Institute for Molecular Biophysics, University of Mainz, Germany.
IUBMB Life. 2011 May;63(5):329-36. doi: 10.1002/iub.459. Epub 2011 Apr 13.
Cooperative effects in ligand binding and dissociation kinetics are much less investigated than steady state kinetics or equilibrium binding. Nevertheless, cooperativity in ligand binding leads necessarily to characteristic properties with respect to kinetic properties of the system. In case of positive cooperativity as found in oxygen binding proteins, a typical property is an autocatalytic ligand dissociation behavior leading to a time dependent, apparent ligand dissociation rate. To follow systematically the influence of the various potentially involved parameters on this characteristic property, simulations based on the simple MWC model were performed which should be relevant for all types of models based on the concept of an allosteric unit. In cases where the initial conformational distribution is very much dominated by the R-state, the intrinsic kinetic properties of the T-state are of minor influence for the observed ligand dissociation rate. Even for fast conformational transition rates, the R-state properties together with the size of the allosteric unit and the allosteric equilibrium constant define the shape of the curve. In such a case, a simplified model of the MWC-scheme (the irreversible n-chain model) is a good approximation of the full scheme. However, if in the starting conformational distribution some liganded T-molecules are present (a few percent is enough), the average off-rates can be significantly altered. Thus, the assignment of the initial rates to R-state properties has to be done with great care. However, if the R-state strongly dominates initially it is even possible to get an estimation of the lower limit for the number of interacting subunits from kinetic data: similar to the Hill-coefficient for equilibrium conditions, a measure for "kinetic cooperativity" can be derived by comparing initial and final ligand dissociation rates.
与稳态动力学或平衡结合相比，配体结合和离解动力学中的协同作用的研究要少得多。然而，配体结合中的协同作用必然导致与系统动力学特性相关的特征性质。在氧结合蛋白中发现的正协同作用的情况下，一个典型的性质是自动催化配体解离行为，导致时间相关的、表观的配体解离速率。为了系统地研究各种潜在相关参数对这种特征性质的影响，基于简单的 MWC 模型进行了模拟，该模型应该与基于变构单元概念的所有类型的模型都相关。在初始构象分布主要由 R 态主导的情况下，T 态的固有动力学性质对观察到的配体解离速率的影响较小。即使对于快速构象转变速率，R 态性质以及变构单元的大小和变构平衡常数也定义了曲线的形状。在这种情况下，MWC 方案的简化模型（不可逆 n 链模型）是对全方案的良好近似。然而，如果在初始构象分布中存在一些配体结合的 T 态分子（只需几个百分比），则平均离解速率可以显著改变。因此，必须小心地将初始速率分配给 R 态性质。然而，如果 R 态最初占主导地位，则甚至可以从动力学数据中估计相互作用亚基的数量下限：类似于平衡条件下的 Hill 系数，可以通过比较初始和最终配体解离速率来得出“动力学协同性”的度量。
