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Solution-phase equilibrium binding interaction of human protein S with C4b-binding protein
1Department of Biochemistry, University of Vermont College of Medicine, Burlington 05405.
Biochemistry
|March 5, 1991
Summary
This study reveals a high-affinity interaction between human protein S (HPS) and C4b-binding protein (C4BP), crucial for complement regulation. The equilibrium dissociation constant (Kd) indicates a strong, physiologically relevant binding interaction.
Area of Science:
- Biochemistry
- Immunology
- Complement System
Background:
- Human protein S (HPS) and C4b-binding protein (C4BP) are key regulators of the complement system.
- Understanding their interaction is vital for comprehending immune response modulation.
Purpose of the Study:
- To accurately determine the binding affinity and stoichiometry between HPS and C4BP in solution.
- To investigate the influence of calcium ions and thrombin cleavage on HPS-C4BP binding.
Main Methods:
- Solution-phase equilibrium binding studies using purified HPS and C4BP.
- Quantification of free C4BP using solid-phase immobilized HPS and an antibody detection system.
- Equilibrium dissociation constant (Kd) and stoichiometry determination.
Main Results:
- High-affinity binding with an equilibrium dissociation constant (Kd) of approximately 6 x 10(-10) M.
- Stoichiometry of approximately 1.7 HPS molecules per C4BP molecule.
- Binding is calcium-dependent and unaffected by thrombin cleavage of HPS.
Conclusions:
- The HPS-C4BP interaction exhibits significantly higher affinity than previously reported.
- The strong binding, with Kd values lower than plasma concentrations, underscores its physiological importance in complement regulation.
- These findings provide a more accurate understanding of HPS-C4BP complex formation in biological systems.