Department of Chemistry & Biochemistry, University of Maryland, College Park, MD 20742, USA.
Biochimie. 2010 Sep;92(9):1227-35. doi: 10.1016/j.biochi.2010.02.013. Epub 2010 Feb 16.
Daily ingestion of iodide alone is not adequate to sustain production of the thyroid hormones, tri- and tetraiodothyronine. Proper maintenance of iodide in vivo also requires its active transport into the thyroid and its salvage from mono- and diiodotyrosine that are formed in excess during hormone biosynthesis. The enzyme iodotyrosine deiodinase responsible for this salvage is unusual in its ability to catalyze a reductive dehalogenation reaction dependent on a flavin cofactor, FMN. Initial characterization of this enzyme was limited by its membrane association, difficult purification and poor stability. The deiodinase became amenable to detailed analysis only after identification and heterologous expression of its gene. Site-directed mutagenesis recently demonstrated that cysteine residues are not necessary for enzymatic activity in contrast to precedence set by other reductive dehalogenases. Truncation of the N-terminal membrane anchor of the deiodinase has provided a soluble and stable source of enzyme sufficient for crystallographic studies. The structure of an enzyme.substrate co-crystal has become invaluable for understanding the origins of substrate selectivity and the mutations causing thyroid disease in humans.
单独摄入碘不足以维持甲状腺激素三碘甲状腺原氨酸和甲状腺素四碘的产生。碘在体内的适当维持还需要其主动转运到甲状腺,并从单碘酪氨酸和二碘酪氨酸中回收,这些物质在激素生物合成过程中过量形成。负责这种回收的碘酪氨酸脱碘酶因其能够催化依赖黄素辅因子 FMN 的还原脱卤反应而具有独特的能力。该酶的初步表征受到其膜结合、难以纯化和较差稳定性的限制。只有在鉴定并异源表达其基因后,该脱碘酶才易于进行详细分析。最近的定点突变表明,与其他还原脱卤酶设定的先例相反,半胱氨酸残基对于酶活性不是必需的。脱碘酶的 N 端膜锚截断提供了足够用于晶体学研究的可溶性和稳定的酶源。酶-底物共晶的结构对于理解底物选择性的起源以及导致人类甲状腺疾病的突变非常有价值。