J. Biol. Chem., Vol. 262, Issue 28, 13472-13475, Oct, 1987
Evidence for self-association of prothrombin fragment 1 in the absence of calcium ions. Implications for the interpretation of cooperativity of calcium binding
CM Jackson, GM Brenckle, PJ Hogg and DJ Winzor
American Red Cross Blood Services, Southeastern Michigan Region, Detroit 48232.
Sedimentation equilibrium studies have demonstrated that prothrombin
fragment 1 from either human or bovine plasma reversibly dimerizes in the
absence of Ca2+ with an equilibrium constant of 1,000 M-1. In the presence
of 10 mM Ca2+ this association constant increased to 10,000 M- 1. A model
for preferential binding of Ca2+ to the pre-existing dimer has been found
capable of accounting quantitatively for the cooperative Ca2+ binding to
this prothrombin fragment, and for the dependence of its sedimentation
coefficient on protein concentration in the presence and absence of metal
ion. Sedimentation equilibrium studies of intact bovine and human
prothrombins have confirmed previous reports that these prothrombins
dimerize. For both prothrombins the association constant is 10,000 M-1,
both in the absence and presence of Ca2+.
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