A thermodynamic study of the interaction between human complement components C3b or C3(H2O) and factor B in solution.

Area of Science:

• Immunology
• Biochemistry

Background:

• The alternative complement pathway is crucial for innate immunity.
• Formation of the C3 convertase is a key step in this pathway.
• Understanding the interactions of complement proteins is vital.

Purpose of the Study:

• To investigate the thermodynamic parameters of factor B binding to activated C3.
• To elucidate the role of magnesium ions and hydrophobic interactions in complex formation.

Main Methods:

• Utilized the fluorescent probe 8-anilino-1-naphthalene sulfonate (ANS) to monitor binding.
• Measured binding affinities (Ka) and thermodynamic parameters (enthalpy, entropy).
• Assessed the effect of magnesium ion concentration on binding.

Main Results:

• Factor B and ANS are competitive ligands for C3b.
• Binding of factor B to C3b and C3(H2O) is endothermic and entropy-driven.
• Magnesium ions enhance complex formation, suggesting a role in hydrophobic interactions.

Conclusions:

• Hydrophobic interactions are dominant in the formation of C3b.B and C3(H2O).B complexes.
• Magnesium ions likely facilitate complex formation by increasing hydrophobic contacts.
• These findings provide thermodynamic insights into alternative complement pathway regulation.