Pb^2+对核糖核酸酶活性及其结构的影响

通过各种光谱学手段研究了Pb^2+对核糖核酸酶活性的影响及其作用机制。结 果表明低浓度的Pb^2+可提高酶活性，高浓度则严重抑制酶活性，这是因为在高浓 度下Pb^2+能完全竞争出核糖核酸酶中的Ca^2+，荧光滴定显示核糖核酸酶可结合3 个Pb^2+。利用EXAFS表征出Pb^2+已结合到核糖核酸酶方链氨基酸残基上，与N或O 发生了配位，Pb-N(或O)键长分别为0.242nm和0.312nm，配位数均为2。圆二色谱测 试进一步表明高 浓度的Pb^2+结合使核糖核酸酶的二级结构遭到严重破坏，α－螺 旋含量、β－折叠及β－转角大量下降，无规则卷曲则明显增加。

Effect of Pb~(2+) on RNase Activity and Its Structure

Activity of RNase from porcine pancreas was enhanced by treatment with Pb~(2+) at low concentration (0.17×10~(-2)～0.6×10~(-2)μmol/L) but was indibited by Pb~(2+) at high concentration (over 0.85×10~(-2) μmol/L). Pb~(2+) at the high concentration could competitively displace Ca~(2+) from RNase and the fluorescence titration showed that one molecule of RNase had three binding sites for Pb~(2+), the association constant κ_(siv for its low-affinity Pb~(2+) -binding site was 6.03×10~6(mol/L)~(-1). The extended X-ray absorption fine structure (EXAFS) spectrum demonstrated that Pb~(2+) coordinated nitrogen or oxygen atoms of the active site of RNase or other amino acid residue of RNase, Pb-N (or O) bond length was 0.242nm and o.312nm, and coordination number was 2, respectively. The secondary structure of RNase was greatly damaged by Pb~(2+) at high concentration resulting in the decrease of α-helix, β-sheet andβ-turn contents and the increase of random coil contents.