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Related Experiment Video

Updated: Jul 15, 2026

Measuring the 50% Haemolytic Complement (CH50) Activity of Serum

Measuring the 50% Haemolytic Complement (CH₅₀) Activity of Serum

Published on: March 29, 2010

Structure of C3b reveals conformational changes that underlie complement activity.

Bert J C Janssen¹, Agni Christodoulidou, Andrew McCarthy

¹Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Faculty of Sciences, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.

|October 20, 2006

Summary

The complement system

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Last Updated: Jul 15, 2026

Measuring the 50% Haemolytic Complement (CH50) Activity of Serum

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Published on: March 29, 2010

High-resolution Melting PCR for Complement Receptor 1 Length Polymorphism Genotyping: An Innovative Tool for Alzheimer's Disease Gene Susceptibility Assessment

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Evaluation of the Interplay Between the Complement Protein C1q and Hyaluronic Acid in Promoting Cell Adhesion

Evaluation of the Interplay Between the Complement Protein C1q and Hyaluronic Acid in Promoting Cell Adhesion

Published on: June 15, 2019

Area of Science:

Immunology
Structural Biology
Biochemistry

Background:

The complement system is crucial for innate and adaptive immunity, aiding in infection resistance and cell debris clearance.
Activated complement protein 3 (C3b) covalently attaches to surfaces, amplifying immune responses and marking cells for phagocytosis.
The conformational changes of C3 into C3b, exposing its binding sites, remain largely unknown.

Purpose of the Study:

To elucidate the conformational changes involved in the proteolytic activation of complement protein 3 (C3) into C3b.
To present the crystal structure of activated C3b and detail domain rearrangements.
To understand how C3b exposes its binding sites for protein and receptor interactions.

Main Methods:

Determined the crystal structure of activated complement protein C3b at 4-Å resolution.
Analyzed the conformational rearrangements of C3b's 12 domains upon proteolytic activation.
Compared the structure of activated C3b with native C3 to identify key changes.

Main Results:

The crystal structure reveals significant domain rearrangements in C3b compared to native C3.
The thioester site in C3b is fully exposed for covalent attachment, a significant shift from its buried position in C3.
Altered molecular surfaces expose cryptic sites, consistent with binding sites for factor B and complement regulators.

Conclusions:

Proteolytic activation of C3 to C3b involves major conformational changes primarily in the initial conversion step.
These structural insights into C3b formation reveal how its functional binding sites are exposed.
Understanding these structural dynamics is vital for developing therapeutic strategies for complement-mediated inflammatory disorders.