Intercalative G-Tetraplex Stabilization of Telomeric DNA by a Cationic Porphyrin

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• 作者：Ihtshamul Haq ; John O. Trent ; Babur Z. Chowdhry ; and Terence C. Jenkins
• 刊名：Journal of the American Chemical Society
• 出版年：1999
• 出版时间：March 10, 1999
• 年：1999
• 卷：121
• 期：9
• 页码：1768 - 1779
• 全文大小：258K
• 年卷期：v.121,no.9(March 10, 1999)
• ISSN：1520-5126

文摘

DNA tetraplex ("quadruplex") structures formed from guanine tetrads and significant to the regulationof telomerase activity have recently been shown to be stabilized by interaction with a cationic porphyrin[Anantha, N. V.; Azam, M.; Sheardy, R. D. Biochemistry 1998, 37, 2709-2714. Wheelhouse, R. T.; Sun, D.;Han, H.; Han, F. X.; Hurley, L. H. J. Am. Chem. Soc. 1998, 120, 3261-3262]. Porphyrin binding to DNAsequences from the Oxytricha and human telomeres and a thrombin-binding aptamer is here examined byisothermal titration calorimetry (ITC) and spectrophotometry under conditions that favor self-assembly to theirrespective intermolecular or intramolecular tetraplex structures. Analysis of the ITC and optical data reveals(i) saturating porphyrin/tetraplex binding stoichiometries of 1:1, 2:1, and 3:1 for d(G₂T₂G₂TGTG₂T₂G₂),d(AG₃[T₂AG₃]₃), and [d(T₄G₄)]₄, respectively, involving near-equivalent sites, (ii) weak binding of only (0.3-2) × 10⁵ M^-1 per site, and (iii) no evidence for stepwise complexation in solutions containing K⁺ ions. Thisstoichiometry is maintained in Na⁺ solutions, if the tetraplex is stable, but nondegenerate sites are implicatedfor the 2:1 and 3:1 complexes where the first porphyrin binds with 20-40-fold greater affinity than anysubsequent ligand(s). Importantly, the stoichiometries correspond to the number of intervals between successiveG-tetrad planes in each tetraplex. The results indicate binding by threading intercalation at each closely similarGpG site, possibly without invoking neighbor exclusion for adjacent sites, rather than through either externalelectrostatic processes or end-pasted stacking modes. This mechanism is supported by dynamic molecularmodeling simulations with two DNA tetraplexes which show that stable intercalated complexes can be realized.A plausible model is developed that accounts for the occupation of adjacent sites, where unfavorable interligandcontacts are avoided by phased asymmetric positioning of the porphyrin molecules within each G-tetradintercalation pocket.