Studies on the effect of calcium in interactions between heparin and heparin cofactor II using surface plasmon resonance

Heparin is the most acidic polysaccharide in the human body and as a result interacts with many cationic species, including ions and proteins, giving rise to myriad biologic activities. Heparin cofactor II (HCII) is a serine protease inhibitor that resembles antithrombin (ATIII) in its ability to be activated by heparin. The interaction of heparin with HCII has been the focus of many studies using affinity chromatography and fluorescence spectroscopy. In this study, surface plasmon resonance (SPR) spectroscopy was used to quantitatively measure the interaction of heparin and HCII using a heparin biochip prepared by covalently immobilizing preformed albumin-heparin conjugate. HCII contains multiple EF hand domains that represent putative calcium ion binding sites. The interactions of HCII with heparin, low-molecular-weight heparin, and heparin oligosaccharides (disaccharide, tetrasaccharide, hexasaccharide) were examined in solution competition experiments using SPR. The results also showed while calcium ions enhanced the heparin/HCII interaction, the activity of heparin-HCII complex against thrombin was not calcium dependent but can be enhanced by the presence of calcium.