Tanford C
Proc Natl Acad Sci U S A. 1982 Nov;79(21):6527-31. doi: 10.1073/pnas.79.21.6527.
The thermodynamic pathway for "chemiosmotic" free energy transduction in active transport is discussed with an ATP-driven Ca2+ pump as an illustrative example. Two innovations are made in the analysis. (i) Free energy dissipated as heat is rigorously excluded from overall free energy bookkeeping by focusing on the dynamic equilibrium state of the chemiosmotic process. (ii) Separate chemical potential terms for free energy donor and transported ions are used to keep track of the thermodynamic state of each substrate through the reaction cycle. These procedures clarify the mechanism of free energy transduction, even without step-by-step analysis. The results show that free energy exchange must occur in its entirety among protein-bound species. Imposition of conditions for an adequate rate of physiological function further indicates (i) that the standard free energy of hydrolysis of protein-bound ATP (to yield protein-bound products) needs to differ substantially from the standard free energy of hydrolysis in solution and (ii) that binding sites for the transported ions must have different affinities when facing opposite sides of the membrane. The results also demonstrate that step-by-step "basic" free energy changes (often used in the form of free energy level diagrams) are inherently unsuited for analysis of the mechanism of free energy transduction.
以ATP驱动的Ca2+泵为例,讨论了主动运输中“化学渗透”自由能转导的热力学途径。分析中有两项创新。(i) 通过关注化学渗透过程的动态平衡状态,将以热形式耗散的自由能严格排除在总自由能计算之外。(ii) 自由能供体和转运离子的化学势项分开使用,以跟踪每个底物在反应循环中的热力学状态。即使不进行逐步分析,这些程序也能阐明自由能转导的机制。结果表明,自由能交换必须在蛋白质结合物种之间整体发生。施加足够生理功能速率的条件进一步表明:(i) 蛋白质结合ATP水解(产生蛋白质结合产物)的标准自由能需要与溶液中水解的标准自由能有很大差异;(ii) 转运离子的结合位点在面对膜的两侧时必须具有不同的亲和力。结果还表明,逐步的“基本”自由能变化(通常以自由能级图的形式使用)本质上不适合用于分析自由能转导机制。