ADAMTS13-VWF interactions. (A) A homology model of the ADAMTS13 metalloprotease (magenta) and disintegrin-like domains (cyan) is shown (MD) to provide a sense of scale, with active site Zn²⁺ ion (green) and 3 structural Ca²⁺ ions (gray) as spheres. The VWF A2 domain is predicted to consist of a 6-stranded β-sheet surrounded by 5 α-helices. Residues Tyr¹⁶⁰⁵-Met¹⁶⁰⁶ (side chains in red) are buried in strand β4. Exposure of this bond to ADAMTS13 requires substantial unfolding of domain A2; more distal segments that interact with specific domains of ADAMTS13 are labeled. The locations of these ADAMTS13 domains relative to the MD moiety are not known. Deletion of strand β5 through helix α4 (dispensable) has a minimal effect on the rate of substrate cleavage. Molecular graphics prepared with PyMOL (DeLano Scientific, Palo Alto, CA). (B) The minimal segment of VWF domain A2 that ADAMTS13 is known to cleave consists of residues Arg¹⁵⁹⁷-Ile¹⁶²³. VWD type 2A mutations that increase cleavage are shown. A VWF polymorphism that may not impair cleavage is shown in parentheses to indicate that cleavability of VWF(1601T) has not been studied directly. Synthetic substrate FRETS-VWF73 is cleaved rapidly with replacement of side chains of Gln¹⁵⁹⁹ by N-methylanthranylate and Asn¹⁶¹⁰ by 2,4-dinitrophenyl.²⁹  Alanine substitutions at 3 positions reduce the rate of VWF cleavage.²⁸  Substrates with deletions VWFd4 and VWFd5 (Figure 1C) have the sequences shown, with altered residues in lowercase and preserved residues in uppercase, and neither substrate is cleaved detectably.