Active Site Binding Is Not Sufficient for Reductive Deiodination by lodotyrosine Deiodinase
Active Site Binding Is Not Sufficient for Reductive Deiodination by lodotyrosine Deiodinase
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DOI：
10.1021/acs.biochem.6b01308
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发表时间：
2017-02-28
期刊：
影响因子：
2.9
通讯作者：
Rokita, Steven E.
中科院分区：
文献类型：
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作者：
Ingavat, Nattha;Kavran, Jennifer M.;Rokita, Steven E.
碘酪氨酸脱碘酶的底物识别和营业额的最低要求进行了检查，以了解其催化特异性的基础。这种酶对碘的体内平衡和脊索动物甲状腺激素的产生至关重要。2-碘苯酚与人体酶的结合非常弱，并且以比碘酪氨酸低4个数量级以上的k（cat）/K-m脱卤。这种区分可能防止甲状腺激素生物合成的碘化和脱碘前体的无效循环。令人惊讶的是，一个非常相似的催化选择性表达的细菌同源物从Haliscomenoprosthydrossis。在该实施例中，区分不是基于亲和力，因为4-氰基-2-碘苯酚以低于碘酪氨酸的Kd结合到细菌脱碘酶，但未被可检测地脱碘。其他酚类包括2-碘苯酚被脱碘，但效率很低。具有和不具有结合的碘酪氨酸的细菌酶的晶体结构几乎是可重叠的，并且与人酶的相应结构非常相似。同样，细菌酶在与底物类似物氟酪氨酸结合后被激活进行单电子转移，如先前在人酶中观察到的。细菌脱碘酶和2-碘苯酚的共晶结构表明，该配体堆叠在活性位点黄素单核苷酸（FMN）的方向类似于绑定的碘酪氨酸。然而，2-碘苯酚缔合不足以激活催化所需的FMN化学，因此细菌酶似乎对卤代酪氨酸具有相似的特异性，即使它们的生理作用可能与人类中的生理作用非常不同。
The minimal requirements for substrate recognition and turnover by iodotyrosine deiodinase were examined to learn the basis for its catalytic specificity. This enzyme is crucial for iodide homeostasis and the generation of thyroid hormone in chordates. 2-Iodophenol binds only very weakly to the human enzyme and is dehalogenated with a k(cat)/K-m that is more than 4 orders of magnitude lower than that for iodotyrosine. This discrimination likely protects against a futile cycle of iodinating and deiodinating precursors of thyroid hormone biosynthesis. Surprisingly, a very similar catalytic selectivity was expressed by a bacterial homologue from Haliscomenobacter hydrossis. In this example, discrimination was not based on affinity since 4-cyano-2-iodophenol bound to the bacterial deiodinase with a K-d lower than that of iodotyrosine and yet was not detectably deiodinated. Other phenols including 2-iodophenol were deiodinated but only very inefficiently. Crystal structures of the bacterial enzyme with and without bound iodotyrosine are nearly superimposable and quite similar to the corresponding structures of the human enzyme. Likewise, the bacterial enzyme is activated for single electron transfer after binding to the substrate analogue fluorotyrosine as previously observed with the human enzyme. A cocrystal structure of bacterial deiodinase and 2-iodophenol indicates that this ligand stacks on the active site flavin mononucleotide (FMN) in a orientation analogous to that of bound iodotyrosine. However, 2-iodophenol association is not sufficient to activate the FMN chemistry required for catalysis, and thus the bacterial enzyme appears to share a similar specificity for halotyrosines even though their physiological roles are likely very different from those in humans.