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The human complement system: assembly of the classical pathway C3 convertase.

M A Kerr

    The Biochemical Journal
    |July 1, 1980
    PubMed
    Summary
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    The classical pathway C3 convertase rapidly loses activity after assembly. Its catalytic subunit is C2a, which interacts with C4b, while C2b aids in enzyme assembly.

    Area of Science:

    • Immunology
    • Biochemistry

    Background:

    • The classical complement pathway is crucial for innate and adaptive immunity.
    • The C3 convertase enzyme initiates complement activation by cleaving C3.

    Purpose of the Study:

    • To investigate the assembly and decay kinetics of the fluid-phase classical pathway C3 convertase.
    • To elucidate the roles of C2 fragments (C2a and C2b) in C3 convertase function.

    Main Methods:

    • Kinetic analysis of enzyme activity at different temperatures and concentrations.
    • Gel-filtration experiments to characterize enzyme complexes.

    Main Results:

    • The C3 convertase rapidly decays with a rate constant of 2.0 min⁻¹ at 37°C.
    • The enzyme is an equimolar complex of C4b and cleaved C2 (C4b x C2a).

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  • C2a is the catalytic subunit, while C2b mediates C4b-C2 interaction but lacks catalytic activity.

    • Conclusions:

    • Optimal C3 convertase activity requires high C1s levels for rapid assembly.
    • Enzyme decay results from C2a release; iodine stabilizes the complex by reinforcing the C4b-C2 interaction.