ADAMTS13 M domain residues exhibit a conformational change in the presence of antispacer domain inhibitors. (A) Depiction of ADAMTS13 M domain (PDB: 6QIG) with the catalytic Glu225 residue (blue) replacing the Gln225 in the crystal structure for illustrative purposes. Active site residues His224, His228, and His234 (blue) coordinate the zinc ion (green sphere). Most active site residues are part of the α4 helix (other helix residues shown in gray). Other M domain regions with evidence of increased deuterium uptake are shown here, namely the α5 helix and the α2 helix (dark gray). (B) M domain active site residues shown in blue as in panel A, with important areas for substrate binding depicted in light green. Specifically highlighted are the S1 pocket (thought to bind VWF residue Tyr1605), S1ʹ pocket (thought to bind VWF Met1606), and S3 pocket (thought to bind VWF L1603). Leu232 is immediately C-terminal to the α4 helix. (C-E) Residues with increased deuterium uptake in the presence of scFv3-1, scFv4-16, and scFv4-20, respectively, involving the active site (magenta) and other regions of the M domain with increased deuterium uptake (red), are illustrated.