Rapid Kinetics of Dehalogenation Promoted by Iodotyrosine Deiodinase from Human Thyroid

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• 作者：Kostyantyn D. Bobyk ; David P. Ballou ; Steven E. Rokita
• 刊名：Biochemistry
• 出版年：2015
• 出版时间：July 28, 2015
• 年：2015
• 卷：54
• 期：29
• 页码：4487-4494
• 全文大小：390K
• ISSN：1520-4995

文摘

Reductive dehalogenation such as that catalyzed by iodotyrosine deiodinase (IYD) is highly unusual in aerobic organisms but necessary for iodide salvage from iodotyrosine generated during thyroxine biosynthesis. Equally unusual is the dependence of this process on flavin. Rapid kinetics have now been used to define the basic processes involved in IYD catalysis. Time-dependent quenching of flavin fluorescence was used to monitor halotyrosine association to IYD. The substrates chloro-, bromo-, and iodotyrosine bound with similar rate constants (k_on) ranging from 1.3 脳 10⁶ to 1.9 脳 10⁶ M^鈥? s^鈥?. Only the inert substrate analogue fluorotyrosine exhibited a significantly (5-fold) slower k_on (0.3 脳 10⁶ M^鈥? s^鈥?). All data fit a standard two-state model and indicated that no intermediate complex accumulated during closure of the active site lid induced by substrate. Subsequent halide elimination does not appear to limit reactions of bromo- and iodotyrosine since both fully oxidized the reduced enzyme with nearly equivalent second-order rate constants (7.3 脳 10³ and 8.6 脳 10³ M^鈥? s^鈥?, respectively) despite the differing strength of their carbon鈥揾alogen bonds. In contrast to these substrates, chlorotyrosine reacted with the reduced enzyme approximately 20-fold more slowly and revealed a spectral intermediate that formed at approximately the same rate as the bromo- and iodotyrosine reactions.