Kinetic Analysis of the Role of Zinc in the Interaction of Domain 5 of High-Molecular Weight Kininogen (HK) with Heparin

Previous investigations have shown that HK and its light chain bind heparin, preventing the enhancement of antithrombin inhibition of thrombin and potentiating the inhibition of plasma kallikrein by antithrombin. We found that both HK and HKa bound heparin, but HK exhibited a greater affinity. We therefore localized the binding sites for heparin on HK. HK domains 5 and 6 of the light chain as well as domain 3 from the heavy chain, expressed as glutathione S-transferase (GST) fusion proteins in Escherichia coli, were tested for binding to immobilized heparin by surface plasmon resonance using a BiaCore 2000 instrument. GST-D5, but not GST-D3, GST-D6, or GST, bound to heparin when the recombinant domains were present at a concentration of 70 nM. To localize more precisely the amino acid sequences on D5, both of the subdomains, histidine-glycine-rich GST-(K420−D474) and histidine-glycine-lysine-rich GST-(H475−S626), were expressed and tested for binding to immobilized heparin. The K d was much lower for GST-(K420−D474) than for GST-(H475−S626) in the presence or absence of Zn2+. GST-(K420−D474) was effective in decreasing the rate of inactivation of thrombin by antithrombin in the presence of heparin and Zn2+, while GST-(H475−S626) had no effect. We conclude that the binding of heparin to HK is a complex function of Zn2+ interacting with histidines in the sequence K420−D474 to create high-affinity binding sites. HK has the potential to be an important modulator of heparin therapy.