APPARENT HEAT-CAPACITY CHANGE ACCOMPANYING A NONSPECIFIC PROTEIN-DNA INTERACTION. ESCHERICHIA-COLI SSB TETRAMER BINDING TO OLIGODEOXYADENYLATES
APPARENT HEAT-CAPACITY CHANGE ACCOMPANYING A NONSPECIFIC PROTEIN-DNA INTERACTION. ESCHERICHIA-COLI SSB TETRAMER BINDING TO OLIGODEOXYADENYLATES
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DOI：
10.1021/bi00209a022
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发表时间：
1994-11-01
期刊：
影响因子：
2.9
通讯作者：
LOHMAN, TM
中科院分区：
文献类型：
--
作者：
FERRARI, ME;LOHMAN, TM
本文研究了温度对大肠杆菌SSB四聚体与一系列单链寡脱氧核苷酸dT（pT）（n）、dC（pC）（n）和dA（pA）（n）（n = 34，55和69）结合的平衡常数K-obs的影响，以探讨大肠杆菌强选择性结合的热力学基础。coli SSB（和其他SSB蛋白）用于结合ss-DNA的聚嘧啶片段。除了预期的K-obs大小的碱基依赖性差异之外，我们还观察到SSB四聚体与寡聚脱氧腺苷酸结合的定性不同的温度依赖性。获得SSB四聚体与dT（pT）（n）和dC（pC）（n）结合的线性范特霍夫图，Δ H度（obs）范围为-50至-100 kcal/mol，取决于寡脱氧核苷酸长度和盐浓度。相比之下，SSB四聚体与dA（pA）（N）结合的所有van 't霍夫图明显是非线性的，其中K-obs的最大值出现在25 ℃附近，表明摩尔热容的明显大的负变化（Δ C度（P，obs）< 0）。因此，对于SSB-dA（pA）（n）相互作用，Δ H度（obs）和Δ S度（obs）都是高度依赖于温度的，但是补偿使得Δ G度（obs）对温度相对不敏感。这些非线性范特霍夫图不是由于SSB组装与dA（pA）（n）结合的偶联，也不是由于其他SSB-PNA结合模式形成中的温度依赖性位移。SSB四聚体与dA（pA）（n）结合的非线性van't霍夫图似乎是由两个过程的耦合引起的：（1）dA（pA）（n）碱基的解堆叠（发生在Delta H度> 0且Delta C度（P）= 0时）和（2）SSB与未堆叠DNA的结合（发生在Delta H度< 0且Delta C度（P）= 0时）。因此，尽管每个孤立的平衡发生在Δ C度（P）接近0的情况下，但由于耦合平衡，总体平衡显示出明显的Δ C度（P，obs）< 0。SSB与dT（pT）（n）和dC（pC）（n）的结合发生在Δ H度< 0和Δ C度（P，obs）= 0的情况下，因为这些ss-DNA分子中的碱基不会明显堆叠。这些结果表明，非特异性蛋白质-DNA相互作用可以显示出大的负表观Δ C度（P）;然而，这种效应似乎不是由于疏水效应，而是由于与蛋白质结合偶联的DNA中的温度依赖性构象转变。这些意见对其他蛋白质-核酸系统的影响进行了讨论。
We have examined the effects of temperature on the equilibrium constant, K-obs, for Escherichia coli SSB tetramer binding to a series of single-stranded (ss) oligodeoxyribonucleotides, dT(pT)(n), dC(pC)(n), and dA(pA)(n) (n = 34, 55, and 69) in order to investigate the thermodynamic basis for the strong preference of E. coli SSB (and other SSB proteins) for binding polypyrimidine stretches of ss-DNA. In addition to the expected base-dependent differences in the magnitude of K-obs, we also observe qualitatively different temperature dependencies for the binding of the SSB tetramer to oligodeoxyadenylates. Linear van't Hoff plots are obtained for SSB tetramer binding to dT(pT)(n) and dC(pC)(n), with Delta H degrees(obs) ranging from -50 to -100 kcal/mol depending on the oligodeoxynucleotide length and salt concentration. In contrast, all van't Hoff plots for SSB tetramer binding to dA(pA)(N) are distinctly nonlinear with maxima in K-obs occurring near 25 degrees C, indicative of an apparent large negative change in molar heat capacity (Delta C degrees(P,obs) < 0). Thus for the SSB-dA(pA)(n) interaction, Delta H degrees(obs) and Delta S degrees(obs) are both highly temperature dependent, but compensate such that Delta G degrees(obs) is relatively insensitive to temperature. These nonlinear van't Hoff plots are not due to coupling of SSB assembly to dA(pA)(n) binding or to temperature-dependent shifts in the formation of other SSB-PNA binding modes. The nonlinear van't Hoff plots for SSB tetramer binding to dA(pA)(n) appear to result from the coupling of two processes: (1) the unstacking of the dA(pA)(n) bases (occurring with Delta H degrees > 0 and Delta C degrees(P) = 0) and (2) the binding of SSB to the unstacked DNA (occurring with Delta H degrees < 0 and Delta C degrees(P) = 0). Therefore, although each isolated equilibrium occurs with Delta C degrees(P) approximate to 0, the overall equilibrium displays an apparent Delta C degrees(P,obs) < 0 due to the coupled equilibrium. The binding of SSB to dT(pT)(n) and dC(pC)(n) occurs with Delta H degrees < 0 and Delta C degrees(P,obs) = 0, since the bases in these ss-DNA moleculesdo not stack appreciably. These results indicate that a nonspecific protein-DNA interaction can display a large negative apparent Delta C degrees(P); however, this effect appears not to be due to the hydrophobic effect, but rather to a temperature-dependent conformational transition in the DNA that is coupled to protein binding. Implications of these observations for other protein-nucleic acid systems are discussed.