Modulator-induced conformational changes in complement C5, implications for function and drug design

Background: The human complement system comprises proteolytic cascades that drive protective inflammatory and cytolytic responses, making it a crucial part of innate immunity. Complement component C5 links the upstream activation pathways to the terminal pathway, and its cleavage by classical and alternative pathway C5 convertases generates potent effectors implicated in autoimmune and inflammatory diseases. Several inhibitors targeting diverse sites on C5 are in clinical use or development. However, the structural basis by which binding at distinct sites achieves comparable functional outcomes remains poorly understood.

Methods and results: We systematically compare twelve C5-modulator complexes using domain superposition, interface quantification, C5a scissile loop conformational analysis, and rigid cluster decomposition. We demonstrate that modulators propagate conformational and dynamic changes well beyond their binding sites, identifying the MG3 domain as a conformational relay that transmits perturbations across the C5 scaffold and MG8 as a domain whose rigidity correlates with global constraint redistribution. Structurally unrelated inhibitors converge on a-helical restructuring of the normally disordered C5a scissile loop, although this feature is neither necessary nor sufficient for complete pathway blockade. Benchmarking against the cobra venom factor (CVF)-bound structure as a proxy for C5 convertase engagement, we show that complete inhibitors, notably eculizumab and the tick-derived OmCI-RaCI complexes, achieve efficacy through distinct structural rearrangements rather than a single shared mechanism, whereas partial inhibitors each lack a different subset of these properties.

Conclusions: These findings establish that C5 inhibition is governed by the nature and extent of conformational perturbation rather than by simple steric hindrance of convertases alone, providing a structural framework for rational design of nextgeneration complement therapeutics.