Department of Chemistry, Stanford University, Stanford, CA 94305.
Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Stanford University, Menlo Park, CA 94025.
Proc Natl Acad Sci U S A. 2018 Sep 11;115(37):9098-9103. doi: 10.1073/pnas.1810706115. Epub 2018 Aug 27.
Ladderane lipids are unique to anaerobic ammonium-oxidizing (anammox) bacteria and are enriched in the membrane of the anammoxosome, an organelle thought to compartmentalize the anammox process, which involves the toxic intermediate hydrazine (NH). Due to the slow growth rate of anammox bacteria and difficulty of isolating pure ladderane lipids, experimental evidence of the biological function of ladderanes is lacking. We have synthesized two natural and one unnatural ladderane phosphatidylcholine lipids and compared their thermotropic properties in self-assembled bilayers to distinguish between [3]- and [5]-ladderane function. We developed a hydrazine transmembrane diffusion assay using a water-soluble derivative of a hydrazine sensor and determined that ladderane membranes are as permeable to hydrazine as straight-chain lipid bilayers. However, pH equilibration across ladderane membranes occurs 5-10 times more slowly than across straight-chain lipid membranes. Langmuir monolayer analysis and the rates of fluorescence recovery after photobleaching suggest that dense ladderane packing may preclude formation of proton/hydroxide-conducting water wires. These data support the hypothesis that ladderanes prevent the breakdown of the proton motive force rather than blocking hydrazine transmembrane diffusion in anammox bacteria.
梯烷脂质是厌氧氨氧化（anammox）细菌所特有的，并且富含在 anammoxosome 的膜中，该细胞器被认为分隔了涉及有毒中间产物联氨（NH）的 anammox 过程。由于 anammox 细菌的生长速度缓慢且难以分离出纯梯烷脂质，因此缺乏关于梯烷的生物学功能的实验证据。我们已经合成了两种天然和一种非天然的梯烷磷脂，并比较了它们在自组装双层中的热性质，以区分[3]-和[5]-梯烷的功能。我们使用联氨传感器的水溶性衍生物开发了一种联氨跨膜扩散测定法，并确定梯烷膜对联氨的通透性与直链脂质双层一样。然而，梯烷膜的 pH 值平衡比直链脂质膜慢 5-10 倍。Langmuir 单层分析和光漂白后荧光恢复的速率表明，密集的梯烷堆积可能会阻止质子/氢氧化物传导水线的形成。这些数据支持这样的假设，即梯烷类物质防止质子动力势的破坏，而不是在 anammox 细菌中阻止联氨的跨膜扩散。
