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首页> 外文期刊>The Journal of Chemical Physics >Proton transport via coupled surface and bulk diffusion
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Proton transport via coupled surface and bulk diffusion

机译：质子通过耦合表面和体扩散的传输

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摘要

Translocation of protons across biological membranes is carried out by special membrane proteins, proton pumps. Surprisingly, the turnover rate of some proton pumps, such as cytochrome c oxidase (CcO), is higher than the bulk diffusion limit (i.e., the rate at which protons can be supplied to the entrance of the proton conducting channel via free bulk diffusion). It has bene suggested that the diffusion of protons along the membrane surface that surrounds the entrance of the proton conducting channel can increase the supply of the protons and therefore explain the puzzling high turnover rates. Here we consider a phenomenological model of proton transport to a proton collecting channel. The model takes into account both the diffusion in the bulk and the coupled diffusion of protons along the membrane surface. In our model a homogeneous membrane surface, which mediates proton diffusion toward the channel entrance, is populated with protolytic groups that can exchange protons with a bulk solution. Equations which describe the coupled surface-bulk proton diffusion are derived and solved. The maximum (diffusion limited) rate at which protons can be delivered to the pump is examined. It is found that there are two regimes of surfacemediated proton transport, depending on the rate of proton exhcange betweent he bulk and the surface. In both regimes proton transport is dominated by the contribution of surface diffusion. Due to two-dimensional character of the surface diffusion, the transport rate depends on the size of the channel entrance in a weak, logarithmic fashion. The theory also provides a simple expression for the maximum distance that a proton can migrate on the surface before it is fully equilibrated with the bulk. This result allows one to examine whether the chemiosmotic coupling between a proton source on a membrane surface, such as CcO, and a sink, such as ATP synthase, occurs via diffusion along the membrane, or involves equilibration with the bulk.
机译：质子跨生物膜的转运是通过特殊的膜蛋白质子泵进行的。令人惊讶的是，某些质子泵（例如细胞色素C氧化酶（CcO））的周转率高于体积扩散极限（即，质子可通过自由体积扩散提供给质子传导通道入口的速率） 。已经暗示，质子沿着围绕质子传导通道的入口的膜表面的扩散可以增加质子的供应，因此可以解释令人费解的高周转率。在这里，我们考虑质子传输到质子收集通道的现象学模型。该模型既考虑了本体的扩散，也考虑了质子沿膜表面的耦合扩散。在我们的模型中，介导质子向通道入口扩散的均质膜表面填充了可以将质子与本体溶液交换的蛋白水解基团。推导并求解了描述耦合的表面体质子扩散的方程。检查质子可以输送到泵的最大速率（扩散受限）。发现存在两种表面介导的质子传输方式，这取决于质子在体与表面之间的质子交换速率。在这两种情况下，质子传输都以表面扩散的贡献为主导。由于表面扩散的二维特性，传输速率以微弱对数的方式取决于通道入口的大小。该理论还提供了一个简单的表达式，表示质子在与主体完全平衡之前可以在表面上迁移的最大距离。这一结果使人们能够检查膜表面上的质子源（例如CcO）和吸收器（例如ATP合酶）之间的化学渗透耦合是通过沿膜的扩散发生还是涉及与主体的平衡。

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