Hernández J A
Sección Biofisica, Facultad de Ciencias, Universidad de la República, 11400 Montevideo, Uruguay.
J Membr Biol. 1998 Oct 1;165(3):235-42. doi: 10.1007/s002329900437.
The four-state simple carrier model (SCM) has been employed to describe facilitative transport of ligands across biological membranes. Two basic mechanisms have been invoked to account for carrier-mediated ligand translocation: (i) binding to a mobile carrier, and (ii) displacement determined by conformational changes of an integral protein. While translatory carriers may be accurately represented by a four-state diagram, it is unlikely that the transport process mediated by a complex membrane protein can be strictly described by the elementary SCM. The purpose of this article is to test whether facilitative transporters with a more complex kinetic design than the SCM can exhibit macroscopic kinetic properties indistinguishable from it. For this, I studied a "general carrier model" (GCM), and evaluated whether the relevant kinetic parameters are subject to the same basic restrictions as in the SCM. The fundamental finding is that there is a general kinetic design embodied with SCM-like properties, that can be shared by many transporters. In particular, the classical SCM is shown here to represent a particular case of the GCM. A main conclusion of this work is therefore that the finding of a macroscopic SCM-like kinetic behavior for a particular process of facilitative transport does not represent a sufficient argument in favor of a particular type of mechanism, like the typical one involving a two-conformational single-site carrier.
四态简单载体模型（SCM）已被用于描述配体跨生物膜的易化运输。人们提出了两种基本机制来解释载体介导的配体转运：（i）与可移动载体结合，以及（ii）由整合蛋白的构象变化决定的置换。虽然平移载体可以用四态图准确表示，但由复杂膜蛋白介导的运输过程不太可能用基本的SCM严格描述。本文的目的是测试具有比SCM更复杂动力学设计的易化转运体是否能表现出与SCM无法区分的宏观动力学特性。为此，我研究了一个“通用载体模型”（GCM），并评估了相关动力学参数是否受到与SCM相同的基本限制。基本发现是，存在一种具有类似SCM性质的通用动力学设计，许多转运体都可以共享。特别是，经典的SCM在这里被证明是GCM的一个特殊情况。因此，这项工作的一个主要结论是，对于特定的易化运输过程，发现宏观上类似SCM的动力学行为并不足以支持某种特定类型的机制，比如涉及双构象单位点载体的典型机制。
