Allosteric conversion of Z DNA to an intercalated right-handed conformation by daunomycin.
Allosteric conversion of Z DNA to an intercalated right-handed conformation by daunomycin.
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发表时间：
1986-07
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通讯作者：
J. Chaires
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作者：
J. Chaires
用吸光度和荧光法测量了抗癌药物柔红霉素与聚(DGdC)在离子条件下的结合，该离子条件有利于聚合物的左手Z构象。药物结合在这些条件下是协同的，可以完全用变构模型来解释，在该模型中，药物优先(但不是排他性地)结合到右旋B构象，并将聚合物从Z移动到插层右旋构象。根据变构模型对结合等温线进行定量分析，可以估计B-Z界面上的碱基对从Z构象转换到B构象的平衡常数，以及在Z构象中的一段螺旋内形成B构象中的碱基对。根据这一数据计算了碱基对Z到B的转换自由能，在2.4-3.5M的氯化钠范围内，范围为+0.03到+0.3kcal/mol。在相同的氯化钠浓度范围内，形成B-Z结的自由能在+4.0kcal/mol时几乎是恒定的。Z-B转变自由能的盐依赖性表明Na+优先结合到Z型，当碱基对从Z构象转换到B构象时，有Na+的净释放。通过这一分析发现，能量上不利的Z到B转变是由柔红霉素与B型DNA的能量有利的相互作用所驱动的。以3.5M氯化钠为例，总反应(Z DNA碱基对)+(柔红霉素)与(右旋络合物)的平衡自由能变化为-7.0kcal/mol，几乎全部由药物与B DNA的结合贡献。用变构模型的分析还表明，每个结合药物分子从Z构象转化为B构象的碱基对的数量与盐有关，并提供了药物分子在右旋构象中分配到聚合物区域的证据。
Absorbance and fluorescence methods were used to measure the binding of the anticancer drug daunomycin to poly (dGdC) under ionic conditions that initially favor the left-handed Z conformation of the polymer. Drug binding was cooperative under these conditions and may be fully accounted for by an allosteric model in which the drug binds preferentially (but not exclusively) to the right-handed B conformation and shifts the polymer from the Z to an intercalated right-handed conformation. Quantitative analysis of binding isotherms in terms of the allosteric model allowed for estimation of the equilibrium constants for the conversion of a base pair at a B-Z interface from the Z to the B conformation and for the formation of a base pair in the B conformation within a stretch of helix in the Z conformation. The free energy of the Z to B conversion of a base pair was calculated from this data and ranges from +0.03 to +0.3 kcal/mol over the NaCl range of 2.4-3.5 M. The free energy for the formation of a B-Z junction was nearly constant at +4.0 kcal/mol over the same range of NaCl concentrations. The salt dependence of the free energy of the Z to B transition indicates preferential Na+ binding to the Z form and that there is a net release of Na+ upon conversion of a base pair from the Z to the B conformation. The energetically unfavorable Z to B transition was found by this analysis to be driven by coupling to the energetically favorable interaction of daunomycin with B form DNA. In 3.5 M NaCl, for example, the free energy change for the overall reaction (Z DNA base pairs) + (daunomycin) in equilibrium with (right-handed complex) is -7.0 kcal/mol, nearly all of which is contributed by the binding of drug to B DNA. Analysis using the allosteric model also shows that the number of base pairs converted from the Z to the B conformation per bound drug molecule is salt dependent and provides evidence that drug molecules partition into regions of the polymer in the right-handed conformation.