Biochemistry
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The release of fibrinopeptides A and B by the slow and fast forms of thrombin was studied over the temperature range from 5 to 45 degrees C and the salt concentration range from 100 to 800 mM. The sequential mechanism for the release of fibrinopeptides originally proposed by Shafer was found to be obeyed under all conditions examined. The origin of preferential binding of fibrinogen and fibrin I to the fast form of thrombin in the transition state is in the second-order rate constant for association, k(l). ⋯ This anion drastically and specifically reduces the thickness of fibrin fibers, as judged by the 10-fold decrease in the equilibrium turbidity of clots developed in NaCl as compared to the turbidity of clots developed in NaF. Hence, the transition from a "coarse" to a "fine" clot induced by an increase in ionic strength as first described by Ferry is, instead, due to the specific binding of Cl- to intermediates in the ensuing polymerization. In fact, no change in the clotting curve is observed when the ionic strength is changed with NaF.