Nikaido H, Rosenberg E Y
J Gen Physiol. 1981 Feb;77(2):121-35. doi: 10.1085/jgp.77.2.121.
Nutrients usually cross the outer membrane of Escherichia coli by diffusion through water-filled channels surrounded by a specific class of protein, porins. In this study, the rates of diffusion of hydrophilic nonelectrolytes, mostly sugars and sugar alcohols, through the porin channels were determined in two systems, (a) vesicles reconstituted from phospholipids and purified porin and (b) intact cells of mutant strains that produce many fewer porin molecules than wild-type strains. The diffusion rates were strongly affected by the size of the solute, even when the size was well within the "exclusion limit" of the channel. In both systems, hexoses and hexose disaccharides diffused through the channel at rates 50-80% and 2-4%, respectively, of that of a pentose, arabinose. Application of the Renkin equation to these data led to the estimate that the pore radius is approximately 0.6 nm, if the pore is assumed to be a hollow cylinder. The results of the study also show that the permeability of the outer membrane of the wild-type E. coli cell to glucose and lactose can be explained by the presence of porin channels, that a significant fraction of these channels must be functional or "open" under our conditions of growth, and that even 10(5) channels per cell could become limiting when E. coli tries to grow at a maximal rate on low concentrations of slowly penetrating solutes, such as disaccharides.
营养物质通常通过扩散穿过大肠杆菌的外膜，扩散是通过由一类特定蛋白质——孔蛋白包围的充满水的通道进行的。在本研究中，亲水性非电解质（主要是糖类和糖醇类）通过孔蛋白通道的扩散速率在两个系统中得以测定：（a）由磷脂和纯化的孔蛋白重构的囊泡，以及（b）突变菌株的完整细胞，这些突变菌株产生的孔蛋白分子比野生型菌株少得多。即使溶质大小完全在通道的“排阻极限”范围内，扩散速率仍受到溶质大小的强烈影响。在这两个系统中，己糖和己糖双糖通过通道的扩散速率分别为戊糖阿拉伯糖扩散速率的50 - 80%和2 - 4%。如果将孔假设为空心圆柱体，将伦金方程应用于这些数据可得出孔半径约为0.6纳米的估计值。该研究结果还表明，野生型大肠杆菌细胞外膜对葡萄糖和乳糖的通透性可以用孔蛋白通道的存在来解释，在我们的生长条件下，这些通道的很大一部分必须是有功能的或“开放的”，并且当大肠杆菌试图在低浓度的缓慢渗透溶质（如双糖）上以最大速率生长时，即使每个细胞有10^5个通道也可能成为限制因素。
