Zhou Xianfeng, Dalai Punam, Sahai Nita
Department of Polymer Science, University of Akron, Akron, OH 44325, USA.
Key Lab of Biobased Polymer Materials of Shandong Provincial Education Department, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China.
Life (Basel). 2020 Apr 14;10(4):39. doi: 10.3390/life10040039.
Two important ions, K and Na, are unequally distributed across the contemporary phospholipid-based cell membrane because modern cells evolved a series of sophisticated protein channels and pumps to maintain ion gradients. The earliest life-like entities or protocells did not possess either ion-tight membranes or ion pumps, which would result in the equilibration of the intra-protocellular K/Na ratio with that in the external environment. Here, we show that the most primitive protocell membranes composed of fatty acids, that were initially leaky, would eventually become less ion permeable as their membranes evolved towards having increasing phospholipid contents. Furthermore, these mixed fatty acid-phospholipid membranes selectively retain K but allow the passage of Na out of the cell. The K/Na selectivity of these mixed fatty acid-phospholipid semipermeable membranes suggests that protocells at intermediate stages of evolution could have acquired electrochemical K/Na ion gradients in the absence of any macromolecular transport machinery or pumps, thus potentially facilitating rudimentary protometabolism.
两种重要的离子，钾（K）和钠（Na），在当代基于磷脂的细胞膜上分布不均，因为现代细胞进化出了一系列复杂的蛋白质通道和泵来维持离子梯度。最早的类生命实体或原始细胞既没有离子致密膜也没有离子泵，这会导致细胞内钾/钠比例与外部环境达到平衡。在此，我们表明，由脂肪酸组成的最原始的原始细胞膜最初是有渗漏的，但随着其膜向磷脂含量增加的方向进化，最终会变得对离子的渗透性降低。此外，这些脂肪酸 - 磷脂混合膜会选择性地保留钾，但允许钠流出细胞。这些脂肪酸 - 磷脂混合半透膜的钾/钠选择性表明，处于进化中间阶段的原始细胞在没有任何大分子运输机制或泵的情况下，可能已经获得了电化学钾/钠离子梯度，从而有可能促进基本的原代谢。
