Functional structures of nucleic acids and their recognition by proteins

核糖核酸酶中能催化生化反应的核酶。由于核酶需要金属离子来发挥催化功能，因此核酶被认为是金属酶之一。核酶反应对金属离子的种类和浓度非常敏感，这表明二价金属离子可能在核酶的化学反应和结构稳定中起重要作用。（前导酶）CUGGGAGUCC和底物GGACCGAGCCAG，并对配合物活性中心的裂解步骤进行了动力学研究，结果表明，Nd^<3+>的加入在Pb^<2+>存在下，尽管除Pb^<2+>外的其它稀土离子或二价离子对RNA切割反应没有促进作用，但铅酶的RNA切割反应产率显著增加。此外，在不同浓度的Nd^<3+>和Pb^<2+>下测量了铅酶反应的动力学。在金属离子总浓度不变的情况下，低浓度的Nd ^<3 +>增加了铅酶与底物复合物的稳定性。相反，在高浓度比的Nd^<3+>，Nd^<3+>的加入降低了配合物的稳定性。当Nd^<3+>与Pb^<2+>的比例为1：1时，裂解步骤的速率常数最大。这些结果表明，在酶与底物的复合物中，存在着影响复合物稳定性的Nd^<3+>离子结合位点，并直接催化裂解反应。在此基础上，提出了Pb^2+和Nd^3+分别起碱催化剂和酸催化剂作用的双金属离子机理。

A ribozyme in an RNA enzyme that can catalyze a biochemical reaction. Because a ribozyme requires a metal ions for a catalytic function, the ribozyme is considered as one of the metalloenzymes. Ribozyme reactions are very sensitive to species and concentration of metal ions, suggesting that divalent metal ions may play important roles for the chemical reaction and structural stabilization of the ribozyme.We have investigated the combined effects of metal ions on both the structural stabilization of the complex between a ribozyme (a leadzyme), CUGGGAGUCC,and a substrate, GGACCGAGCCAG,and the cleavage step in the active center of the complex has been also investigated kinetically.The results showed that the addition of Nd^<3+> in the presence of Pb^<2+> increased significantly the yield of the RNA cleavage reaction by the leadzyme, although other rare earth ions or divalent ions except Pb^<2+> did not promote the reaction. Further, kinetics for the leadzyme reaction was measured at various concentrations of Nd^<3+> and Pb^<2+>. At low concentration rations of Nd^<3+> under a constant total concentration of metal ions, Nd^<3+> increased the stablility of the complex between the leadzyme and the substrate. In contrast, at high concentration ratios of Nd^<3+>, the addition of Nd^<3+> decreased the stability of the complex. The rate constant of the cleavage step was maximized when the ratio of Nd^<3+> to Pb^<2+> was 1 : 1. These results suggest that the complex between the leadzyme and the substrate has binding sites for Nd^<3+> ion taht influence complex stability and catalyze directly the cleavage reaction. On the basis of the results, we propose a two-metal-ion mechanism in which Pb^<2+> and Nd^<3+> play the roles of base and acid catalyst, respectively.