Yang Huaiyu, Xu Yechun, Zhu Weiliang, Chen Kaixian, Jiang Hualiang
Drug Discovery and Design Center, State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Shanghai Institutes for Biological Science and Graduate School, Chinese Academy of Sciences, Shanghai 201203, China.
Biophys J. 2007 Feb 1;92(3):877-85. doi: 10.1529/biophysj.106.090191. Epub 2006 Nov 10.
The mechanism by which the ammonium transporter, AmtB, conducts NH4+/NH3 into the cytoplasm was investigated using conventional molecular dynamics (MD) simulations. These simulations revealed that the neutral molecule, NH3, passes automatically through the channel upon its arrival at the Am2 site and that the function of the channel as a one-way valve for passage of NH3 is not determined by the cytoplasmic exit gate but, rather, by the periplasmic entrance gate of the channel. The NH3, produced by deprotonation of NH4+ at the entrance gate, is spontaneously conveyed to the central region of the channel via a hydrogen-bond network comprising His-168, His-318, Tyr-32, and the NH3 molecule. Finally, the NH3 molecule exits the channel through the exit gate formed by Phe-31, Ile-266, Val-314, and His-318. In addition, Ser-263 is shown to play a critical role in the final stages, acting as a pivoting arm to shunt the NH3 molecule from the cytoplasmic exit gate of the channel out into the cytoplasm. This finding is further supported by another simulation which shows that NH3 fails to be translocated through the channel formed by the Ser-263-Ala mutation. Thus, this study casts new insights on the mechanism of AmtB-mediated passage of NH3 across cellular membranes.
利用传统分子动力学（MD）模拟研究了铵转运蛋白AmtB将NH4+/NH3导入细胞质的机制。这些模拟结果表明，中性分子NH3在到达Am2位点时会自动通过通道，并且通道作为NH3单向通过阀门的功能并非由细胞质出口门决定，而是由通道的周质入口门决定。在入口门处由NH4+去质子化产生的NH3通过由His-168、His-318、Tyr-32和NH3分子组成的氢键网络自发地输送到通道的中心区域。最后，NH3分子通过由Phe-31、Ile-266、Val-314和His-318形成的出口门离开通道。此外，Ser-263在最后阶段起着关键作用，充当一个旋转臂，将NH3分子从通道的细胞质出口门分流到细胞质中。另一项模拟进一步支持了这一发现，该模拟表明NH3无法通过由Ser-263-Ala突变形成的通道转运。因此，本研究为AmtB介导的NH3跨细胞膜转运机制提供了新的见解。
