Busath D D
Department of Physiology, Brown University School of Medicine, Providence, Rhode Island 02912.
Annu Rev Physiol. 1993;55:473-501. doi: 10.1146/annurev.ph.55.030193.002353.
The various details provided by physical methods have been integrated into a coherent picture of the cation transport process. The free energy profile describes simultaneously the thermodynamic and kinetic factors that govern conductance. Localization of ion binding sites near each end of the channel by NMR and X-ray crystallographic techniques demonstrates that there is an energy barrier separating the two ends. There is no a priori reason to suspect any barrier to ion entry into the channel, and recent molecular modeling computations confirm this intuition. Therefore, the channel is best described as a two-site, one-barrier channel. However, the movement of ions across the central barrier over a distance of 1.9 nm is really a multistep process best described as Brownian motion (even the short steps from one pair of carbonyls to the next is more diffusive than activated for ions as large as Na+), the entry step is probably best considered as a compound step requiring correction for interfacial polarization and diffusion limitation, the binding sites in the channel ought not be considered to be in equilibrium with the bath, and quasi-knock off behavior (binding of a second ion at the entry facilitates release of a first ion from the exit) is probably the rule at physiological permeant ion concentrations and higher. Progress will probably focus on channel kinetics. The channel backbone probably undergoes conformational changes as the ions pass which, according to solid state NMR results, may last long enough to provide the channel with a sort of memory and which may give rise to excess single channel noise. These conformational changes are further reflected in shifts in side chain positions which, in turn, may be partly responsible for changes in the single-channel lifetime, which appears to be exquisitely sensitive to side chain-lipid interactions. Reasonable explanations for the impermeance of divalent cations, anions, and medium-sized organic cations such as guanidinium have proven elusive at first, but it now appears that divalent cations bind water too tightly, anions bind water in an orientation that produces unfavorable contacts between water oxygens and peptide carbonyl oxygens at the channel entry, and iminium ions bind strongly to the flexible peptide carbonyls at the channel entry.
物理方法提供的各种细节已被整合到阳离子转运过程的连贯图景中。自由能分布图同时描述了控制电导的热力学和动力学因素。通过核磁共振（NMR）和X射线晶体学技术确定通道两端附近的离子结合位点，表明存在一个将两端隔开的能量屏障。没有先验理由怀疑离子进入通道存在任何屏障，最近的分子模型计算证实了这一直觉。因此，该通道最好描述为双位点、单屏障通道。然而，离子在1.9纳米的距离上穿过中央屏障的移动实际上是一个多步过程，最好描述为布朗运动（即使是从一对羰基到下一对羰基的短距离移动，对于像Na+这样大的离子来说，扩散性也比活化性更强），进入步骤可能最好被视为一个复合步骤，需要校正界面极化和扩散限制，通道中的结合位点不应被认为与浴液处于平衡状态，并且准敲除行为（第二个离子在入口处结合促进第一个离子从出口释放）在生理渗透离子浓度及更高浓度下可能是规则。进展可能会集中在通道动力学上。当离子通过时，通道主链可能会发生构象变化，根据固态核磁共振结果，这种变化可能持续足够长的时间，为通道提供一种记忆，并且可能会产生过量的单通道噪声。这些构象变化进一步反映在侧链位置的移动上，而侧链位置的移动反过来可能部分导致单通道寿命的变化，单通道寿命似乎对侧链 - 脂质相互作用非常敏感。二价阳离子、阴离子以及中等大小的有机阳离子（如胍盐）的不可渗透性起初难以找到合理的解释，但现在看来，二价阳离子与水结合过于紧密，阴离子以一种导致通道入口处水氧与肽羰基氧之间产生不利接触的取向结合水，而亚胺离子在通道入口处与柔性肽羰基强烈结合。
