Energetics of the thrombin-fibrinogen interaction

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Abstract
The kinetic mechanism of thrombin-fibrinogen interaction has been elucidated by steady-state measurements of synthetic substrate hydrolysis by human a-thrombin in the presence of human fibrinogen used as a competitive inhibitor and sucrose used as a viscogenic agent. Sucrose greatly affects the FKm for thrombin-fibrinogen interaction, without altering the intrinsic properties of the system. Under conditions of pH 7.5 and 0.1 M NaCl, fibrinogen behaves like a sticky substrate for thrombin, with acylation being comparable to dissociation in the temperature range 20-37 OC. In the same temperature range, deacylation is much faster than acylation. The van't Hoff enthalpy of binding for thrombin-fibrinogen interaction is -24 f 3 kcal/mol and the entropy is -55 f 11 cal mol-' deg-'. A chemical compensation effect is present in the binding of fibrinogen and synthetic amide substrates to thrombin, with the AH and AG values being linked through a linear relationship.

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