BioCyc Id	RXN-10604
Mass Balance Status	Balanced.
Direction Note	The reaction direction shown is in accordance with the Enzyme Commission rules (the physiological direction of the reaction, if known, is indicated by the arrow direction).
EC Number	1.21.99.4 -- thyroxine 5′-deiodinase
Supersedes EC Number	1.97.1.10
Enzyme Commission Synonyms	diiodothyronine 5′-deiodinase [ambiguous] iodothyronine 5′-deiodinase iodothyronine outer ring monodeiodinase type I iodothyronine deiodinase type II iodothyronine deiodinase thyroxine 5-deiodinase [misleading] L-thyroxine iodohydrolase (reducing)

Enzyme Commission Summary

The enzyme activity has only been demonstrated in the direction of 5'-deiodination, which renders the thyroid hormone more active. The enzyme consists of type I and type II enzymes, both containing selenocysteine, but with different kinetics. For the type I enzyme the first reaction is a reductive deiodination converting the -Se-H group of the enzyme into an -Se-I group; the reductant then reconverts this into -Se-H, releasing iodide.