Reductive Dehalogenation in Mammals by Iodotyrosine Deiodinase

描述（由申请人提供）：有机卤化物通过自然和人类活动在环境中无处不在。大多数生物体通过酶介导的水解、消除或氧化来解毒和降解这些化合物。某些微生物也能够促进还原性脱卤，尽管这主要限于厌氧代谢。哺乳动物为这一普遍现象提供了一个有趣的例外。必需激素甲状腺素（3-[4-[4-羟基-3,5-二碘苯氧基]-3,5-二碘苯基]丙氨酸）在多种组织中被含硒半胱氨酸的酶还原去碘化。令人惊讶的是，在甲状腺中采用了一种完全不同的策略来脱碘3-碘和3,5-二碘酪氨酸。在这种情况下，一种独特的黄蛋白，碘酪氨酸脱碘酶，负责减少碘化氨基酸，以回收碘化物，用于甲状腺素的生物合成。现在提出的研究，以确定这种哺乳动物脱碘酶的前所未有的化学和机制。这种酶的新特性将扩展已知的黄素依赖性催化和生物学中可用的处理卤化化合物的途径。我们对催化的描述将集中在C-I键的还原和IYD中还原黄素的伴随氧化。光谱和产品分析将用于区分一种和两种电子过程，并首次揭示由IYD处理的衬底的全部范围。底物和黄素辅助因子的激活将通过独立测量酶突变体和底物类似物的识别和催化特性来描述。同时，黄素化学的底物依赖性控制将通过其氧化还原性质的变化来检测。这些研究也将通过继续对IYD的晶体学研究而得到丰富。最后，这种不寻常的脱碘的起源将通过表达和表征来自生物的同源基因来检验，这些基因在“生命之树”中依次离哺乳动物更远。公共卫生相关性：碘是我们饮食的必要组成部分，用于在甲状腺中产生一种含碘激素，这种激素是调节我们整个身体代谢率所必需的。我们将研究从激素生物合成过程中形成的副产物中回收碘的过程，这对了解某些导致甲状腺功能减退的先天性缺陷的基础至关重要。

DESCRIPTION (provided by applicant): Organohalides are ubiquitous in the environment through natural and human activities. Most organisms detoxify and degrade these compounds by enzyme-mediated hydrolysis, elimination or oxidation. Certain microbes are also capable of promoting reductive dehalogenation, although this is primarily limited to anaerobic metabolism. Mammals provide a fascinating exception to this general observation. The essential hormone, thyroxine (3-[4-[4-hydroxy-3,5-diiodophenoxy]-3,5-diiodophenyl]alanine), is reductively deiodinated in a variety of tissues by selenocysteine-containing enzymes. Quite surprisingly, an entirely different strategy has been recruited in the thyroid to deiodinate 3-iodo- and 3,5-diiodotyrosine. In this case, a unique flavoprotein, iodotyrosine deiodinase, is responsible for reducing the iodinated amino acids in order to salvage iodide for reuse in thyroxine biosynthesis. Investigations are now proposed to identify the unprecedented chemistry and mechanism of this mammalian deiodinase. The novel properties of this enzyme will extend the known repertoire of flavin-dependent catalysis and pathways available in biology to process halogenated compounds. Our description of catalysis will focus on the reduction of the C-I bond and concomitant oxidation of the reduced flavin in IYD. Spectroscopic and product analyses will be used to differentiate between one and two electron processes and, for the first time, reveal the full range of substrates that are processed by IYD. Activation of both the substrate and flavin cofactor will be described by independently measuring recognition and catalytic properties of enzyme mutants and substrate analogues. Concurrently, substrate-dependent control of the flavin chemistry will be detected by changes in its redox properties. These investigations will be enriched as well by continuing crystallographic studies of IYD. Finally, the origins of this unusual deiodination will be examined by expressing and characterizing homologous genes from organisms that are successively more distant from mammals in the "Tree of Life." PUBLIC HEALTH RELEVANCE: Iodide is a necessary component of our diet and used to produce an iodide-containing hormone in the thyroid that is required for regulating the metabolic rate of our entire body. The process by which we recycle iodide from byproducts formed during hormone biosynthesis will be investigated and is crucial to understand the basis for certain congenital defects leading to hypothyroidism.