Law A Y, Cherian M G, Stillman M J
Biochim Biophys Acta. 1984 Jan 18;784(1):53-61. doi: 10.1016/0167-4838(84)90172-9.
The absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectra of native rat liver and crab (Scylla serrata) Cd,Zn-metallothionein have been measured and the data are compared. The MCD data indicate that there are close similarities in the geometries of the cadmium-binding sites in both of these proteins; however, the CD spectra are quite different for the rat liver and crab proteins. The CD spectrum for the crab metallothionein is unlike any previously reported for a cadmium-containing metallothionein. This suggests that the CD spectrum is sensitive to the different bridging pattern used in the binding sites in the crab compared with the rat-liver metallothionein. Cadmium binding to the metal-free metallothionein is demonstrated for both proteins and it is shown that there are only minor structural differences between the native and remetallated proteins. The structural changes that occur near to the cadmium-binding sites during cadmium loading to the native proteins have been followed using absorption and CD spectroscopy. Marked changes are observed in the CD spectrum which can be associated with a two-phase reaction: initially Zn2+ is displaced by the Cd2+, then at higher concentrations of Cd2+ the tetrahedral geometry of the Cd2+-binding sites is lost as more Cd2+ is bound using the same thiolate groups. While this latter reaction results in considerable change to the CD spectrum, only minor changes are observed in the absorption spectrum. A significant red shift is observed in the S leads to Cd charge transfer transition region of the MCD spectrum (230-270 nm) following both cadmium loading of native rat liver, Cd,Zn-metallothionein and the metallation of metal-free metallothionein with cadmium. There are two contributions to this effect in Cd,Zn-metallothionein: (i) there is a S leads to Zn band underlying the S leads to Cd band; and (ii) the occupation of zinc sites by cadmium changes the energy of the S leads to Cd transition.
已测量了天然大鼠肝脏和蟹（锯缘青蟹）镉、锌金属硫蛋白的吸收光谱、圆二色性（CD）光谱和磁圆二色性（MCD）光谱，并对数据进行了比较。MCD数据表明，这两种蛋白质中镉结合位点的几何结构非常相似；然而，大鼠肝脏和蟹蛋白质的CD光谱却大不相同。蟹金属硫蛋白的CD光谱与之前报道的任何含镉金属硫蛋白的光谱都不同。这表明，与大鼠肝脏金属硫蛋白相比，CD光谱对蟹结合位点中使用的不同桥连模式敏感。两种蛋白质都证明了镉与无金属金属硫蛋白的结合，并且表明天然蛋白质和重新金属化蛋白质之间只有微小的结构差异。利用吸收光谱和CD光谱跟踪了天然蛋白质镉负载过程中镉结合位点附近发生的结构变化。在CD光谱中观察到明显变化，这可能与一个两相反应有关：最初Zn2+被Cd2+取代，然后在更高浓度的Cd2+下，随着更多的Cd2+使用相同的硫醇盐基团结合，Cd2+结合位点的四面体几何结构丧失。虽然后一个反应导致CD光谱发生相当大的变化，但在吸收光谱中只观察到微小变化。在天然大鼠肝脏镉、锌金属硫蛋白镉负载以及无金属金属硫蛋白用镉金属化后，MCD光谱（230 - 270 nm）的S→Cd电荷转移跃迁区域观察到显著的红移。在镉、锌金属硫蛋白中，这种效应有两个贡献：（i）在S→Cd带下方有一个S→Zn带；（ii）镉占据锌位点改变了S→Cd跃迁的能量。
