Kinetics of activated thrombin-activatable fibrinolysis inhibitor (TAFIa)-catalyzed cleavage of C-terminal lysine residues of fibrin degradation products and removal of plasminogen-binding sites

Abstract

Partial digestion of fibrin by plasmin exposes C-terminal lysine residues, which comprise new binding sites for both plasminogen and tissue-type plasminogen activator (tPA). This binding increases the catalytic efficiency of plasminogen activation by 3000-fold compared with tPA alone. The activated thrombin-activatable fibrinolysis inhibitor (TAFIa) attenuates fibrinolysis by removing these residues, which causes a 97% reduction in tPA catalytic efficiency. The aim of this study was to determine the kinetics of TAFIa-catalyzed lysine cleavage from fibrin degradation products and the kinetics of loss of plasminogen-binding sites. We show that the kcat and Km of Glu1-plasminogen (Glu-Pg)-binding site removal are 2.34 s-1 and 142.6 nM, respectively, implying a catalytic efficiency of 16.21 μM-1 s-1. The corresponding values of Lys77/Lys78-plasminogen (Lys-Pg)-binding site removal are 0.89 s-1 and 96 nM implying a catalytic efficiency of 9.23 μM-1 s-1. These catalytic efficiencies of plasminogen-binding site removal by TAFIa are the highest of any TAFIa-catalyzed reaction with a biological substrate reported to date and suggest that plasmin-modified fibrin is a primary physiological substrate for TAFIa. We also show that the catalytic efficiency of cleavage of all C-terminal lysine residues, whether they are involved in plasminogen binding or not, is 1.10 μM-1 s-1. Interestingly, this value increases to 3.85 μM-1 s-1 in the presence of Glu-Pg. These changes are due to a decrease in Km. This suggests that an interaction between TAFIa and plasminogen comprises a component of the reaction mechanism, the plausibility of which was established by showing that TAFIa binds both Glu-Pg and Lys-Pg. © 2011 by The American Society for Biochemistry and Molecular Biology, Inc.