Revisões, Atualizações e Artigos Originais February 01, 2004 February 01, 2004

Triplets! Unexpected structural similarity among the three enzymes that catalyze initiation and termination of thyroid hormone effects

DOI: 10.1590/S0004-27302004000100004

The three iodothyronine deiodinases catalyze the initiation (D1, D2) and termination (D3) of thyroid hormone effects in vertebrates. A recently conceived 3-dimensional model predicts that these enzymes share a similar structural organization and belong to the thioredoxin (TRX) fold superfamily. Their active center is a selenocysteine-containing pocket defined by the b1-a1-b2 motifs of the TRX fold and a domain that shares strong similarities with the active site of iduronidase, a member of the clan GH-A fold of glycoside hydrolases. While D1 and D3 are long-lived plasma membrane proteins, D2 is an endoplasmic reticulum resident protein with a half-life of only 20min. D2 inactivation is mediated by selective UBC-7-mediated conjugation to ubiquitin, a process that is accelerated by T₄ catalysis, thus maintaining local T₃ homeostasis. In addition, D2 interacts with and is a substrate of the pVHL-interacting deubiquitinating enzymes (VDU1 and VDU2); thus deubiquitination regulates the supply of active thyroid hormone in D2-expressing cells.