Wang Yan-Ni, Topol Igor A, Collins Jack R, Burt Stanley K
Advanced Biomedical Computing Center, National Cancer Institute at Frederick, NCI/NIH, P.O. Box B, Frederick, MD 21702, USA.
J Am Chem Soc. 2003 Oct 29;125(43):13265-73. doi: 10.1021/ja0279794.
Phosphate hydrolysis by GTPases plays an important role as a molecular switch in signal transduction and as an initiator of many other biological processes. Despite the centrality of this ubiquitous reaction, the mechanism is still poorly understood. As a first step to understand the mechanisms of this process, the nonenzymatic hydrolysis of mono-phosphate and tri-phosphate esters were systematically studied in gas phase and aqueous solution using hybrid density functional methods. The dielectric effect of the environment on the energetics of these processes was also explored. Theoretical results show that for mono-phosphate ester, the dissociative pathway is much more favorable than the associative pathway. However, the reaction barriers for the dissociative and associative pathways of tri-phosphate hydrolysis are very close in aqueous solution, though the dissociative pathway is more favorable in the gas phase. High dielectric solvents, such as water, significantly lower the activation barrier of the associative pathway due to the greater solvation energy of the associative transition states than that of the reactant complex. By contrast, the barrier of the dissociative pathway, with respect to the gas phase, is less sensitive to the surrounding dielectric. In the associative hydrolysis pathway of the tri-phosphate ester, negative charge is transferred from the gamma-phosphate to beta-phosphate through the bridging ester oxygen and results in Pgamma-O bond dissociation. No analogous charge transfer was observed in the dissociative pathway, where Pgamma-O bond dissociation resulted from proton transfer from the gamma-phosphate to the bridge oxygen. Finally, the active participation of local water molecules can significantly lower the activation energy of the dissociative pathway for both mono-phosphate and tri-phosphate.
GTP酶催化的磷酸水解作为信号转导中的分子开关以及许多其他生物过程的启动子发挥着重要作用。尽管这种普遍存在的反应至关重要，但其机制仍知之甚少。作为理解这一过程机制的第一步，我们使用混合密度泛函方法在气相和水溶液中系统地研究了单磷酸酯和三磷酸酯的非酶水解。还探讨了环境的介电效应对这些过程能量学的影响。理论结果表明，对于单磷酸酯，解离途径比缔合途径更有利。然而，在水溶液中，三磷酸水解的解离途径和缔合途径的反应势垒非常接近，尽管在气相中解离途径更有利。高介电常数的溶剂，如水，由于缔合过渡态的溶剂化能比反应物络合物的溶剂化能大得多，显著降低了缔合途径的活化势垒。相比之下，解离途径的势垒相对于气相而言，对周围的介电常数不太敏感。在三磷酸酯的缔合水解途径中，负电荷通过桥连酯氧从γ-磷酸转移到β-磷酸，导致Pγ-O键解离。在解离途径中未观察到类似的电荷转移，在解离途径中，Pγ-O键解离是由于质子从γ-磷酸转移到桥连氧。最后，局部水分子的积极参与可以显著降低单磷酸酯和三磷酸酯解离途径的活化能。
