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Related Experiment Videos

Dynamic equilibria between subcomponents of C1, the first component of human complement.

K C Ingham1, D J Milasincic, T F Busby

  • 1American Red Cross Biomedical Research and Development Laboratory, Rockville, MD 20855.

Molecular Immunology
|January 1, 1992
PubMed
Summary

The complement system

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Area of Science:

  • Biochemistry
  • Immunology
  • Molecular Biology

Background:

  • The complement system is a crucial part of innate immunity.
  • Activated complement 1 (C1) comprises C1r, C1s, and C1q subcomponents.
  • C1r and C1s form a stable tetramer in the presence of calcium ions.

Purpose of the Study:

  • To investigate the assembly and subcomponent exchange of the C1 complex.
  • To characterize the thermodynamic properties of C1q binding to the C1r2S2 tetramer.
  • To understand the role of calcium ions in C1 complex formation.

Main Methods:

  • Fluorescence anisotropy (A) of fluorescein-labeled C1s (s*) was used to monitor assembly.
  • Experiments were conducted in a buffered saline solution with calcium.

Related Experiment Videos

  • Kinetic and thermodynamic parameters were determined by varying concentrations and temperature.

    • Main Results:

    • C1q binding to the C1r2S2 tetramer caused a measurable increase in fluorescence anisotropy.
    • The assembly reaction was rapid at 37°C but measurable at 4°C.
    • Dissociation constants (Kd) showed an inverse temperature dependence, indicating an entropy-driven process.
    • The enthalpy change (ΔH) was -15 kcal/mol, and the free energy change (ΔG) was +11 kcal/mol at 37°C.

    Conclusions:

    • The association of C1q with the C1r2S2 tetramer is an entropy-driven process.
    • This entropy increase is likely due to the release of structured water molecules at the protein-protein interface.
    • The findings provide insights into the molecular mechanisms of complement activation.