuk J, Matosiuk D, Gawlik-Dziki U (2019) Thyroid peroxidase activity is inhibited by phenolic compounds-impact of interaction. Molecules 24.

    [Crossref] [Google scholar] [PubMed]

13. Duplomb S, Benoit A, Mechtouff-Cimarelli L, Cho TH, Derbel O, et al. (2019) Unusual adverse event with vandetanib in metastatic medullary thyroid cancer. J Clin Oncol 30:e21-e23.

    [Crossref] [Google scholar] [PubMed]

14. Goundan PN, Lee SL (2020) Thyroid effects of amiodarone: Clinical update. Curr Opin Endocrinol Diabetes Obes 27:329-334.

    [Crossref] [Google scholar] [PubMed]

15. Raza MA, Jain A, Mumtaz M, Mehmood T (2022) Thyroid storm in a patient on chronic amiodarone treatment. Cureus 14:e24164.

    [Crossref] [Google scholar] [PubMed]