Quantitation of gC1qR binding to FXII using SPR. Plots of SPR sensorgrams measured in response units on the y-axis are shown illustrating wt-gC1qR (A) and gC1qR variants (B-D) binding to immobilized full-length FXII at increasing concentrations indicated. (B) gC1qR variant with 4 Ala substitutions made in the region of the G1 pocket (T228A, D229A, W233A, and Y236A). (C) gC1qR G2-pocket variant (S106A and D249A) and (D) gC1qR variant H187A (Zn²⁺-binding site ablation). (E) Cartoon diagram showing a close-up view of the gC1qR Zn²⁺-binding site, with key residues shown as sticks and electrostatic interactions shown as dashed lines (purple). The Zn²⁺ (gray) and water molecules (red) are shown as solid spheres. (F) Superposition of the FXIIFnII-bound gC1qR (colored wheat with the FXII side chain Arg36 in orange) with the unbound gC1qR structure (cyan) in the region of the G1 pocket illustrating conformational changes. The Arg207 side chain replaces the Zn²⁺ in coordinating the Asp185 side chain in the ligand-free gC1qR structure. Black arrows indicate gC1qR side-chain movements from ligand-free to FXIIFnII bound. Electrostatic interactions are shown as purple dotted lines.