Heparin exerts its anticoagulant activity by catalysing the inhibition of coagulation proteases by antithrombin (AT). Its main target is thrombin but it also catalyses the inhibition of the other serine-proteases of the coagulation cascade, such as factor IXa (fIXa). The aim of this study was to compare the catalysis of inhibition of blood fIXa by antithrombin in the presence of several sulfated polysaccharides with anticoagulant activity, i.e. heparin, three widely used in therapeutics low molecular weight heparins (LMWH) and fucoidan. Plots of the second-order rate constants of the fIXa-antithrombin reaction vs. the concentration of added heparin and LMWH are bell-shaped and fit the kinetic model established for thrombin-antithrombin reaction by Jordan R., Beeler D., Rosenberg R. (1979) J. Biol. Chem., 254, 2902-2913. In the ascending branch, the catalyst (C) binds quickly to the inhibitor (I) to form a catalyst-inhibitor (CI) complex which is more reactive towards the enzyme (E) than the free inhibitor, leading to the formation of an inactive enzyme-inhibitor complex (EI) and the release of free catalyst, in a rate-limiting second step. After a maximum corresponding to an optimal catalyst concentration, the decrease in the reaction rate was in keeping with the formation of a catalyst-enzyme (CE) complex, whose inactivation by the CI complex was slower than that of the free enzyme. Maximum second-order rate constants for the inhibition of fIXa by AT were 105, 6.8, 12.24 and 22 microM-1 min-1 with heparin, Enoxaparin, Fraxiparin and Fragmin, respectively, leading to 3500-, 225-, 405- and 728-fold increases in the inhibition rate in the absence of polysaccharide, respectively. Fucoidan yielded 23-fold increase in the fIXa-antithrombin interaction rate. The kinetic profiles obtained with this polysaccharide exhibited ascending branch which correlated well with the kinetic model based on the formation of binary complexes (CI or CE). Fucoidan was covalently conjugated with a fluorescent probe (DTAF) and used in conjunction with fluorescence anisotropy to follow its binding to antithrombin, heparin cofactor II (HCII), thrombin and fIXa. The binding of fucoidan to these proteins occurred with low affinities when compared to heparin and LMWH. Fucoidan had higher affinity for the inhibitor HCII compared to antithrombin and enzymes. These data suggest that binding of heparins and fucoidan to the inhibitor (CI) is required for the polysaccharide-dependent enhancement in the rate of neutralization of the enzyme by the inhibitor.