Laboratory of Bioinorganic Chemistry, University of Bologna, Italy.
Acc Chem Res. 2011 Jul 19;44(7):520-30. doi: 10.1021/ar200041k. Epub 2011 May 4.
Transition metals are both essential to enzymatic catalysis and limited in environmental availability. These two biological facts have together driven organisms to evolve mechanisms for selective metal ion sensing and utilization. Changes in metal ion concentrations are perceived by metal-dependent transcription factors and transduced into appropriate cellular responses, which regulate the machineries of competitive metal ion homeostasis and metallo-enzyme activation. The intrinsic toxicity of the majority of metal ions further creates a need for regulated intracellular trafficking, which is carried out by specific chaperones. The Ni(2+)-dependent urease enzymatic system serves as a paradigm for studying the strategies that cells use to handle an essential, yet toxic, metal ion. Although the discovery of urease as the first biological system for which nickel is essential for activity dates to 1975, the rationale for Ni(2+) selection, as well as the cascade of events involving metal-dependent gene regulation and protein-protein interactions leading to enzyme activation, have yet to be fully unraveled. The past 14 years since the Account by Hausinger and co-workers (Karplus, P. A.; Pearson, M. A.; Hausinger, R. P. Acc. Chem. Res. 1997, 30, 330-337) have witnessed impressive achievements in the understanding of the biological chemistry of Ni(2+) in the urease system. In our Account, we discuss more recent advances in the comprehension of the specific role of Ni(2+) in the catalysis and the interplay between Ni(2+) and other metal ions, such as Zn(2+) and Fe(2+), in the metal-dependent enzyme activity. Our discussion focuses on work carried out in our laboratory. In particular, the structural features of the enzyme bound to inhibitors, substrate analogues, and transition state or intermediate analogues have shed light on the catalytic mechanism. Structural and functional information has been correlated to understand the Ni(2+) sensing effected by NikR, a nickel-dependent transcription factor. The urease activation process, involving insertion of Ni(2+) into the urease active site, has been in part dissected and analyzed through the investigation of the molecular properties of the accessory proteins UreD, UreF, and UreG. The intracellular trafficking of Ni(2+) has been rationalized through a deeper understanding of the structural and metal-binding properties of the metallo-chaperone UreE. All the while, a number of key general concepts have been revealed and developed. These include an understanding of (i) the overall ancillary role of Zn(2+) in nickel metabolism, (ii) the intrinsically disordered nature of the GTPase responsible for coupling the energy consumption to the carbon dioxide requirement for the urease activation process, and (iii) the role of the accessory proteins regulating this GTPase activity.
过渡金属是酶催化所必需的，同时在环境中的可用性也受到限制。这两个生物学事实共同促使生物体进化出选择性金属离子感应和利用的机制。金属离子浓度的变化被金属依赖的转录因子感知，并转化为适当的细胞反应，从而调节竞争性金属离子稳态和金属酶激活的机制。大多数金属离子的固有毒性进一步要求进行受调控的细胞内运输，这是通过特定的伴侣蛋白来完成的。镍（Ⅱ）依赖性脲酶酶系统是研究细胞用于处理必需但有毒的金属离子的策略的范例。尽管脲酶作为第一个镍对其活性至关重要的生物系统的发现可以追溯到 1975 年，但镍（Ⅱ）选择的基本原理，以及涉及金属依赖性基因调控和蛋白-蛋白相互作用导致酶激活的事件级联，尚未完全阐明。自豪辛格及其同事的综述（Karplus，P. A.；Pearson，M. A.；Hausinger，R. P. Acc. Chem. Res. 1997，30，330-337）发表以来的过去 14 年见证了对脲酶系统中镍（Ⅱ）生物化学的理解方面令人瞩目的成就。在我们的综述中，我们讨论了对镍（Ⅱ）在催化中的特定作用以及镍（Ⅱ）与其他金属离子（如锌（Ⅱ）和铁（Ⅱ））在金属依赖性酶活性中的相互作用的更深入理解方面的最新进展。我们的讨论重点是在我们实验室进行的工作。特别是，与抑制剂、底物类似物和过渡态或中间态类似物结合的酶的结构特征阐明了催化机制。通过研究辅助蛋白 UreD、UreF 和 UreG 的分子特性，将结构和功能信息相关联以了解由镍依赖转录因子 NikR 介导的镍（Ⅱ）感应。脲酶的激活过程，包括将镍（Ⅱ）插入脲酶活性位点，部分通过研究辅助蛋白 UreD、UreF 和 UreG 的分子特性进行了剖析和分析。通过深入了解金属伴侣蛋白 UreE 的结构和金属结合特性，对镍（Ⅱ）的细胞内运输进行了合理化。同时，揭示并发展了一些关键的一般概念。这些概念包括对（i）锌（Ⅱ）在镍代谢中的整体辅助作用的理解，（ii）负责将能量消耗与脲酶激活过程中二氧化碳需求耦合的 GTP 酶的固有无序性质，以及（iii）调节该 GTP 酶活性的辅助蛋白的作用的理解。
