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The parvalbumin metal ion-binding sites differ at the +z and -x residues: Whereas the CD site employs serine and glutamate (or aspartate), respectively, the EF site employs aspartate and glycine. Although frequently indistinguishable in Ca2+- and Mg2+-binding assays, the CD and EF sites nonetheless exhibit markedly different preferences for members of the lanthanide series [Williams et al. (1984) J. Am. Chem. Soc. 106, 5698-5702], underscoring an intrinsic nonequivalence. This nonequivalence reaches its pinnacle in the mammalian beta-parvalbumin (oncomodulin). Whereas the oncomodulin EF site exhibits the expected Ca2+/Mg2+ signature, the Ca2+ affinity of the CD site is severely attenuated. To obtain insight into the structural factors responsible for this reduction in binding affinity, oncomodulin variants were examined in which the CD and EF site ligand arrays had been exchanged. Our data suggest that binding affinity may be dictated either by ligand identity or by the binding site environment. For example, the Ca2+ affinity of the quasi-EF site resulting from the combined S55D and D59G mutations is substantially lower than that of the authentic EF site. This finding implies that other local environmental variables (e.g., binding loop flexibility, electrostatic potentials) within the CD binding site supersede the influence of ligand identity. However, the CD site ligand array does not acquire a high-affinity signature when imported into the EF site, as in the D94S/G98D variant. Instead, it retains its Ca2+-specific signature, implying that this constellation of ligands is less sensitive to placement within the protein molecule. The D59G and D94S single mutations substantially lower binding affinity, consistent with removal of a liganding carboxylate. By contrast, the S55D and G98D mutations substantially increase binding affinity, a finding at odds with corresponding data collected on model peptide systems. Significantly, the Ca2+ affinity of the oncomodulin CD site is increased by mutations that weaken binding at the EF site, indicating a negatively cooperative interaction between the two sites.  相似文献   

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Green-emitting Sr2LiSiO4F:Eu2+ and blue-emitting Sr2MgSi2O7:Eu2+ phosphors were synthesized by the conventional high temperature solid-state route,respectively.Their structures and photoluminescenee properties were comparatively investigated.It was found that the mixture phases of Sr2MgSi2O7 and SrF2 were obtained when a part of Sr2+ in Sr2LiSiO4F was replaced by some amount of Mg2+ in order to design the possible SrMgLiSiO4F:Eu2+ phosphor.Based on the photoluminescence analysis,Sr2LiSiO4F:Eu2+ phosphor exhibited a green broad emission band of main peak at 513 nm under the excitation of 365 nm,while the Sr2MgSi2O7:Eu2+ and SrMgLiSiO4F:Eu2+ phosphor showed blue emission centered at 467 nm.The temperature dependent photoluminescence properties and room temperature decay time for the three kinds of phosphors were also discussed in this paper.  相似文献   

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石建稳  郑经堂  胡燕  赵玉翠 《稀土》2007,28(3):68-70
以钛酸四正丁酯为原料,利用溶胶-凝胶法制备了纯的和不同Eu含量掺杂的TiO2纳米粒子,利用XRD对样品进行了表征,并以甲基橙的光催化降解研究了样品的光催化性能.发现Eu的掺杂抑制了TiO2粒径的增长,细化了晶粒;同时,Eu掺入到TiO2的晶格中,引起了晶格的畸变和膨胀.结果表明,适量Eu的掺杂可提高TiO2的光催化性能,在本试验进行的条件下,当掺杂摩尔分数为0.1%时,光催化性能最好.  相似文献   

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