Lysine 114 of antithrombin is of crucial importance for the affinity and kinetics of heparin pentasaccharide binding

Lys(114) of the plasma coagulation proteinase inhibitor, antithrombin, has been implicated in binding of the glycosaminoglycan activator, heparin, by previous mutagenesis studies and by the crystal structure of antithrombin i n complex with the active pentasaccharide unit of heparin. In the present w ork, substitution of Lys(114) by Ala or Met was shown to decrease the affin ity of antithrombin for heparin and the pentasaccharide by similar to 10(5) -fold at I 0.15, corresponding to a reduction in binding energy of similar to 50%,. The decrease in affinity was due to the loss of two to three ionic interactions, consistent with Lys(114) and at least one other basic residu e of the inhibitor binding cooperatively to heparin, as well as to substant ial nonionic interactions. The mutation minimally affected the initial, wea k binding of the two-step mechanism of pentasaccharide binding to antithrom bin but appreciably (> 40-fold) decreased the forward rate constant of the conformational change in the second step and greatly (> 1000-fold) increase d the reverse rate constant of this step. Lys(114) is thus of greater impor tance for the affinity of heparin binding than any of the other antithrombi n residues investigated so far, viz. Arg(47), Lys(125), and Arg(129). It co ntributes more than Arg(47) and Arg(129) to increasing the rate of inductio n of the activating conformational change, a role presumably exerted by int eractions with the nonreducing end trisaccharide unit of the heparin pentas accharide. However, its major effect, also larger than that of these two re sidues, is in maintaining antithrombin in the activated state by interactio ns that most likely involve the reducing end disaccharide unit.