Structure and activity of C1r and C1s

Area of Science:

• Biochemistry
• Immunology
• Structural Biology

Background:

• The classical complement pathway's first component (C1) involves zymogen subcomponents C1r and C1s, which become active proteases.
• Activated C1r cleaves C1s, enabling C1s to activate C4 and C2, crucial steps in the complement cascade.
• Understanding the structural and functional basis of C1s substrate specificity is vital for comprehending immune response regulation.

Purpose of the Study:

• To investigate the structural basis of C1s enzyme specificity for its substrates C4, C3, and C5.
• To elucidate the molecular interactions governing the cleavage of C4 by C1s.
• To compare the structural features of C4a with homologous complement anaphylatoxins C3a and C5a.

Main Methods:

• Amino acid sequence analysis of the proteolytic chains of C1r and C1s.
• Homology modeling of C1r and C1s sequences to the three-dimensional structure of chymotrypsin.
• Modeling of C4a amino acid sequences and docking to the active site of C1s, using C3a coordinates.

Main Results:

• C1r and C1s are identified as serine proteinases with conserved structural cores similar to chymotrypsin.
• Key catalytic residues and a trypsin-like cleavage specificity (Asp-189) are conserved in C1r and C1s.
• Distinct distribution of basic residues near C4a's C-terminal arginine, compared to C3a and C5a, suggests a role in C1s recognition.

Conclusions:

• Specific interactions between basic residues on C4a and unique glutamate residues in C1s likely mediate C1s's ability to discriminate C4 from C3 and C5.
• Structural modeling provides insights into the molecular mechanisms underlying complement component activation and specificity.
• These findings contribute to a deeper understanding of the classical complement pathway's regulatory mechanisms.