title: The effect of calcium (II) on the binding of anticoagulation factor I with activated coagulation factor X.
abstract: Anticoagulation factor I (ACF I) from the venom of Agkistrodon acutus forms a 1:1 complex with activated coagulation factor X (FXa) in a Ca2+-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 Ca2+ ions was quantitatively analyzed by HPLC, and the result showed that the maximal binding of ACF I to FXa occurred at concentration of Ca2+ ions of about 1 mM. The binding of Ca2+ ions to ACF I was investigated by equilibrium dialysis and two Ca2+-binding sites with different affinities were identified. At pH 7.6, the apparent association constants K1 and K2 for these two sites were (1.8 +/- 0.5) x 10(5) and (2.7 +/- 0.6) x 10(4) M(-1) (mean +/- SE, n = 4), respectively. It was evident from the observation of Ca2+-induced changes in the intrinsic fluorescence of ACF I that ACF I underwent a conformational change upon binding of Ca2+ ions. The occupation of both Ca2+-binding sites in ACF I required a concentration of Ca2+ ions of about 1 mM, which is equal to the effective concentration of Ca2+ 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 Ca2+-binding sites in ACF I with Ca2+ ions and subsequent conformational rearrangement might be essential for the binding of ACF I to FXa.
authors.affiliation: Department of Chemistry, University of Science and Technology of China, Hefei, Anhui.
authors.forename: X
authors.initials: X
authors.lastname: Xu
authors.forename: Q
authors.initials: Q
authors.lastname: Liu
authors.forename: Y
authors.initials: Y
authors.lastname: Xie
mesh_terms: D001665:Binding Sites; D002118:Calcium / Q000737:chemistry / Q000378:metabolism*; D005170:Factor X / Q000737:chemistry / Q000378:metabolism*; D005340:Fibrinogen / Q000737:chemistry / Q000378:metabolism*; D005453:Fluorescence; D013050:Spectrometry, Fluorescence
chemical_list: D005170:Factor X; D005340:Fibrinogen; D002118:Calcium