The Effect of Calcium (II) on the Binding of Anticoagulation Factor I with Activated Coagulation Factor X

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• 作者：Xu ; Xiaolong ; Liu ; Qingliang ; Xie ; Yongshu
• 刊名：Journal of Protein Chemistry
• 出版年：2001
• 出版时间：2001
• 年：2001
• 卷：20
• 期：1
• 页码：33-37
• 全文大小：38 KB

文摘

Anticoagulation factor I (ACF I) from the venom of Agkistrodon acutus forms a 1:1 complex with activated coagulation factor X (FXa) in a Ca$^2+$-dependent fashion and thereby prolongs the clotting time. In the present study, the dependence of the binding of ACF I with FXa on the concentration of Ca$^2+$ ions was quantitatively analyzed by HPLC, and the result showed that the maximal binding of ACF I to FXa occurred at concentration of Ca$^2+$ ions of about 1 mM. The binding of Ca$^2+$ ions to ACF I was investigated by equilibrium dialysis and two Ca$^2+$-binding sites with different affinities were identified. At pH 7.6, the apparent association constants K$\_1$ and K$\_2$ for these two sites were (1.8 ± 0.5) × 10$^5$ and (2.7 ± 0.6) × 104 M$^-1$ (mean ± SE, n = 4), respectively. It was evident from the observation of Ca$^2+$-induced changes in the intrinsic fluorescence of ACF I that ACF I underwent a conformational change upon binding of Ca$^2+$ ions. The occupation of both Ca$^2+$-binding sites in ACF I required a concentration of Ca$^2+$ ions of about 1 mM, which is equal to the effective concentration of Ca$^2+$ ions required both for maximal binding of ACF I to FXa and for the maximal enhancement of emission fluorescence of ACF I. It could be deduced from these results that the occupation of both Ca$^2+$-binding sites in ACF I with Ca$^2+$ ions and subsequent conformational rearrangement might be essential for the binding of ACF I to FXa.