EVIDENCE FOR SELF-ASSOCIATION OF PROTHROMBIN FRAGMENT 1 IN THE ABSENCE OF CALCIUM IONS: IMPLICATIONS FOR THE INTERPRETATION OF COOPERATIVITY OF CALCIUM BINDING
EVIDENCE FOR SELF-ASSOCIATION OF PROTHROMBIN FRAGMENT 1 IN THE ABSENCE OF CALCIUM IONS: IMPLICATIONS FOR THE INTERPRETATION OF COOPERATIVITY OF CALCIUM BINDING
复制标题
在没有钙离子的情况下凝血酶原片段 1 自缔合的证据：对钙结合协同性解释的影响
作者：
C. Jackson;G. Brenckle;Philip J. Hogg;D. Winzor
不再需要根据两种完全不同的机制模型来考虑凝血酶原及其片段 1 与金属离子的结合。 Ca+T 与凝血酶原和凝血酶原片段 1 结合的协同性可能反映了蛋白质的异构化和/或自缔合。沉降平衡研究表明，在没有 Ca++ 的情况下，凝血酶原及其片段 1 都会可逆地二聚化。基于这种预先存在的平衡，我们发现了一个 Ca++ 与二聚体优先结合的模型，能够定量解释 Ca2+ 与片段 1 的相互作用。这种现象由关系式 r ={pkA[A][S] (1 + kA[S])p-1 + qkc X [A]2[S](1 + kc [S])q-1}/[A] 描述，其中 X (1,000 M-1)表示预先存在的单体-二聚体平衡的缔合常数，p, kA (10, 100 M-1) 和 q, kc (20, 2,000 M-1) 分别是 Ca++ 与单体和二聚体片段 1, A 相互作用的化学计量和内在结合常数。排阻色谱和沉降速度实验也有证据表明，凝血酶原及其片段 1 存在不同的异构体状态。因此，有人提出，基于异构体和二聚体蛋白质状态共存的模型将使片段 1 和凝血酶原的 Ca++ 介导的反应的定量差异仅根据两个系统中相同相互作用的平衡常数的相对大小的差异来合理化。
The necessity to consider the binding of metal ion by prothrombin and its fragment 1 in terms of two entirely different mechanistic models has been removed. Cooperativity of Ca+T binding to prothrombin and prothrombin fragment 1 may reflect isomerization and/or self association of the protein. Sedimentation equilibrium studies have demonstrated that both prothrombin and its fragment 1 reversibly dimerize in the absence of Ca++. Based on this pre-existing equilibrium, a model for preferential binding of Ca++ to the dimer has been found capable of accounting quantitatively for the interaction of Ca2+ with fragment 1. This phenomenon is described by the relationship r ={pkA[A][S] (1 + kA[S])p-1 + qkc X [A]2[S](1 + kc [S])q-1}/[A] in which X (1,000 M-1) denotes the association constant for the pre-existing monomer-dimer equilibrium, and p, kA (10, 100 M-1) and q, kc (20, 2,000 M-1) are the respective stoichiometries and intrinsic binding constants for the interactions of Ca++ with monomeric and dimeric fragment 1, A. There is also evidence from exclusion chromatography and sedimentation velocity experiments that different isomeric states of prothrombin and its fragment 1 exist. It is therefore proposed that a model based on co-existence of isomeric and dimeric protein states will enable quantitative differences in the Ca++-mediated responses of fragment 1 and prothrombin to be rationalized solely in terms of differences in the relative magnitudes of equilibrium constants for the same interactions in the two systems.