Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.
J Phys Chem B. 2012 Jun 21;116(24):7213-20. doi: 10.1021/jp303840a. Epub 2012 Jun 6.
Although a minor component of the lipidome, phosphatidic acid (PA) plays a crucial role in nearly all signaling pathways involving cell membranes, in part because of its variable electrical charge in response to environmental conditions. To investigate how charge is regulated in domains of PA, we applied surface-sensitive X-ray reflectivity and fluorescence near-total-reflection techniques to determine the binding of divalent ions (Ca(2+) at various pH values) to 1,2-dimyristoyl-sn-glycero-3-phosphate (DMPA) and to the simpler lipid dihexadecyl phosphate (DHDP) spread as monolayers at the air/water interface. We found that the protonation state of PA is controlled not only by the pK(a) and local pH but also by the strong affinity to PA driven by electrostatic correlations from divalent ions and the cooperative effect of the two dissociable protons, which dramatically enhance the surface charge. A precise theoretical model is presented providing a general framework to predict the protonation state of PA. Implications for recent experiments on charge regulation by hydrogen bonding and the role of pH in PA signaling are discussed in detail.
尽管磷脂酸 (PA) 在脂质组中只占很小的比例，但它在几乎所有涉及细胞膜的信号通路中都起着至关重要的作用，部分原因是它的可变电荷会对环境条件做出响应。为了研究 PA 域中的电荷是如何调节的，我们应用了表面敏感的 X 射线反射率和荧光近全反射技术来确定二价离子（不同 pH 值下的 Ca(2+)）与 1,2-二肉豆蔻酰-sn-甘油-3-磷酸（DMPA）以及更简单的脂质二十六烷基磷酸（DHDP）在空气/水界面上作为单层分散时的结合情况。我们发现，PA 的质子化状态不仅受 pK(a)和局部 pH 值的控制，还受来自二价离子的静电相关性以及两个可离解质子的协同作用所驱动的与 PA 的强亲和力所控制，这极大地增强了表面电荷。提出了一个精确的理论模型，为预测 PA 的质子化状态提供了一个通用框架。详细讨论了最近关于氢键调节电荷和 pH 值在 PA 信号转导中作用的实验的意义。
