Requirement of lysine residues outside of the proposed pentasaccharide binding region for high affinity heparin binding and activation of human antithrombin III

Variant forms of human antithrombin III with glutamine or threonine substitutions at Lys(114), Lys(125), Lys(133), Lys(136), and Lys(139) were expressed in insect cells to evaluate their roles in heparin binding and activation. Recombinant native ATIII and all of the variants had very similar second order rate constants for thrombin inhibition in the absence of heparin, ranging from 1.13 x 10(5) M(-1)min(-1) to 1.66 x 10(5) M(-1)min(-1). Direct binding studies using I-125-fluoresceinamine-heparin yielded a K-d of 6 nM for the recombinant native ATIII and K136T, whereas R114Q and K139Q bound heparin so poorly that a K-d could not be determined. K125Q had a moderately reduced affinity. Heparin binding affinity correlated directly with heparin cofactor activity. Recombinant native ATIII was nearly identical to plasma-purified ATIII, whereas K114Q and K139Q were severely impaired in heparin cofactor activity. K125Q and K136T were only slightly impaired. Based on these data, Lys(114) and Lys(139) which are outside of the putative pentasaccharide binding site, play pivotal roles in the high affinity binding of heparin to ATIII and the activation of thrombin inhibitory activity.