The allosteric modulation of complement C5 by knob domain peptides.

Area of Science:

• Immunology
• Structural Biology
• Drug Discovery

Background:

• Bovine antibodies possess unique ultra-long heavy chain complementarity determining regions with cysteine-rich knob domains.
• Autonomous knob domains (3-6 kDa) were previously isolated for high-affinity peptide production.

Purpose of the Study:

• To investigate the therapeutic potential of knob domain peptides targeting complement C5.
• To elucidate the allosteric mechanisms of knob domain binding to C5.
• To identify novel inhibitors of complement C5.

Main Methods:

• Peptide design and synthesis of knob domains.
• Biophysical characterization including co-crystallography and solution methods.
• Functional assays to assess C5 inhibition and pathway selectivity.

Main Results:

• Four knob domain peptides demonstrated binding to complement C5, inducing various allosteric effects.
• One peptide selectively inhibited C5 cleavage by the alternative pathway C5 convertase.
• Co-crystal structures revealed C5-knob domain interactions, with allosteric effects propagating over 50 Å.

Conclusions:

• Knob domain peptides represent a novel class of low molecular weight antibody fragments with therapeutic potential.
• The study expands the therapeutic scope of complement C5 inhibition.
• A targetable mechanistic difference between classical and alternative pathway C5 convertases was identified.