Center for Integrated Nanotechologies, Sandia National Laboratories, Albuquerque, New Mexico, USA.
J Phys Chem B. 2010 Sep 2;114(34):11061-8. doi: 10.1021/jp1055182.
The fusion between two lipid bilayers involves crossing a complicated energy landscape. The limiting barrier in the process appears to be between two closely opposed bilayers and the intermediate state where the outer leaflets are fused. We have performed molecular dynamics simulations to characterize the free energy barrier for the fusion of two liposomes and to examine the molecular details of barrier crossing. To capture the slow dynamics of fusion, a model using coarse-grained representations of lipids was used. The fusion between pairs of liposomes was simulated for four systems: DPPC, DOPC, a 3:1 mixture of DPPC/DPPE, and an asymmetric lipid tail system in which one tail of DPPC was reduced to half the length (ASTail). The weighted histogram method was used to compute the free energy as a function of separation distance. The relative barrier heights for these systems was found to be ASTail >> DPPC > DPPC/DPPE > DOPC, in agreement with experimental observations. Further, the free energy curves for all four can be overlaid on a single curve by plotting the free energy versus the surface separation (differing only in the point of fusion). These simulations also confirm that the two main contributions to the free energy barrier are the removal of water between the vesicles and the deformation of the vesicle. The most prominent molecular detail of barrier crossing in all cases examined was the splaying of lipid tails, where initially a single splayed lipid formed a bridge between the two outer leaflets that promotes additional lipid mixing between the vesicles and eventually leads to fusion. The tail splay appears to be closely connected to the energetics of the process. For example, the high barrier for the ASTail is the result of a smaller distance between terminal methyl groups in the splayed molecule. The shortening of this distance requires the liposomes to be closer together, which significantly increases the cost of water removal and bilayer deformation. Before tail splay can initiate fusion, contact must occur between a tail end and the external water. In isolated vesicles, the contact fraction is correlated to the fusogenicity difference between DPPC and DOPC. Moreover, for planar bilayers, the contact fraction is much lower for DPPC, which is consistent with its lack of fusion in giant vesicles. The simulation results show the key roles of lipid tail dynamics in governing the fusion energy landscape.
两层脂质双层的融合涉及穿越一个复杂的能量景观。该过程中的限制壁垒似乎是在两个紧密相对的双层之间，以及在外层融合的中间状态。我们已经进行了分子动力学模拟，以描述两个脂质体融合的自由能势垒，并研究势垒穿越的分子细节。为了捕捉融合的慢动力学，使用了一种使用脂质粗粒化表示的模型。模拟了四种系统的脂质体对之间的融合：DPPC、DOPC、DPPC/DPPE 的 3:1 混合物和 DPPC 的一个尾部缩短到一半长度的不对称脂质尾系统（ASTail）。使用加权直方图方法将自由能作为分离距离的函数进行计算。这些系统的相对势垒高度发现为 ASTail >> DPPC > DPPC/DPPE > DOPC，与实验观察结果一致。此外，通过绘制自由能与表面分离（仅在融合点上有所不同）的关系，可以将所有四个系统的自由能曲线叠加在单个曲线上。这些模拟还证实，自由能势垒的两个主要贡献是囊泡之间的水的去除和囊泡的变形。在所检查的所有情况下，势垒穿越的最突出的分子细节是脂质尾部的张开，其中最初一个张开的脂质形成两个外层之间的桥，促进囊泡之间的额外脂质混合，最终导致融合。尾部张开似乎与过程的能量密切相关。例如，ASTail 的高势垒是由于张开分子末端甲基之间的距离较小所致。缩短此距离需要脂质体更靠近，这大大增加了水去除和双层变形的成本。在尾部张开可以引发融合之前，尾部末端必须与外部水接触。在孤立的囊泡中，接触分数与 DPPC 和 DOPC 之间的融合性差异相关。此外，对于平面双层，DPPC 的接触分数要低得多，这与其在巨大囊泡中缺乏融合一致。模拟结果表明，脂质尾部动力学在控制融合能景观方面起着关键作用。
