Dorairaj Sudha, Allen Toby W
Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA.
Proc Natl Acad Sci U S A. 2007 Mar 20;104(12):4943-8. doi: 10.1073/pnas.0610470104. Epub 2007 Mar 13.
Biological membranes consist of bilayer arrangements of lipids forming a hydrophobic core that presents a physical barrier to all polar and charged molecules. This long-held notion has recently been challenged by biological translocon-based experiments that report small apparent free energies to insert charged side chains near the center of a transmembrane (TM) helix. We have carried out fully atomistic simulations to provide the free-energy profile for moving a TM helix containing a protonated Arg side chain across a lipid bilayer. Our results reveal the fundamental thermodynamics governing the stability of charged side chains in membranes and the microscopic interactions involved. Despite local membrane deformations, where large amounts of water and lipid head groups are pulled into the bilayer to interact with Arg, the free-energy barrier is 17 kcal/mol. We provide a rationale for the differences in our microscopic free energies and cell biological experiments using free-energy calculations that indicate that a protonated Arg at the central residue of a TM helix of the Leader peptidase might reside close to the interface and not at the membrane center. Our findings have implications for the gating mechanisms of voltage-gated ion channels, suggesting that movements of protonated Arg residues through the membrane will be prohibited.
生物膜由脂质的双层排列组成，形成一个疏水核心，对所有极性和带电分子构成物理屏障。这一长期以来的观念最近受到了基于生物转运体的实验的挑战，这些实验报告称，将带电侧链插入跨膜（TM）螺旋中心附近时，表观自由能较小。我们进行了全原子模拟，以提供一个包含质子化精氨酸侧链的TM螺旋穿过脂质双层的自由能分布。我们的结果揭示了控制膜中带电侧链稳定性的基本热力学以及所涉及的微观相互作用。尽管存在局部膜变形，即大量的水和脂质头部基团被拉入双层与精氨酸相互作用，但自由能垒为17千卡/摩尔。我们利用自由能计算为微观自由能和细胞生物学实验中的差异提供了一个解释，结果表明，引导肽酶TM螺旋中心残基处的质子化精氨酸可能位于靠近界面处，而不是膜中心。我们的发现对电压门控离子通道的门控机制有影响，表明质子化精氨酸残基穿过膜的运动将被禁止。
