Gabel Christopher V, Berg Howard C
Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138, USA.
Proc Natl Acad Sci U S A. 2003 Jul 22;100(15):8748-51. doi: 10.1073/pnas.1533395100. Epub 2003 Jul 11.
A protonmotive force (pmf) across the cell's inner membrane powers the flagellar rotary motor of Escherichia coli. Speed is known to be proportional to pmf when viscous loads are heavy. Here we show that speed also is proportional to pmf when viscous loads are light. Two motors on the same bacterium were monitored as the cell was slowly deenergized. The first motor rotated the entire cell body (a heavy load), while the second motor rotated a small latex bead (a light load). The first motor rotated slowly and provided a measure of the cell's pmf. The second motor rotated rapidly and was compared with the first, to give the speed-pmf relation for light loads. Experiments were done at 24.0 degrees C and 16.2 degrees C, with initial speeds indicating operation well into the high-speed, low-torque regime. Speed was found to be proportional to pmf over the entire (accessible) dynamic range (0-270 Hz). If the passage of a fixed number of protons carries the motor through each revolution, i.e., if the motor is tightly coupled, a linear speed-pmf relation is expected close to stall, where the work done against the viscous load matches the energy dissipated in proton flow. A linear relation is expected at high speeds if proton translocation is rate-limiting and involves multiple steps, a model that also applies to simple proton channels. The present work shows that a linear relation is true more generally, providing an additional constraint on possible motor mechanisms.
跨细胞内膜的质子动力（pmf）为大肠杆菌的鞭毛旋转马达提供动力。已知当粘性负载较大时，速度与质子动力成正比。在此我们表明，当粘性负载较小时，速度同样与质子动力成正比。在细胞缓慢去能的过程中，对同一细菌上的两个马达进行了监测。第一个马达使整个细胞体旋转（重负载），而第二个马达使一个小乳胶珠旋转（轻负载）。第一个马达旋转缓慢，可作为细胞质子动力的一种度量。第二个马达旋转迅速，并与第一个马达进行比较，以得出轻负载下速度与质子动力的关系。实验在24.0摄氏度和16.2摄氏度下进行，初始速度表明其运行处于高速、低扭矩状态。发现在整个（可达到的）动态范围（0 - 270赫兹）内，速度与质子动力成正比。如果固定数量的质子通过使马达每旋转一圈，即如果马达紧密耦合，那么在接近失速时预计会有线性的速度 - 质子动力关系，此时克服粘性负载所做的功与质子流中耗散的能量相匹配。如果质子转运是限速的且涉及多个步骤，那么在高速时预计会有线性关系，该模型也适用于简单的质子通道。目前的研究表明，线性关系更普遍地成立，这为可能的马达机制提供了额外的限制条件。
