Alhadeff Raphael, Ganoth Assaf, Arkin Isaiah T
Department of Biological Chemistry, The Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem, Edmond J. Safra Campus Givat Ram, Jerusalem, 91904, Israel.
Proteins. 2015 Jun;83(6):1107-17. doi: 10.1002/prot.24796. Epub 2015 Apr 28.
In mammals, the apical sodium-dependent bile acid transporter (ASBT) is responsible for the reuptake of bile acid from the intestine, thus recycling bile acid that is secreted from the gallbladder, for the purpose of digestion. As bile acid is synthesized from cholesterol, ASBT inhibition could have important implications in regulation of cholesterol levels in the blood. We report on a simulation study of the recently resolved structures of the inward-facing ASBT from Neisseria meningitidis and from Yersinia frederiksenii, as well as of an ASBT variant from Yersinia frederiksenii suggested to be in the outward-facing conformation. Classical and steered atomistic simulations and comprehensive potential of mean force analyses of ASBT, both in the absence and presence of ions and substrate, allow us to characterize and gain structural insights into the Na(+) binding sites and propose a mechanistic model for the transport cycle. In particular, we investigate structural features of the ion translocation pathway, and suggest a third putative Na(+) binding site. Our study sheds light on the structure-function relationship of bacterial ASBT and may promote a deeper understanding of transport mechanism altogether.
在哺乳动物中，顶端钠依赖性胆汁酸转运蛋白（ASBT）负责从肠道重新摄取胆汁酸，从而循环利用从胆囊分泌的胆汁酸以用于消化。由于胆汁酸是由胆固醇合成的，抑制ASBT可能对调节血液中的胆固醇水平具有重要意义。我们报告了一项模拟研究，该研究涉及脑膜炎奈瑟菌和费氏耶尔森菌向内取向的ASBT的最新解析结构，以及费氏耶尔森菌中一个被认为处于向外取向构象的ASBT变体。对ASBT进行的经典和引导式原子模拟以及全面的平均力势分析，无论有无离子和底物存在，都使我们能够表征并深入了解Na⁺结合位点，并提出运输循环的机制模型。特别是，我们研究了离子转运途径的结构特征，并提出了第三个假定的Na⁺结合位点。我们的研究揭示了细菌ASBT的结构 - 功能关系，并可能促进对运输机制的更深入理解。
