植物螯合肽(PC)是在植物中合成的含有重复γ-Glu-Cys基序的聚Cys肽,藻类,某些真菌,和蠕虫通过还原型谷胱甘肽的PC合成酶。已经表明,过量的有毒金属离子会诱导它们的生物合成,并且它们负责解毒过程。对它们在无毒下参与必需金属结合的情况知之甚少,PC合酶具有活性的基础条件。本研究介绍了与PC2-PC5系列的光谱和热力学相互作用,主要集中在Zn(II)络合物稳定性与细胞Zn(II)可用性之间的关系。研究采用了质谱法,紫外-可见光谱,电位计,用锌探针进行竞争分析,和等温滴定量热法(ITC)。所有的肽形成ZnL复合物,而ZnL2仅在PC2中发现,在配位球中含有两到四个硫供体。在Zn(II)的情况下,不会形成Cd(II)-PC配合物的典型双核物种。结果表明,对Zn(II)的亲和力从PC2到PC4线性增加,范围从微至低皮摩尔。进一步的伸长不会显著增加稳定性。稳定性提高主要由与螯合效应和构象限制有关的熵因素驱动,而不是与硫供体数量增加有关的焓因素驱动。研究的PC的亲和力落在植物中观察到的可交换Zn(II)浓度范围内(数百pM),首次支持PC在正常条件下缓冲和胞质Zn(II)浓度中的作用,不暴露于锌过量,在许多研究中都发现了短PC。此外,我们发现,Cd(II)-PC配合物显示出显着更高的金属容量,由于多核物种的形成,缺乏锌(II),支持PCs在Cd(II)储存(解毒)和Zn(II)缓冲和消声中的作用。我们关于植物螯合素的配位化学和热力学的结果对于锌生物学和理解镉毒性的分子基础是重要的,为未来的研究留下了空间。
Phytochelatins (PCs) are poly-Cys peptides containing a repeating γ-Glu-Cys motif synthesized in plants, algae, certain fungi, and worms by PC synthase from reduced glutathione. It has been shown that an excess of toxic metal ions induces their biosynthesis and that they are responsible for the detoxification process. Little is known about their participation in essential metal binding under nontoxic, basal conditions under which PC synthase is active. This study presents spectroscopic and thermodynamic interactions with the PC2-PC5 series, mainly focusing on the relations between Zn(II) complex stability and cellular Zn(II) availability. The investigations employed mass spectrometry, UV-vis spectroscopy, potentiometry, competition assays with zinc probes, and isothermal titration calorimetry (ITC). All peptides form ZnL complexes, while ZnL2 was found only for PC2, containing two to four sulfur donors in the coordination sphere. Binuclear species typical of Cd(II)-PC complexes are not formed in the case of Zn(II). Results demonstrate that the affinity for Zn(II) increases linearly from PC2 to PC4, ranging from micro- to low-picomolar. Further elongation does not significantly increase the stability. Stability elevation is driven mainly by entropic factors related to the chelate effect and conformational restriction rather than enthalpic factors related to the increasing number of sulfur donors. The affinity of the investigated PCs falls within the range of exchangeable Zn(II) concentrations (hundreds of pM) observed in plants, supporting for the first time a role of PCs both in buffering and in muffling cytosolic Zn(II) concentrations under normal conditions, not exposed to zinc excess, where short PCs have been identified in numerous studies. Furthermore, we found that Cd(II)-PC complexes demonstrate significantly higher metal capacities due to the formation of polynuclear species, which are lacking for Zn(II), supporting the role of PCs in Cd(II) storage (detoxification) and Zn(II) buffering and muffling. Our results on
phytochelatins\' coordination chemistry and thermodynamics are important for zinc biology and understanding the molecular basis of cadmium toxicity, leaving room for future studies.